CHAPTER IV POPULATION DYNAMICS

4. POPULATION DYNAMICS

INTRODUCTION

The main purpose of studies on fish population dynamics is to provide advice

on the optimum exploitation of aquatic living resources such as finfishes, prawns

and molluscs. It has been noticed that a number of commercial fisheries such

as the Peruvian anchovetta, California sardine, Japanese sardine and African

pilchard have been driven to commercial extinction due to over exploitation. It

is feared that the same fate may befall on some of the commercial fisheries of

India like prawns, oil sardine, mackerel and whitebait anchovies along the

southwest coast. It is therefore imperative that a fishery should be properly

managed to sustain its benefits to the future generations.

Studies on fish stock assessment involve estimation of information on age,

rate of growth, mortality and other factors which cause variations in fish stocks.

The estimation of mortality rates is a basic requirement in fish stock assessment

studies. The rate at which the numbers of a population is decreasing is the

mortality. Usually in an exploited stock there are two sources of mortality - the

natural mortality M and the instantaneous rate of fishing mortality F. The former

covers events such as predation, diseases and deaths due to old age. Thus we

have Z = F + M, where Z is the instantaneous rate of total mortality or the

mortality coeffcient or simply the total mortality rate, F is the fishing mortality

coefficient and M is the natural mortality coefficient. An essential characteristic

of a stock is that its population parameters remain constant throughout its area

of distribution (Gulland, 1969, 1983). The easiest way to describe the change

in a fish stock is oflen to follow the fate of a cohort (Sparre et a/., 1989). This

means that all fish of a cohort are assumed to have the same age at a given time

so that they all attain the recruitment age, tr at the same time. Due to mortality

there is a continuous decrease in the number of survivors. In the context of

mortality rates the number of survivors from a cohort is estimated as a function

of time.

Stock assessment of tropical fish resources has gained momentum in the

last one and a half decade mainly through the works of Pauly (1979, 1980.

1984a, 198413. 1984c), Pauly and Gaschutz (1979). Saila and Roedel (1980),

Pauly and David (1981), Pauly et a1(1981), Garcia and Le Reste (1981), Munro

(1983), Larkin etal. (1984), Pauly and Soriano, 1986, Morgan and Pauly (1987)

and Sparre and Venema (2992). The introduction of special software for fish

stock assessment in particular those based on length frequency data such as

LFSA (Sparre, 1987), COMPLEAT ELEFAN (Gayanilo, Sorianoand Pauly, 1988)

and FEAT (Gayanilo, Sparre and Pauly, 1996) has also contributed to the rapid

development of stock assessment studies on tropical fish stocks.

There have been several studies on the population dynamics of marine fin

fishes from Indian waters. Devaraj (1983) and Alagaraja (1984. 1989) have

given accounts on stock assessment models and estimation of parameters suited

for assessing tropical fish stocks. Srinath (1990, 1991) has proposed some

methods for estimating natural mortality coefficient. Till 1980 very few attempts

were made which included the works on Pseudosciaena diacanthus (Rao,

1971); Sardinella longiceps (Annigeri, 1972) ; Rastrelliger kanagurfa (Banerji ,

1973); S. albella (Sekharan, 1976); Nemipterus japonicus (Krishnamoorthi,

1976) and seer fishes (Devaraj, 1977). A large number of studies have been

undertaken since 1980, the more important among them are on silver bellies

(Venkataraman etal., 1981); mackerel (Yohannan. 1983); Nernipterusjaponicus

(Murty, 1983, 1987 and Vivekanadan and James, 1986); tunas (Silas et al.,

1986), Johnius (Johnius) caruffa) (Murty, 1986); Harpodon nehereus (Khan,

1986); cat fishes (Alagaraja and Srinath, 1987); Leiognathus bindus (Murfy.

1988); Scornberomorus commersoni (Kasim and Ameer Hamsa, 1989);

Otolithes cuvieri (Chakraborty, 1989); Sardinella gibbosa and S. dayi (Annigeri,

1989a, 1989b); Secutor insidiator (Murty, 1990) and Trichiurus lepturus

(Chakraborty, 1990). Murty, 1985 also studied the dynamics of the major species

of fin fishes in the multispecies demersal fisheries at Kakinada. Some of the

more recent publications on the subject are those of Murty (1991) on Decapterus

russellii. Annigeri et a1 1992 on Sardinella longiceps

James et a/. (1992) on tunas, Kurian and Kurup (1992) on Harpodon

nehereus, Murty etal. (1992, 1994) on threadfin breams, Reuben etal. (1992)

on carangids, Yohannan et a/. (1992) on Scomberomorus commersoni, Ameer

Hamsa et a\. (1994) on Nemipterus delagoae, Devaraj et a/. (1994) on

Rastrelliger kanagurta, Menon et al. (1994) on cat fishes, Pillai et a/. (1994)

on Scomberomorus commersoni, Rao eta/. (1994) on sciaenids, Chakraborty

(1995) on N. japonicus and Thiagarajan et a1 and Narasimham (1995) on

Trichiurus lepturus.

As the study on the population dynamics of whitebait anchovies along the

Indian coast is mainly limited to the studies made by Luther et al. (199213) an

attempt is made here to estimate the mortality parameters , yield and stock

assessment of the major species of whitebait anchovies such as Encrasicholina

devisi, Stolephorus waitei and E. punctifer.

MATERIAL AND METHODS

The landing centre was visited once a week and the data on effort, catch

and length composition of Encrasicholina devisi, Stolephorus waitei and E.

punctifer was collected on each observation day. The data on species

composition and length composition collected on each day was first weighted to

the estimated total catch of the group (whitebaits) and species respectively on

that day and such estimates in a month were pooled and then raised to the

estimated catch of the month following the procedure ofAlagaraja (1984). The

monthly estimates were pooled to get annual estimates. The gmwth parameters

(L oo and K) and the relative age estimated by ELEFAN method for E. devisi, S.

waitei and E. puncfifer have been employed for the present study.

A number of methods is available for the estimation of total instantaneous

mortality coefficient Z [ Heincke, 1913; Beverton and Holt, 1957; Chapman,

1961 ; Robson and Chapman, 1961; Pope, 1972; Ssentego and Larkin, 1973;

Ricker, 1975; Jones andvan Zalinge, 1981; Pauly, 1982,1983and Wetherall et

a/., 19871 of which the following methods have been employed in the present

investigations.

1) Length converted catch curve method (Pauly. 1982)

2)Cumulative catch curve method (Jones and van Zalinge, 1981)

3) Beverton and Holt method (1 957)

4) Ault and Ehrhardt method (1991)

5) Powell - Wetherall method (1987)

The estimation of natural mortality coefficient M was made based on

1) Pauly's empirical formula (1980)

2) Rikhter and Effanov's formula (1976)

3) Srinath's method (1990)

A recruitment pattern ,ie, of a graph showing the seasonality of recruitment

into the stock investigated was derived based on the method of Pauly (1982).

The following methods were used for the assessment of

whitebait anchovy stocks.

1) Jones' length based cohort analysis (Jones, 1974 and Jones and

van Zalinge, 1981)

2) Length based Thompson and Bell analysis (Sparre and Venema, 1992)

3) Relative Yield per recruit analysis ( Beverton and Holt, 1966)

ESTIMATION OF TOTAL MORTALITY COEFFICIENT 'Z'

Due to mortality (fishing or natural) there is a continuous decrease in the

number of survivors. At birth a cohort has age zero and from this age to Tr (the

minimum size at recruitment) it is in the pre-recruitment phase. The rate of

change in numbers per gear NI /\ t, N :

The unit of Z is per year or in general per time unit. If Z remains constant

throughout the life of acohort, the earlier equation is mathematically equivalent to

N (t) = N (tr) * exp [-Z * (t-Tr) ]

which is called the exponential decay model. The fraction surviving after

one year is called survival rate (Ricker, 1975):

N (tr + 1)

exp (-Z) =

N (tr)

1) LENGTH CONVERTED CATCH CURVE METHOD

This method discussed in Pauly (1982, 1983, 1984a and 198413) is also

referred to as linearized length converted catch curve. One of the methods to

estimate the total mortality rate is to construct catch curves by plotting the natural

logarithm of fish numbers against their age, where Z is the slope, with sign

changed of the descending part of the curve (Ricker. 1975). Here the time

taken for average fish to grow from length L1 (lower limit) to L2 (upper limit) and

the age interval mid-points are derived from the inverse von Bertallanffy equation

1 t (L) = to - - *In (1- UL 00) for making use of the equation K

where C = numbers caught in each length class, t =time taken to grow

from lower limit L1 to upper limit L2 in each length class. This is a linear equation

where

c (LI, L2) LI+L2 y = In and x = t(- 1

t (L1 ,L2) 2

which is the form of y = a + bx, where the slope 'b' = -Z, with the sign

changed Z is obtained.

Scatter plot of the values oft (Ll + L2)/2 against

In C (L1 , L2)i t (LI, L2) are used to identify the straight portion belonging

to the descending part of the catch curve. The first few length groups

representing the ascending limb consists of fish not yet fully recruited to the

fishery. Thus given a set of length frequency data and the growth parameters

L oo and Kit is possible to obtain an estimate of Z

2) CUMULATED CATCH CURVE METHOD

Jones and van Zalinge ( 1981) showed that

where c (L, L 00) stands for the cumulated catch of fish of length L and

above. Plotting In c (L, L 00) on In (Loo - L) thus gives a slope equal to ZIK, Z

being assumed as constant. If linearity is not maintained over thewhole range

of sizes examined only the linear section can be used for regression. The

usable interval must however represent a sufficient proportion of the life span to

allow a certain significance to be attached to the calculated value of 2.

3) BEVERTON AND HOLT METHOD

One of the simplest method to estimate the total mortality rate Z is from the

mean size in the catch as suggested by Beverton and Holt (1957). They showed

that the functional relationship between Z and L is

where i is the mean length of fish of L' and longer, while L' is some length

for which all fish of that length and longer are under full exploitation. L' is the

lower limit of the corresponding length interval. K and Loo are constants of the

growth equations.

4) Ault and Ehrhardt method

This is a slight modification of the Beverton and Holt method. Ault and

Ehrhardt (1991) showed that

Z/K

L oo - L max Z (L' - Lmax) + K (L 00- L mean)

[ I =

Loo-L ' Z (Lmax -Lmean) + K (L 00- L mean)

where L oo is the asymptotic length. K is the curvature parameter of the

VBGF, L mean is the mean length of the fish in a sample representing a steady

state population, L' is the cut off length or the lower limit of the smallest length

class included in the computation and L maxis the largest fish in the sample.

5) POWELL - WETHERALL METHOD

Powell (1979), discussed in Wetherall ef al .(1987) suggested a special

application of the Beverton and Holt equation by which L oo and Z/K can be

estimated. As L' can take any value equal to and above the smallest length

under full exploitation, the said equation can give a series of estimates of Z, viz,

one for each choice of L'.This makes it possible to turn the Beverton and Holt

equation based on length data into a regression analysis with L' as the

independent variable. With the corresponding mean length (L) of size groups

exceeding only the selection length (C) when plotted gives a linear relationship

equivalent to

L - L '= a + b * C

where Z/K = -(l+b)/b and L oo = -alb

or b = -WZ+K and a = -b * L oo

In the present study computations for the Beverton and Holt method and

Powell - Wetherall method for estimation of Z were executed by the programme

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'BHZWET' in the LFSA package of microcomputer programmes.

ESTIMATION OF NATURALMORTALITY COEFFICIENT M

Natural mortality is the mortality due to all causes other than fishing, e.g.,

predation including cannibalism, diseases, spawning stresses, starvation and

old age. The same species may have different natural mortality rates in different

areas and seasons depending on the density of predators and competitors. As

direct measurement of M are impossible to obtain, it has been attempted to

identify quantities which can be assumed proportional to M which are easier to

measure. As a general rule fish species with a high K value has a high M

value. Beverton and Holt (1959) found that values of the ratio MIK mostly lie in

the range of 1.5 to 2.5. Natural mortality is

linked directly to longevity (Tanaka, 1960; Holt, 1965; Cushing, 1968;

Saville, 1977; Pauly. 1980 and Alagaraja, 1984). It is also related to other

growth parameters like Loo (Sparre eta/., 1989; Srinath, 1991), W oo (Sparre

ef a/., 1989), maturity (Rikhter and Efanov, 1976), gonad weight (Gunderson

and Dygert, 1988) and mean environmental temperature (Pauly, 1980). Several

simple methods are available to estimate mortality based on the above principles.

1) RIKHTERAND EFANOV'S FORMULA

Beverton and Holt (1959) investigated a relationship between longevity,

Tm and the LmIL oo ratio, where Lm is the length at first spawning. Holt (1962)

noted that this ratio is generally about 0.66. Rikhter and efanov (1976) showed

a close association between M and TmSO%, the age when 50% of the population

is mature.

189

0.720

M = 1.521/(Tm50% ) - 0.155 per year..

They also suggested that Tm50% should be equal to the 'optimum age'

defined as the age at which the biomass of a cohort is the maximum.

2) PAULY'S EMPIRICAL FORMULA

Pauly (1980) made a regression analysis of M on K, L oo in cm and T

(average annual sea surface temperature in degrees centigrade) based on data

from 175 different fish stocks and estimated the empirical linear relationship.

Pauky (1983) suggested a correction factor for schooling by multiplication

of M by 0.80 so that for schooling species the estimate becomes 20% lower.

M = 0.8* exp [-0.0152- 0.279 * In L oo + 0.6543' In K+ 0.463* In T ]

3) SRINATH'S EMPIRICAL FORMULA

Srinath (1990) proposed the following empirical formula to estimate natural

mortality:

M = 0.4603 + I .4753 K

where K is the growlh coefficient.

ESTIMATION OF FISHING MORTALITY COEFFICIENT 'F'

As the total mortality rate Z is the sum total of natural mortality rate M and

fishing mortality rate F Q = M + F), Fis computed by the subtraction of Mfrom 2.

STOCK ASSESSMENT

Assessment of the stocks of exploited fish populations enables estimation

of maximum sustainable yield (MSY) which is the most commonly used biological

reference point for management of fishery. In stock assessment studies the

following parameters are generally used.

EXPLOITATION RATIO (U)

The exploitation ratio is defined as the fraction of fish present at the start of

a year that is caught during the year. It is computed by the equation given by

Beverton and Holt (1957) and Ricker (1975) as

EXPLOITATION RATE (E)

It refers to the ratio between fish caught and the total mortality (Ricker,

1975) or the exploitation rate or the fraction of deaths caused by fishing (Sparre

and Venema, 1992). It is computed by the equation

The exploitation rate gives an indication whether a stock is overfished or

not on the assumption that the optimal value of E = 0.5 assuming that the

sustainable yield is optimised when F - M (Gulland, 1971).

YIELD

Yield is the fraction of fish population by weight taken by the fishery and is

denoted by 'Y'.

STANDING STOCK (YIF)

The standing stock is a concentration of fish population for a given area at

a given time. It is measured in terms of numbers, weight and is estimated from

the relation : YIF, where 'Y' is the yield and F is the coefficient of fishing mortality.

TOTAL STOCK OR ANNUAL STOCK OR BIOMASS (YIU)

It refers to the total weight or number of fish population available for a given

area at a particular time. It is estimated from the relation YIU, where Y is the

yield and U is the exploitation ratio.

In the present investigations on the stock and yield of whitebait anchovies

the following methods have been employed.

1) JONES' LENGTH BASED COHORT ANALYSIS (JONES, 1974; JONES AND VAN ZALINGE, 7981).

Several methods have been brought under cohort analysis which deal with

a method of sequential computation of fishing mortality and population size

(Ricker, 1975). Pope (1972) proposed a simplified cohort analysis method

which was later on modified by Jones (1974) to enable direct application to

catch data by length classes (annual catch by length class). Jones and van

Zalinge (1981) used this method to estimate the population size of penaeid

prawns in Kuwait waters. Population biomass can be estimated by simple

multiplication of the average weight of the animals in the size groups with the

number of individuals in the size group. In this method it is assumed that the

picture presented by all lengths or year classes caught during one year reflects

that of a single cohort during its entire life span. The length based cohort

analysis is written as

where N (LI) = the number of fish that attains length L1

N (L2) = the number of fish that attains length L2

C (LI, L2) = the number of fish caught of lengths between L1 and L2

'LCOHOR' programme in the LFSA package of microcomputer programmes

(Sparre, 1987) was used for this analysis.

2) LENGTH BASED THOMPSON AND BELL MODEL

Thompson and Bell model (1934) is an age strudured model for prediction

of catch and stocksize for a given fishing pattern. The length based Thompson

and Bell model (Sparre, 1985) takes its inputs from a length based cohort

analysis. The inputs consist of the fishing mortalities by length group, the growth

parameter K and the natural mortality factor by length group, which must be the

same as used in the cohort analysis. The outputs are for each length group the

number at the lower limit of the length group N (LI), the catch in numbers, the

yield inweight, the biomass multiplied by t, i.e., the time required to grow from

193

the lower limit to the upper limit of the length group. Finally the totals of the

catch, yield, and mean biomass *tare obtained.

The equation C (11, L2) in the Jones' length cohort analysis is rewritten as

(FIZ) * N (LI) - N (L2) and given as

N (LI) = N (L2) * (LI, L2) +(F/Z) N(L1) - N(L2) * (LI, L2)

Solving this equation with respect to N(L2) gives

N (L2) = N (LI) ' [IIX (L1 , L2) - FIZ [X (LI, L2) - FIZ in which

X (L1 , L2) = [(L oo - Ll)/(L oo - L2) ] as before. In its simplest form

the length converted Thompson and Bell analysis used the F - array estimated

in cohort analysis as the reference F - array and assesses the effect of raising or

reducing all F's by acertain factor. In general case when all F values are raised

or reduced by the factor XX the general step becomes

N (Li+l) = N (Li)* l l x (Li, Li+l)-E (Li, Li+l) Ix* (Li, Li +I)-

E (Li, Li+l)

where

E (Li, Li+l) = xx * F (Li, Li+l)/Z (Li, Li+l)

Z (Li, Li+l) = xx * F (Li, Li+l) + M

C (Li, Li+l) = xx' F (Li, Li+l)'N (Li)- N (Li+l)lZ (Li, Li+l)

The yield (catch in weight) in length group i is Yield

(Li, Li+l) = C (Li, Li+l)* W (Li. Li+l)

where w (Li, Li+l) is the mean weight of f6h of lengths between Li and

Li+l . It may be calculated from

194

W (Li, Li+l) = a * (Li b + Li+l b)lZ , where 'a' and 'b' are the parameters in

the length-weight relationship

The mean number of survivors in the length group i is N MEAN

(Li. Li+l) = N (Li) - N (Li+l)/Z (Li, Li+l) and the corresponding mean biomass is

BlOM (Li, Li+l) =N MEAN (Li, Li+l) * W (Li, LI+l)

The prediction made by length converted Thompson and Bell analysis is a

prediction of the average long term catches assuming recruitment to remain

constant.

'MIXFISH' programme of the LFSA package (Sparre, 1987) was

used for the yield analysis of whitebait anchovies.

BEVERTON AND HOLT'S RELATIVE YIELD PER RECRUIT MODEL

For fisheries management purposes it is important to be able to determine

changes in the YRfor different values of F. The absolutevalues of YIR expressed

in grammes per recruit are not important forthis purpose. Therefore Beverton

and holt (1966) also developed a relative yield per recruit model which can provide

the kind of information needed for management. In the present investigations

the relative yield per recruit analysis was also performed for comparison

The relative yield per recruit model is defined by

I -E where m = -- = KIZ

U = 1- LclLoo the fraction of growth to be completed entry into the

exploited phase

E = FIZ the exploitation rate or the fraction of deaths caused by

fishing

(YIR) ' is considered a function of U and E and the only parameter is M/K.

The equation gives a quantity which is proportional to YlR. It can be shown that

(YIR)' = (Y/R)* exp [-M*(Tr-to)]NVoo,

(YIR)' can be calculated for given input values of M/K, Loo and Lcfor values

of E ranging from 0 to 1, corresponding to F values ranging from 0 to 00.

The plot of (YIR)' against E gives a curve with a maximum value, E MSY, for

a given value of LC . Thus when LC, F and z are known for a certain fishery the

actual exploitation rate can be compared with E MSY level and management

measures be proposed as necessary.

MAXIMUM SUSTAINABLE YIELD

The greatest average catch or catch rate that can be obtained for a given

cost of fishing from a stock under existing condition is called as maximum

equilibrium catch (MEC) or simply as sustainable yield or sustainable catch.

In the present studies MSY was estimated by the following two methods.

I) CORTEN'S FORMULA (1974)

x2 Y1

Y1 = - where X I is the Y/R which corresponds to catch in tonnes

X I

and X2 is the YIR at F max.

2) GULLAND'S FORMULA (1979)

Py = Zt * 0.5 " Bt, where Zt is the exponential rate of total mortality in the

year t and Bt being the standing stock size in that year.

RESULTS

ENCRASICHOLINA DEWS1

The total instantaneous mortality coefficients of E. devisicomputed for the

years 1993 and 1994 by the length converted catch curve method, Jones and

van Zalinge method, Beverton and Holt method, Ault and Ehrhardt method and

Powell - Wetherall method based on the length frequency data from trawl and

ring seine catches are given in Tables. 4.1 .a. and 4..l .b. The length converted

catch curve and the Jones and van Zalinge plot of the species based on length

composition data from trawl catches are presented in Figs. 4.1. to 4.4. The

annual mortality coefficient of the species in trawl fishery estimated by length

converted catch curve method was 5.82 for 1993 and 5.92 for 1994, with an

average of 5.87. The Jones and van Zalinge plot gave an annual mortality

estimate of 4.51 in 1993 and 4.55 in 1994 with a mean of 4.53. The mean

annual Z estimates by Beverton and Holt and Ault and Ehrhardt methods for the

two seasons were 4.26 and 4.22 respectively. The annual average Z estimate

by Powell-Wetherall method was 4.34. The average annual mortality coeficient

for the two seasons computed by the different methods was 4.64 and this was

taken for further use in stock assessment studies.

The length converted catch curve and the Jones and van Zalinge plot of the

species from ring seine catches are shown in Figs. 4.5. to 4.8. The total

instantaneous mortality coefficient calculated from the ring seine data by the

length converted catch curve method for 1993 and 1494 were 8.15 and 7.94

respectively, with an average of 8.03 for the two seasons. The Jones and van

Zalinge Z estimate gave an average value of 5.24 for the two seasons. The

annual average Z estimate obtained from mean length by Beverton and Holt

and Ault and Ehrhardt methods were 7.33 and 7.21 respectively. The Powell-

Wetherall Z estimate gave an average value of 4.07 for the ivvo seasons. The

average annual Z estimated by the different methods for the two seasons was

6.39 and this value was utilised for further studies.

The natural mortality coeficient M of E. devisi estimated by Rikhter and

Efanov's formula, Pauly's empirical formula and Srinath's method are given in

Table. 4.2. The estimate of M ranged from 1.87 (Rikhter and Efanov's method)

to 2.53 (Srinath's method). The average for these methods was found to be

2.26 which was used for further studies.

The average annual estimate of fishing mortality coefiicient F was obtained

by subtracting 'M' from the annual average 2. The average annual fishing

mortality coeficient F of E. devisifor the two seasons was computed to be 2.38

in the trawl fishery and 4.13 in the ring seine fishery.

198

The midpoint of the smallest length group in the catch during the period of

study was taken as iength at recruitment (Lr). The length corresponding to

the first value in the descending limb of the length converted catch curve was

taken as an estimate of the length at first capture (LC). The tr and tc values

corresponding to Lr and LC were calculated by the inverse von Bertalanffy

equation. The mean length at first capture was 82.5 mm and the mean age at

first capture was estimated as 1.08 years. The smallest fish of length 42.5

mm recorded during the period under study was taken as the length at

recruitment and the age was estimated as 0.41 years. Woo was estimated by

converting L oo (104 mm) using the length-weight relationship equation -

log W 00 = -5.72231 + 3.28131 log L

The raised annual length frequencies of E. devisi for the years 1993 and

1994for both the arlisanal sector (ring seine) and commercial sector (trawl) and

were used as inputs for length structured virtual population analysis (VPA). The

results are presented in Table. 4.3. and Figure. 4.9. The fishing mortality which

has been negligible in small size groups increased to 1.08 at 60-65 mm size

groups, afler which it showed a slight decline. The F increased to a maximum

of 3.39 at 80-85 mm size group.

Tables. 4.4. and 4.5. show the summary results of the length based

Thompson and Bell analysis of the species for 40 different F factors (X) in the

commercial trawl fishery and the artisanal ring seine fishery. The results are

presented also in Figs. 4.10. and 4.11. Since the whitebait anchovy resources

of the state are being exploited mainly by these gears the catches provided by

the length based Thompson and Bell analysis has been partitioned in a way as

199

shown in the Tables. The graphs and the tables clearly showthat in commercial

trawls the present level of fishing effort forthe species is almost nearthe Ffactor

that gives the maximum sustainable yield (MSY). The effort has to be increased

by 60% to obtain the maximum sustainable yield of 486 tonnes giving only a

5.4% increase in actual yield which is an uneconomic proposition. However

the maximum economic yield is at a slightly higher effort (1.3 F) which is also not

economical as the increase in value is to the tune of only 1.6%. It is known that

when the prices per kg of fish differ from one length group to another the F

factor giving MSY usually differs from the F factor giving MSE.

In the ring seine fishery the results indicate a progressive increase in yield

with increase in F factor. The maximum sustainable economic yield also has

been showing a similar trend. However, it can be seen that a doubling of

fishing effort gives only a 40% increase in yield which gives only about 15%

increase in value. Afour fold increase in effort results in only about 80% increase

in yield. It may be stated that any gains or loss in yield should be considered

together with a corresponding change in CPUE, which can be taken as roughly

proportional to the biomass calculated by Thompson and Bell analysis. The

cumulative biomasscurve above 1.3F isvery low indicating that further increase

in effort will adversely affect the stock biologically resulting in low production

and decline in catch rate.

The relative yield per recruit and relative biomass per recruit of

Encrasicholina devisias a function of exploitation rate for an M/K value of 1.61

and different values of Lc/Loo are given in Table. 4.6. and Figs.4.12.a. and 4.12.b.

The average fishing mortality of the species was estimated at 2.38 for commercial

trawls and 4.13 for commercial ring seines. The corresponding figures for

estimated exploitation rates were 0.51 and 0.64 respectively. The relative

yield per recruit Y'IR indicated a steady increase with increasing exploitation

rate at the current LclLoo value of 0.79, with a maximum Y'IR value of 0.0399624.

For an IcILoo value of 0.57 (calculated for an LC value of 60 mm) the maximum

Y'fR value of 0.049222 was reatised for an exploitation rate of 0.85. This

shows that the exploitation rate E can be increased further to realise the maximum

sustainable yield provided that it is possible to reduce the LC. However, it will

result in a drastic decline in relative biomass per recruit which is not an advisable

proposition.

Maximum sustainable yield derived fmm Corten's formula (1974) and

Gulland's method (1979) was 660 tonnes and 657 tonnes respectively.

According to Gulland the equation can be applied to only lightly exploited stocks.

The exploitation rate (E) for E. devlsi has been estimated to be 0.0.51 for trawl

fishery indicating near optimal exploitation and 0.64 for comercial ring seines

indicating heavy exploitation. Hence the MSY derived from Corten's formula

is considered for the present study. However, there has been no significant

difference between the MSY estimated by the two methods.

STOLEPHORUS WAlTEl

The total instantaneous mortality coefficient '2' of S. waitei computed for

the years 1993 and 1994 and the average for the two years by length converted

catch curve method, Jones and van Zalinge plot, Beverton and Hdt method,

Ault and Ehrhardt method and Wetherall eta/. method based on the length

frequency data from the commercial trawl and ring seine catches are given in

Table. 4.2.a and 4.2.b.

The length converted catch curve and the Jones and van Zalinge plot of S.

waitei based on length composition data from trawl catches are shown in Figs.

4.13. to 4.16. The annual average Z estimate in the trawl fishery derived by

length converted catch curve method, Jones and van Zatinge plot and Wetherall

eta/. were 6.03, 5.66 and 2.84 respectively. The estimates by Beverton and

Holt method and Ault and Ehrhardt method were 7.63 and 7.62 respectively.

The Z estimates by the different methods were found to be compatible and the

average of these values is 5.95 which is considered for further studies.

The length converted catch curve and the Jones van Zalinge plot of the

species based on the data from ring seine fishery are presented in Figs. 4.1 7 to

4.20. The annual average Z estimate for the two years in the ring seine fishery

calculated by the length converted catch curve method, Jones and van Zalinge

plot and Wetherall etal. methodwere 7.65,6.75 and 6.42 respectively. The Z

estimates calculated by Beverton and Holt and Ault and Ehrdart methods were

9.85 and 9.84 respectively. The Z estimates by the different methods were

compatible and the average value computed for the two seasons in the ring

seine fishery was 8.10 which was used in subsequent studies.

The natural mortality coefficient M of Stolephoms waifei estimated by

different methods are presented in Tabled. 4. The average M value obtained

by these methods is 1.96 which has been taken for further use in the stock

assessment studies.

The input data for the length structured virtual population analysis of

Sfolephorus waiteiwas also obtained from the raised annual length frequencies

of the ring seine and trawl catches for the years 1993 and 1994. The results of

the analysis are given in Table. 4.8. and Figure. 4.21. For S. waitei F

increased to 3.06 at 60-65 mm, which is followed by a slight decline and then

increased to a peak of 7.71 at 90-95 mm.

The results of the Thompson and Bell analysis of the species are given in

Tables. 4.9. and 4.10. which have been plotted in Figs. 4.22 and 4.23. The

results for commercial trawls clearly show that the present level of fishing effort

is well above that gives the maximum sustainable yield of 232 tonnes and that a

decrease in fishing effort by 20% will give a slightly higher yield of about 2%.

Siurnilarly the maximum sustainable economic yield MSE is afso obtained by

decreasing the F factor by 30%, which also increases the mean biomass by 30%.

The Thompson and Bell analysis for the ring seine fishery indicates that the

present effort is near the maximum sustainable yield, even slight decreases in

effort not showing any significant change in yield. Similarly doubling or trebling

of F factor also did not result in any significant increase in landings, a three fold

increase in F showing an increase of only 0.85% in the landings. It is also

seen that the maximum sustainable economic yield can be obtained by slightly

decreasing the effort by 30°h, which gives a 65% increase in value. The biomass

curve has been showing a drastic decline upto the present level of fishing effort.

The studies show that the mean biomass can be increased by reducing the

effort, with out significantly affecting the current production.

The relative yield per recruit and relative biomass per recruit of Stolephorus

waitei as a function of exploitation rate for an MIK value of 1.51 and different

values of LclLoo are presented in Fig. 4.24.a. and 4.24.b. and Table.4.11.

The mean fishing mortality F of the species was estimated at 4.0 for commercial

trawls and 6.14 for ring seines. The corresponding exploitation rate of the

species was computed to be 0.67 for trawls and 0.76 for ringseines. The

relative yield per recruit indicated a steady increase with increasing exploitation

rate at the existing LclLoo value of 0.68 with a maximum of 0.0562734.

However there is a drastic decline in relative biomass per recruit which is not

advisable. For this species a reduction in LclLoo value has not shown any

increase in Y'IR value, the maximum being only 0.048622 at an exploitation

rate of 0.65 which is almost the current fishing effort for trawls and slightly less

than that for ring seines.

Maximum sustainable yield (MSY) derived from Corten's formula and

Gulland's method was estimated at 725 tonnes and 396 tonnes respectively.

The exploitation rate (E) for S. waifei was estimated to be 0.67 in the commercial

trawl fishery and 0.76 in the ring seine fishery which indicates heavy

exploitation. Hence the MSY derived from Corten's formula is considered for

the present investigation.

ENCRASICHOLINA PUNCTIFER

The total instantaneous mortality coefficient Z of E. punctifer computed by

different methods for 1993 and 1994 based on length frequency data collected

from trawl and ring seine catches is presented in Table.4.12.a. and 4.12.b.

The results of the Z estimates by the length converted catch curve and Jones

van Zalinge plot based on length composition data from trawl and ring seine

catches are shown in Figs. 4.25. to 4.32. The average annual mortality

coefficients of the species for the two seasons in the trawl fishery estimated by

length converted catch curve, Jones and van Zalinge plot and Powell- Wetherall

method were 5.69,4.30 and 2.44 respectively. The Z estimates computed

by Beverton and Holt and Ault and Ehrhardt methods were 6.57 and 6.56

respectively. The average Z value computed by the different methods was 5.11

which was utilised for further stock assessment studies.

In the ring seine fishery the annual average total mortality coefficient Z for

the two seasons computed by length converted catch curve, Jones and van

Zalinge method and Powell - Wetherall method were 11.77, 7.85 and 5.79

respectively. The Z estimates computed by Beverton and Holt method and

Ault and Ehrdart method were 6.46 and 4.67 respectively. The average

annual mortality coeflicient computed by the different methods for the two seasons

was 7.30 which was utilised for further studies.

The natural mortality coefficient M for the species computed by different

methods are given in Table. 4. 2. The average annual M value was 2.04

which was used for further stock assessment studies.

The results of the length based Virtual Population Analysis of E. punctifef

(Table 4. 13. and Fig. 4.33.) based on input data from commercial trawl and

ringseine catches indicated a gradual increase in fishing mortality F upto 75-

80 mm, after which there was a sudden decline in F.

The Thompson and Bell long term forecast for commercial trawls

(Table.4.14. and Fig. 4.34) indicated that the level of effort expended at present

has to be increased by 30% to produce the MSY of 130 tonnes which is only

14% higher than the current production. But this reduces the mean biomass

by 34.7% over the present level and 61.8% over the virgin biomass.

The Thompson and Bell yield stock prediction for E. punctifer in the

commercial ring seine fishery (Table.4.15. and Fig. 4.35) showed that the fishing

effort can be increased to realise higher production. However it can be seen

that a cent percent increase in effort realised only a 28% increase in the landings,

whereas a 200% increase in effort yielded only a 38% increase in total production.

The mean biomass will be 41.3% and 56.3% lower than the mean biomass

available at current F and 72.4% and 80.4% lower than the virgin biomass, thus

rendering considerable loss in the strength of biomass and potential breeders.

The relative yield per recruit and relative biomass per recruit of

Encrasicholina punctifer as a function of exploitation rate for an MIK value of

1.46 and different values of LclLoo are shown in Table 4.16. and Figs. 4.36.a.

and 4.36.b. The average fishing mortalities of the species were estimated at3.07

for trawls and 5.10 for ring seines. The corresponding exploitation rates were

0.60 and 0.71. The relative yield per recruit Y'lR of this species also as in the

case of E. devisiand S. waitei exhibited a steady increase with higher exploitation

rate at the existing LclLoo value of 0.68. However for an LclLoo value of 0.52

the maximum relative Y'IR of 0.545385 was observed for an exploitation rate of

0.75. This means that the present exploitation rate of commercial trawls can be

increased by 0.15% to increase the yield. But since the relative biomass per

recruit will be drastically reduced at this exploitation rate this cannot be

implemented and the present E appears to be very near the optimum.

Maximum sustainable yield for E. punctifer derived by Corten's method

and Gulland's method were %4 tonnes and 358 tonnes respectively. Since

the exploitation rates (E) were 0.60 and 0.71 which indicate heavy exploitation

the MSY derived by Gulland's method was not considered, as according to him

this method is applicable to only lightly exploited stocks. Hence the MSY

computed by Corten's formula was used in the present study.

DISCUSSION

It is well known that estimation of natural mortality rate M in exploited fish

populations is difficult (Cushing.1968; Alagaraja, 1984). In the absence of

knowledgeof effectiveeffort pertaining to a pa~cularspecies it is not possible to

estimate M with the help of the regression of Z against effort. White bait anchovy

is not a targeted resource for most of the major gears in which they are usually

caught. However, it forms one of the major resource in the commercial ring

seine fishery during certain seasons. In the present investigations M values

were estimated following the methods of Rikhter and Efanov, Pauly and Srinath.

According to Pauly (1984) the M value obtained by his equation may be biased

upward in the case of strongly schooling fishes for which he suggested a

multiplication factor of 0.8. Since significant differences were not observed in

the M calculated by different methods, the average of these values has been

used in the present study for stock assessment.

In the present investigations the length corresponding to the first value in

the descending limb of the catch curve was taken as an estimateof the length at

first capture LC. The values arrived were 82.5 mm for Encrasicholina devisi

and stolephorus waiteiand 77.5 mm for E. punctifer. The depth ratios (Standard

lengthlmaximum body depth) for E. devisi, S. waitei and E. punctifer were

calculated as 5.4, 4.8 and 5.6 respectively. Using the nomogram given by

Pauly (1983) based on data in Sinoda et a/. 1979) and Meeneskul(1979), the

selection factor for these species were read as 2.75,2.5 and 2.9 respectively.

Using the average cod end mesh size of the commercial trawls as 16 mm the LC

value can be calculated as 44 mm for E. devisi, 40 mm for S. waiteiand 46 mm

for E. punctifer. The LC obtained by the length converted catch curve method is

therefore much greater than the theoretically possible value. It may be mentioned

in thisconnection that mechanised trawling is mainly prawn biased, with seasonal

targeting for cephalopods and threadfin breams. The whitebaits anchwyform

a targeted resource for ring seines only during a very limited period in an year.

Thus the effort is not uniformly distributed in the fishing grounds and this can

result in nonrepresentation in the catches of lengths in the population, ie, fishes

of smaller size groups are not available in areas where fishing activity is

concentrated., as otherwise smaller fishes will be retained in large numbers in

the gear since the mesh size is very small. Though the LC values estimated

based on the length composition data of ringseine catches were relatively smaller

57.5 mm for E. devisi and 62.5 mm for S. waitei and E. punctifer, these were

also observed to be higher than the expected LC values for the small mesh sized

(8 mm in netholivala) ring seines. Moreover whitebait anchovies make large

scale seasonal horizontal migrations which is closely related to the coastal

currents under the influence of monsoon. These could probably the reasons

for the difference in the LC values of whitebaits noticed in the present

investigations.

The relative yield per recruit analysisshows that there is need to decrease

the present LC and then to increase the exploitation rate to realise increased

YWIR. However, because of the factors already discussed elsewhere (uneven

distribution of fishing effort in fishing grounds and migration) these regulations

cannot be implemented.

Pauly (1982) calculated the yield per recruit for three species of anchovies

occurring in San Miguel Bay, Philippines, viz, Stolephorus heterolobus, S.

indicus andS. commeresoni and showed that the yield per recruit would increase

considerably if mesh sizes were increased to 20 mm. Pauly and lngles (1984)

obtained estimates of M from Pauly's formula using a mean water

otemperature of 27 C and Z from length converted catch curves to obtain

approximate value of fishing mortality which were used to compute exploitation

rates. The exploitation rates for the anchovies from Manila Bay were found to

be high for species such as Stolephorus heterolobus and S. zollingen which

were suggestive of overexploitation during most of the period of study. For S.

indicus the exploitation rate was near optimum.

The natural mortality coefficient M estimated by Luther et a/. (1992b)

employing Pauly's empirical formula for Stolephorus waitei and Encrasicholina

devisi was relatively high. The exploitation ratio as well as the results of

Thompson and Bell analysis and the Beverton and Holt relative yield per recruit

analysis indicated that there was scope for increasing the exploitation rate

especially for E. devisialong the west coast. The studies by them also indicated

that the present yield of S. waitei along the southwest coast is almost at MSY

level. However it has been shown that for realising an increase of 31.9% in

estimated yield for the west coast a six fold increase in fishing effort is needed,

which is not a tenable proposition. Luther et a!. (1992b) also observes that

although the results of Thompson and Bell analysis show the need for a high

increase in effort level to realise maximum sustainable yield, such an advice can

never be implemented in the multispecies - multigear context as in the present

case.

The present investigations also have shown that any substantial increase

in the yield of the major species of whitebait anchovies such as Encrasicholina

devisi, Stolephorus waifei and E. puncfifer is not possible by increasing the

fishing effort.

Fig. 4.1. Length converted catch curve of Enrrasirholina devisi based on length composition data from Trawl. 1993. Closed circles indicate points used for deriving Z .

F i lename : DEV93 Wt.mode (3a)

0.00

Reyresskon statistics

n = 4 Y-intercept (a) = 22.26 slope i h ) = -5.82 CO;~. coef . (r =- .986 Z from'catch curve = 5.q2

(CI af Z : 8.q4 to '2.79)

CRTCH CURVE

~ ' - 7 ~ F stolephorous devisi ra b 4q - I length mm ob{ervat inns

Other f i le ident if iers : Class size : F ( Imported from LFSfi: unit is not def ined 1 F 5 ' 'mu;

-1 1

16

rr 12

3 \ Z

: 8 - - 4

- &..

0 0 c3 u --.* !?....

'..? 05. -

Q

u & '-.

-

ii 1 2

R e l a t i v e age <years- t ($ )

Fig. 4.3. Jones and van Zalinqe plot of Enrrasicholina devisi, based on length composition data of trawl 1993.

JONES RND URN ZALINGE Z PLOT

16 - 15 - 14 -

A 13 f"

3 6 - u 5 .

d 3 2 . 1 -

L 0 -' 0 2.5 5

l o y < L o o - L)

Filename: DEW93 Wt.mode (3.3)

devisi Y = 1.33 + C 4.509 )*X r=0.991 other' f i fe identifiers Estimate of Z = 4.589 using growth

pi.rameters, Loo =184.881 K = 1.400.

24

n

7 P1 + X 2 12 w

u W k

0

. 1 o

9

0

C>

a 0

Q

2 0 0 -

.-.I < 4 35 , 66.75 98.5

Midlength (nn)

Fig. 4.4. Jones and van Zalinge plot of Encrasicholina deuisi based on length composition data of trawl 1994.

F i lename : TEMP Wt.mode ( l a )

t edeutr'94 Other f i ie ident i f i e r s k (imported from LFSCi: u n i t

- 1

JONES AND UAN ZALINGE z PLOT

18

n

7 a 4 X = 9 - V

P W K I*

0

18 17 16

n 15 ; 14. u 13 - 12 a 11 ' 10-

9 8 - 7 .

u 6 - a 5 .

2 2 1 0

9

0 0

9

i,

U 0

<a 0 0 a .'

- - v - -

- - - i - /

:I,' - - .'

35 66.75 98.5 Midlength

0 2.5 5 log<lon - L)

Fig. 4..5. Length converted catch curve o f Encrasicholina devisi, based on length composition data f r o m r i n g seine 1993. Closed circles indicate points used for deriving 2 .

Filename: EDRIN93 Wt.mode (3a)

.00

Regress ton statistics

11 = 3 Y- intercept la 1 = 23.38 slaye th) = -8.15 ~or'r . coef . (r 1 =- .997, Z f'rom 'catch curve = 8.15

( C I of Z : 16.f0 to '0.20) I

-. Range of. length

CATCH CURVE

16

h 12

2 \ z " C 8 - 3

4

-

" '2 a 0 0 -

b

-

a 0 i 2

Relative age (years-to)

Species name : t e devisi rinseine 1993 Other f i le ident if iers : t (Imported from LFSA: unit is not defined) -

Fig . 4.6. Length converted catch curve of Encrasicholina d e v i s i , based on length co~npos i t ion data from r ing s e i n e 1994. Closed c i r c l e s indicate p o i n t s used for der iv ing Z .

CATCH CURVE Filename: EDRIN94 Wt.mode (3a)

. oo

Regression statistics

n = 6 Y-intercept (a) = 20 .a0 s l o p e (h) = -7.91 ~or'r. coef . (r) =- .975 Z prom 'catch curue = 7.q'l

(CI of Z : 10.44 to 5.39) I

16

,. 12 z

8 - C - 4

95 k e deuisi ring seine 1994 Other file identitier?:

---C

-

0"% :\

- 0 0

o

-

O B 1 2

R e l a t i v e age ( y e a r s - t @ )

Fig. 4..7. Jones and van zalinge plot of Encrasicholina devisi based on length composition data of ring seine 1993.

F i lename : EDR IN93 Wt.mode (3a)

b e devisi rinseine 1993 Other file ident if iers b (imported from LFSCI: urn it

-1 -

JONES BND UBN ZBLINGE Z PLOT

24

h

v < 1P 4 X = 12 V

cj W K I*

B

14

13 12

" 11 i 0 10 Y 6 9 - v 8 -

7 ' 6 . : 5 - V b ? - 4 : 0 :3

::

. 1

Q

9 0

0 2 0

-+-+--el 2

- - - / - -

-

- -

35 ' 63.75 92 .5 Midlength

L

' B 2 . 5 5 loy<Loo - L)

Fig. 4.8. Jones and van Zalinqe plot of Encrasicholina devisi bared on length composition data of ring seine 1W3.

F i lename : EDR IN94 W t .mode ( )

1

REGRESSION EQUATION: b e deui'si r i n g s e i n ~ 1994

Other f i l e ident i f i e r i from L F S ~ : u n i t

I

36

h

v < a -I X 18 "

v W K k4

JONES RND UfiN ZRLINGE Z PLOT

. Q 0

-

2 0 0 0

.' '3 " 0

14 13 12

" 11 L U 10 * 6 9 - 0 8 -

7 6 - 5

U 5 . V m 9 - 0 :3 d '

1 . . -

35 ' 63.7.1; 9 2 . 5 Midlength]

- . - - -

-

.

a @ 2.5 5 log<Loo - L)

-

Fig. 4.9. Length cohort analysis of Enrrasichol ina d e v i s i based on length composition data of trawl and ringseine during 1993 and 1994.

PARAMETERS LENGTH STRUCTURED VIRTUAL POPULATION ANALYSIS

File t EDRSTR

L-inf inity t 104 mm

K constant t 1.4

Nat. mortaI ity t 2.26 Terminal F t 0.590

Mean E t fl.228 Mean F

0.666 LEGENDS :

E deuisi (neryed 1993 and 1994 ring seine & trawl)

4 - A 2 -

w 5 : 6 < .* a r( 4 ffi : - Y : 2

0-- 9 7 . 5

Lensth I classes

!!!? catch Nat.losses

m s u r u i uors -:-f jshfny

Aortal ity merged 1993 and

INSTqUCTIONS

Spec jes name : E deujs: otl'lel! file ident if ierr: :

a*e i-ne K traw 1 .I994 rind .'. I :

Fig. 4.10. Length based Thompson and Bell analysis of Enrrasicholina devisi based on data from commercial trawls during 1993 and 1994.

- Cumulative Curve

.6 d

@ I*

3

9 . 5 1 1.5 2 2 . 5 3 3 . 5 4 f -f ac tor

b?

! r 0 .*

E9 a .5 1 1.5 2 2.5 3 3 . 5 4

f - fac tor

Specie? : E devisi Fleet ' : Ring seine

Tra At f-fdctor = 1 . B - I I - - Comb i n a t ion Cumu lat i ve q Yield : 3

Fig. 4-11. Length based Thompson and Bell analysis Encrasicholina devisi based on data from commercial ringseines during 1993 and 1994.

Fig. 4.12.a. Beverton and Halt's Relative Yield per Recruit for Encrasicholina d e v i s i for different values of L c / L w 1. Lc/Loo = 0.79.

n 4

N

& 3 . 5 4 " 3 - Y Id

3 2:5 Sd '

: 2 - Sd \ a 1.5 d . * .d 1 n

.5 d * K

1. 0000 Lc/Loo = 8.79 Rel. yield/recruit 1. 0000 .M/K = 1.61 Kn if e-edge opt ion

E-- - 5 : 0.4484

- ,--=

-

.

-

. /

-

I*

4 3 - I

an .25 .5 .75 1

Exploi tation rate

1 - 2 * K 1

- I

- I

@la .25 .5 ' .75 1

\ Exploitation rate

. 7 5 , -\-

1

Exploitation rate Exploitation rate

Optima : ~ m h x : 0.8400 Lc/Loo = 0.57 Re1 . y ield/recruit E-jl : 0.7582 M/K = 1 .61 Knife-edge option E-'5 : 0.3844

Fig. 4.12.b. Beverton and Holt's relative yield per recruit for Encrasicholina devisi for different values of Lc/Loo 2. 1 c / l w = 0.57

Fig. 4.13. Length converted catch curve of S t o l e p h o r u s w a i t e i based on length composition data from trawl 1993. Closed circles indicate points used for deriving Z.

Filename: TEMP Wt .mode (lb

4.00 . 0.00

Regression statistics

n = 8 Y-intercept (a) = 23.03 slope (h) = -6.44 ~o$r. coef . (r) =- .924 Z from 'catch curve = 6:1f4

(CI of Z : 9.i'Oto "3.78) I

Range oc length

CATCH CURUE

2 0

,-. 15 $ \ z :la CI

5

- 00

0

0 0 00 \Yo - \

0 I-. O +--lo

-

-

iaaa 1 2

Relative age {years-tB>

Species name : k swtr93 Other f i le identifiers : k (Imported from LFSFI: unit is not defined)

Fig . 4.14. Length converted catch curve of Stolephorus vaitej based on length composition data from trawl 1994. Closed circles indicate points used for deriving Z.

Filename: TEMP Wt.mode (lb)

Regression statistics

n = 7 Y-intercept (a) = 23.46 s l a p e (b) = -5.61 ~or'r. coef . (r) =- ,994 Z f'rom 'catch curve = 5.6'1

(CI of Z : 6.30 to "4.91) I

CnTCH CURVE

110 b length requency stalephorus waitei 1994

2 0

n 1 5

4 \ z ; 1 0 CI

5

-

2,- 0

- 0 Q 'L. :\

Q \ - Ib

-

23- 1 2 3

Relative age (years-t0)

Fig. 4. 15. Jones and van Zalinge plot of Stolephorus u i f i t e i based on length composition data from trawl 1993.

JONES O N D V A N Z a L I N G E Z PLOT

18 - 17 - 16 -

n 15 -

2 14 -

o 13 - 2 12 . 11 - ' 18.

9 - ; 8 - 7 .

u 6 - 5 .

0 4

1 - 8 - I

8 2.5 5 l o g C L o o - L>

F i lename : TEMP W t .mode (la) 1

0th1$ f i l'e i d e n t i f i e r s t Ffmported from LFSfi: u n i t --

92

c. 'n < m 4 X 46 V .

v W K I4

(3

. 0

0

. Q

C>

0 0 0 0

0 <I

2 7 ,-

8 35 76.75 118.5 Micl lengt l l

-.

Fig . 4-16. Jones and van Zalinqe p l o t of Stolephorus uaitei based on length colnposition data from trawl 1994.

JONES aND V A N ZALINGE z PLOT

22 r 21 - 20 - 19

h l a

0 6 -

a - a - 5 -' L

' 1 a 2.5 5 logCLoo - L)

F i lename : TEHP Wt.mode ( l h )

I-: lenythl requency s t o l e y h o r i s w a i t e i 1994 = 2 . 1 9 + ( 4 . 9 3 1 )+X r=0.997

Other f i l'e i d e n t i f i e r ? I I ( Estimate of Z = 4 . 9 3 1 us ing growth b ( imported from LFSF;; u n i t i s not def ined).!meters. Loo =I18 .DO; K = 1.2013.

I

22 . 1

n F- < m 4 X

11 Y

4 W K L*

0

0

9

0 9 .

0 0 0

0 en 3 2 -' r 3 : e L r b

38 ' 68.75 167.5 Midlength

Fig. 4.17. Length converted catch curve of Stolephorus wa i te i based on length composition data from ring seine 1993. Closed circles indicate points used for deriving 2 .

Filename: TEMP Wt.mode (Zb)

. 1.30 UP : 8.00

Regression statistics

11 = 5 Y-intercept (a) = 14.29 slape (b1 = -8.84 CO&. coef. (rl =- .99q Z prom 'catch curve = 8 . €(4

(CI of Z : 10.29 to '6.89)

CC~TCH CURVE

observatians F swaitei ring seine 1993 cochin Other file identifiers : I

8

n 5 - 4 . \ z " c '? d

.> ?'

-

0 *\ -

-

43 6 1 2

Relative age (years-tb3)

Fig. 4.18. Length converted catch curve of Stolephorus uaitei based on length composition data from rinq seine 1994. Closed circles indicate points used for deriving Z.

Filename: TEMP Wt.mode (lb)

1.30 U P : 0.00

- Regressfon statistics

n = 10 Y-intercept (a) = 25.26 s l a p e (h) = -6.45 ~or'r. coef . (r) =- .966 Z f'rom 'catch curve = 6.45

(CI of Z : 7.q7 to '5.03)

C ~ T C H CURVE

Species name : Range OF length b s waitei ring seine 1994 cochin Other file identifiers: b (Imported from LFSA: unit is not defined

--L

24

A 18

$ \ z :I2 #M

6

-

c x l -

0

-

-

@ 8 1 2

Relative aye <years-t0)

Fig. 4.19. Jones and van Zalinge plot o f Stolephorus waitai based on length composition data f r o m r ing s e i n e 1993.

JONES ~ N D ufiN z~LINGE Z PLOT

8 -

7 - A

6 . U Y

4 i

I

0 2 . 5 5 log<Loo - L>

Filename: TEMP Wt.mode ( Z h )

1

ring seine 1993 cochin I :

Othiir f i Te ident if iers from L F S ~ ; 1 L

-.

72

n rl

& 4 X 35 w . .

Q W P: Ir

,3

. Q

. ca

Q 0

0

Q 0 2 --- = - *

- . 8 35 68.75 102.5

Midlength

-

Filename: TEMP Wt.mode (lh) J O N E S BND U B N Z B L I N G E Z PLOT

14 . 1 0

0 n

7 a 4 X 0

= 7 , V

Q W 0 K h 0 0 0

0 Q

--Lu L

6

0 2 0 0

35 71.75 568.5 2.5 5 Midlength lor<Loo - L5

I

Y = -1.56 + ( 5.592 )*X ; r=0.981 : I

I I -

Fig. 4. 20. Jones and van Zalinge plot of Stolephorus uaitei based on length composition data from ring seine 1994.

F i g . 4.21. Length c o h o r t a n a l y s i s o f S t a l e p h o r u s u a i t e i based on l e n g t h composi t ion d a t a from trawl and r i n g s e i n e c a t c h e s during 1993 and 1994.

PARAMETERS LENGTH STRUCTURED VIRTUAL POPULATION ANALYSIS

F i l e b SWRINTRL

L-infinity F 130

K constant b 1 . 3

Nat . mortal i t y b 1.96

Terminal F b 0.600

Mean E b 0 . 4 4 3 . .

Mean F 1.738

LEGENDS. :

-SUI?~J iuors' -f tns '

l e n g t h requency s t o l e p ~ r o r u s w a i t e i 1 9 9 4 ( ( I m p o r t e d from LFSR; u n i t i s n o t d e f i n e d ) )

rn 6 - Y

8 - Cd . i i <

Id . , d

7 - a 5 - 4

6 6 - . Y I . * 4 -

5 - " a r - 3 -

a ; .. I c - Y .* : : d 2 - C 2 - 3

UI ; :

. . n -

I I 3 7 . 5 1 1 7 . 5

Length c l a s s e s

INSTFpCT IONS

( Spttcles name length r(!tir.t4nc9 s t o l e y ~ t ~ j k l ~ f i l e ident i f i e r s : ( Irnpa'rte3. f torn LFSA; un i G ' :s not def ined) r r i

Fig. 4.22. Length based Thompsm and B e l l analysis 0f

Stolephorus waitei based on the data from commercial trawls for the period 1993-1-4-

Fiq. 4.23. Length based Thompson and Be11 analysis of Stolephorus waitei based on the data from commercial ring seines for the period 1993-1994-

Opt :ma : mix : 1. 0000 Lc/Loo = 0.68 E-jl : I . 00~10 H/K = 1.51

Rel. yield/recruit Knife-edge option

Fig. 4.24.a. Beverton and Holt's Relative Yield per Recruit curve for Stolephorus wai tei for different values of Lc/Loo. I. Lc/Loo = 0.68

Fi9. 4-24... Beverton and H01t.s Relative yield per R ~ c v u ~ ~ curve for Stolephorus wai tei for different values of Lc/Loo. 11. Lc/Loo = 0.46

Fig. 4.25. Length converted catch curve of Enrrasicholina punctifer based on length c o a p o s i t i ~ data of trawl 1993. Closed circles indicate points used for deriving Z.

F i lealame : TEMP Ut.mode (lb)

Reyressfon statistics

n = 6 Y-intercept (a) = 25.36 s l o p e ch) = -6.49 C O ~ . coef . (r) =- .9@1 Z Qrom 'catch curve = 6.49

( C I af Z : 10.8;5 to '2.14) ,

CATCH CURVE

observations

0thhr file ident if iers : t (imported from LFSA; unit is not defined) t 5 : ' 1

18 n

z \ = 12 V

C M

5

- n Q 0 '.

"-'-.\. 9 0

-

-

!3; 1 2

Relative aye <years-t0>

F i g . 4.26. Length converted catch curve of Encrasichol ina punc t i f e r based on lenqth composition data of trawl 1994. Closed circles indicate points used for deriving 2 .

CRTCH CURVE Filename: TERP Wt.mode (lb)

. 1.40 WP : 0.00

Regresston statistics

n = 4 Y-intercept [a) = 20.73 s l o p e (h1 = -4.89 ~ o i r . coef. (r) Z f'rom 'catch curve = '- -9 4. 9

(CI af Z : 14.S1 to '4.34)

16

h

$12 \ Z Y

C 8 -

4

-

0

- U

==0,0 0 ==

-

e 1 2

Relative age <years-tB)

Fig. 4.27. Jones and van Zalinge plot of Encrasicholina punrtiferbased on length composition data o f Trawl 1993.

F i lename : TEMP Wt.mode clh)

1

Y = 3.81 + ( 4.016 )*X r=O.965

A

52

h

;" a 4 X = 26 u .

Q W P: L*

(3

JONES &7ND UbN ZfiLINGE Z PLOT

28 19 18 17

h 16 15 14 13

5 8 . u 7 -

6 -

d

. 0

o <I

3

9

0

0 2 Q .--e - - L*

3 2 1

'35 . 73.75 112.5 M i d l e n g t h

---

- - -

0 bL

I

0 2.5 5 l o g < L o o - L)

Fig. 4.28. J o n e s and van Zalinqe plot of Encrasicholina punctifer based on length campositim data of trawl 1994.

Filename: TEMP Wt .mode (lb)

. QO

Regresston statistics

n = 6 Y-intercept (a) = 21.23 slope (b1 = -4.58 ~oir. coef . (r =- .955 Z f'rom'catch curve = 4.58

(CI of Z : 6.5'6 to '2.61) I

C ~ T C H CURUE

Species name : C. epunt94 Other f i le identifiers :

20

A 15

? \ z .; la -

5

-

- 0

%&oQ

-

-

e I 2

R e l a t i v e age <years-t0)

Fig. 4.29. Length converted catch curve of Encrasicholina punrtifer based on length composition data Of ring seine 1993. Closed circles indicate points used for deriving 2 .

Filename: TEHP Wt.mode (la)

. 80

Regress ?on statistics

n = 4 Y-intercept (a) = 29 -82 s lope (b) = -13.9U ~ o + r . coef. (r) =- .962 Z f'rom 'catch curve = 13.70

CCI of Z : 25.q6 to 1.94) I

CnTCH CURUE

Species name : b punc93 Other f i le ident if iers : b (Imported from LFSA! unit is not defined)

--

2 0

A 15

$ '. z .;la -

5

-

-

-

-

B 1 2

Relative age (years-t0)

Fig. 4.30. Length converted catch curve of Encrasirholina punrt i fer based on length composition data of ring seine 1994. Closed circles indicate points used for deriving 2 .

Filename: TEMP

. 1.48

Regresskon statistics

n = 4 Y-intercept (a) = 24.81 s lope (b) = -9.65 cork. coef . Ir ) =- .787 Z f'rom 'catch curve = 9.65

(CI of Z : 32.6'4 to -13.35) ,

CC~TCH CURUE

---L

18 n

$ \ 212 v

c M

5

0 0 0

-

. -

-

; 1

Relative age (years-to>

Flg. 4.31. Jones and van Zalinge plot Encrasicholina punctifer based on length composition data of ring seine 1993.

F i lename : TENP Wt.mode 0 I

b punc93 Other f i lie ident if iers b (imported from LFSA:

- -

JONES f i ~ ~ URN Z ~ L I N G E Z PLOT

28

LI 'a < m r( X 5 1 4 .

v W K h

0

20 14 18 17 16

,a 1 5 $ 1 4 - 6 13

. 1 0 . f 9 . 5 8 . 0 7 . " 6 - a 5 . 2 4: 3

2 1 ' 0

. o

*

2 P

0

- m .----0 s =

- - - - - .

: .

- -' I

45 ' 68.75 - 92.5 Midlength

I - 0 2 . 5 5

l o g < L o o - L:, I

Fig. 4.32. Jones and van Zalinge plot of Encrasicholina punrti fer based on length composition data of ring seine 1994.

r

REGRESSION EQUATION : t ppunr in94 Other' f i le identifiers Estimate of Z = 8.338 using growth t (imported from LFS~; m i . - is not define{)ameters, Loo =110.80; K = 1.498.

. I 1

Filename: TEHP JONES AND V A N ZALINGE Z PLOT

14

n r- < a 4 X = 7 . "

e W K k

0

. 20 - 19 . 18 . 17 -

2 ;; : 1 4 -

2 13 . i f : 18.

E 9 - 5 8 - 0 7 - " 6 -

z z : 2

3 . 2 -

0 .- =!a * 1 . i 2 82'i5 0

-' 45 . 6 3 . 7 5 0 2.5 5

Midlength . . Ios(Loo - L)

Fjle ,'- EPTRRSPL L-'-inf inity i- 120 mm

K 'cohstant r - 114 N&. mortality

r? 2.04 ~;:rhinal F

f a ' hl .588 Il;:an. E li 'u.300

H;:an. F i- '0.374

catch Vat.losses

rf3j:,;urv i v o r s I

-.-.f i s t ~ . n y morfality

I

LENGTH STRUCTURED VIRTUAL POPULATION ANALYSIS E punctifer

<Trawl K Ring seine, 1993 K 94)

Length classes 1 E punctif er Trawl & Ring seine, 1993 Pi 94

Fiq. 4.33. Length cohort analysis of Encrasicholina punct i fe r based on length composition data from trawl and ring seine during 1993 and 1994.

Fig. 4.34. Length based Thompson and Bell analysis of Encrasichol i n a puncti fer based on data f r- commercial trawls for the period 1993-1994 .

Thompson and Bell Routine CE punctifer) Cumulative Curve

a - I*

3 - 1.5 2

< rn 4

f -f ac tor

X 4l

2 r 0 .* m

0 .5 1 1.5 2 2.5 3 3.5 4 f -f actor f -f actor

- I 1 1 1 m

lSpedie*l : 1E punctif er I

~kegt : Ring Seine fi.t :?-f il.l:t~~r = 1 .a

I I Comhinat I jon Cumulatfue - : ) . 8?41~!.08 4 .96<10t+08 ,I .6h ' (7~. ' BE3 C).3754t+B8 3 ,824 , . 1~-"08 ,. 4.9680k+08 .

Fiq. 4.35. Length based Thompson and Bell analysis of Enrrasicholina punctifer based on the data from commercial ring seines for the period 1993-1994.

Fig. 4.36.a. Beverton and Holt's Relative Yield per Recruit curve for Encrasicholioa puflctifer for different values of Lc/Loo. I. Lc/Loo = 0.68

-. 0 P: . l - I

0 .25 .5 .75 1

Exyloi tation rate

* 6

4 " 4.5

* 4 .* 2 3.5

3 \ 2.5

0 8- 1.5 . n

1 .

5 0

.

Opt :ma: ~mAx : 1 . 8888 Lc/Loo = 8 .68 E - ? l : 1. 8000 M/K = 1.46 E-'5 : 0.4161

P: .25 .5 .75 1

Exploitation rate

Rel. yield/recruit Kn if e-edge opt ion

Fig. 4.36.b. Beverton and Holt'c Relative Yield per Recruit curve for Encrasichol jna puncti fer for different values of Lc/Loo. 1 1 . Lc/Loo = 0.52

6 - h

" 4 .5 - * 4 - Id 2 3.5 -

I I

a l .d 1.5 - I n

I I I,

.5 .75 1

Exploitation rate

1

.d 5 8 . SI U .7 - 01

\" .6. w ffi

. 4

3. .2 01 I u .1 I

0 .25 .5 .75 1

Exploitat~on rate

Opt jma : Ern;tx : 0 . 7380 Lc/Loo = 8 .52 E-'1 : 0.6504 H/K = 1.46 E-'5 : 0.3686

Rel. yield/recruit Knife-edge option

Q 4 . : Estiaated total annual wrtality coefficient 111 osmq d~ffermt methcds m6 annual averaqe Z of Encrasicholina dcvisi in trarl fishery

_____._________-___-------------------------------------d------------------

Hrthcds 1993 lW4 Rveraqc

Length converted catch curve S .a 5.- 5.87

lanes and Van lalings plot 4.51 4.55 4.53

Yetherall et 11, fm PWell- 5.35 3.34 4.W Yetherall plat

Bevertm h Holt Z frol nean lenqth 4.B 4.31 4.26

Table 4.l.b Estinated totdl annual mrtdl~ty coefficient (11 uslng dlfferent methods md annual average 1 af Eflcrrsicholina devisi in ring selne fishery

Length converted catch curve 8.15 7.91 8.03

Jmes and Van Zalinqs plat 6.20 4,28 5.24

Yetherall et al. fm Paell- Yetherall plot

Bevertm 1 hlt 1 frw .em length 7.02 7.63 7.33

Z fm Rult L Ehrhastdt 6.79 7.62 7.21

Table 4.2. Annual natural mortality coefficient M of major species of white bait anchovies estimated by different methods

_______-_______-___-------------------------------------------- Name o f species Rikhter Pauly Srinath Average

and Ef anov ...............................................................

E. devisi 1.87 2.39 2.53 2.26

S . waitei 2.10 1.41 2.38 1.96

E. punctifer 2.10 1.51 2.52 2.04

T.PRIF 3 : The F S ~ F U ~ S & # M pPQ~edure for yield and average biocnass i n Jones' Length Cohort Chalysis of Encrasicholina devis i a t Cochin.

~engtn-structured VPA results for

LENGTH CLASS

Total catch : Mean E Mean F

CATCHES (N) (c*loA 2 )

282,021,344 .OO 0.228 (from Lmin 0.666 to Lmax)

POPULATION F. MORTALITY (N*IO* 3 )

Natural mort. : 2.26 LOO : 104 mm K : 1.4

[ FiSAT Output :11-25-1997(16:49:27) 1

VPA I1 results for (a= 1.893-06 ; b= 3.2813 )

............................................................

ML DELTA T MEAN N CATCH STEADY-STATE (mm) (years) (tonnes ) BIOMASS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

42.50 0.058 67417816.00 0.65 28.21 47.50 0.063 63884104.00 1.31 38.47 52.50 0.069 59632128.00 16.02 49.83 57.50 0.077 53894424.00 44.01 60.68 62.50 0.086 46677868.00 75.22 69.07 67.50 0.098 39445408.00 65.16 75.11 72.50 0.114 33062210.00 61.32 79.57 77.50 0.135 26548058.00 84.34 79.51 82.50 0.167 18510420.00 139.80 68.05 87.50 0.218 9495130.00 143.65 42.34 92.50 0.316 3392098.25 54.73 18.15 97.50 0.579 1465110.25 5.50 9.32

............................................................. TOTAL 423424800.00 691.70 618.29 .............................................................

Table. 4.4. Thompson and Be11 long term forecast of Encrasirholina devisi at Cochin. (Trawl 1993- 1994). HSY = Haximum Sustaianble Yield. MSE = Haximum Sustainable Economic Yield.(Value).

E devisi and Trawl Cochin Fisheries Harbour (f-factor varies from 0 to 4 )

f-factor Yield Biomass Value (10- 6 ) (10- 71 (loA 8 )

e. 4.5. Thcmpsan and Bell long term forecast of EncrasichoJina d c v i s i at Ccwhin (Ring seine 1993-1994). M Y = tiaximum Sustainable Yield. r(SE = flaximurn Sustainable ECMM~C Yield (Value).

E dev i s i and Ring s e i n e (f-factor varies from 0 to 4 )

f-factor Yield Biomass Value (10- 6 ) (10- 7 ) (10- 8 )

Table 4 Relative Yield per Recruit fY/R) and relative Biomass per Recruit IB/R) of Encrasicholina devisi at Cochin as a function of exploitation rate E tor different values of Lc/Loo.

-

Parameters : Lc/Loo = .57 M/K = 1.61

Opt irna : Emax = 0.840 E-.1 = 0.758 E-.5 : 0.384

[FiSAT Output :12-01-1997 (14:38:23) 1

RELATIVE YIELD/RECRUIT : Knife-edge Parameters : Lc/Loo = .79 M/K = 1.61

Optima: Emax = 1.000 E-.l = 1.000 E-.5 : 0.448

[FiSAT Output :12-01-1997 (14:38:28) I

Table 4-7a E s t i u t e d to ta l annual m r t a l i t y coeff icient ( 2 ) using d i f f t ren t methods and annual average Z of Stalephorus waitei in t r u l fizhery

------- - ------------------ ~e thods 1995 1994 Averaqe ------------- Length converted catch curve 6.44 5.61 6.03

Jones and Van Zalings p lo t

Yetherall e t a. f ro r P a e l l - Yetherall plot

Beverton I Halt Z frm mean length 7.52 7.74 7.63

Z f m Ault & Ehrhastdt 7.51 7.73 7.62

Average o

Table 4.7.b Estimated total annual m r t a l i t y coefficient (21 using d ~ f f e r e n t methods and annual averaqe Z of Stalephorus na i te i in ring seine fishery

flethods 1995 1994 Rveraqe

Length converted catch curve 8.84 6.45 7.65

Jones and Van Zalinqs plot 7.91 5.59 6.75

Yetherall e t a. f m Powell- Yetherall plot

Beverta I Halt 2 f r m rean length 11.56 8.14 9.85

Z f r m Ault & Ehrhastdt 11.55 8.13 9.84

Table 4.8. The calculation procedure for y i e l d and average biomass in Jones' Length Cohort M a l y s i s of Stolephorus w a i t e i at Cochin.

VPA I1 results for (a= 4.41E-06 ; b= 3.10904 )

........................................................... ML DELTA T MEAN N CATCH STEADY-STATE (cm) (years) (tonnes) BlOMASS

- _ _ _ - _ _ - _ - _ _ _ - _ - _ ____-__-- - - - - - - - -==-- - -_- - - - - - - - - - - - . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

37.50 0.042 19260202.00 0.00 6.68 42.50 0.044 18701216.00 0.54 9.56 47.50 0.047 18071892.00 1.76 13.05 52.50 0.050 17085624.00 14.66 16.83 57.50 0.053 15207651.00 46.63 19.87 62.50 0.057 12642650.00 65.59 21.40 67.50 0.062 10273228.00 58.06 22.09 72.50 0.067 8462173.00 47.73 22.72 77.50 0.. 073 6906752.50 54.51 22.81 82.50 0.081 4979641.00 97.08 19.97 87.50 0.091 2932507.00 89.28 14.12 92.50 0.103 1414166.38 62.47 8.09 97.50 0.119 597774.31 26.74 4.03 102.50 0.140 246914.27 11.42 1.94 107.50 0.172 118238.55 2.42 1.08 112.50 0.221 183525.00 1.16 1.93 117.50 0.312 0.00 0.00 0.00 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

TOTAL 137084128.00 580.04 206.19 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Length-structured VPA results for

LENGTH CLASS CATCHES (N) POPULATION F. MORTALITY (C*lOA 2 1 (N*lOA 2 )

Total catch : 213,547,776.00 Natural mort. : 1.96 Mean E : 0.443 (from Lmin Loo : 130 cm Mean F : 1.558 to Lrnax) K : 1.3

Table. 4.9. Thompson and Bell long term forecast of Stolephorus uritei at Cachin. (Trawl 1993- 1994). HSY = Haximum Surtaianble Yield. HSE = Haximum Sustainable Econoric Yield. (Value ) .

Combinations :

length requency stolephorus wa and Trawl Cochin Fisheries' Harbour (f-factor varies from 0 to 4 )

f-factor Yield Biomass Value (loA 61 (10- 71 (loA 8 )

Table. 4.10. Thompson and Be11 long term forecast of S t o l ephorus wai tei a t Cochin (Ring seine 1993-1994)- MSY = Maximum Sustainable Yield. M E = Maximum Sustainable Economic Yie ld (Value)

combinations:

length requency stolephorus wa and Ring seine (£-factor varies from 0 to 4 )

f -factor Yield Biomass Value ( l o A 6 ) ( l o A 7 ) (loA 8 )

Table 4.11. Relative Yield per Recruit (Y/R) and relative Biomass per Recruit (B/R) of Stolepharus uaitri at Cochin as a function of exploitation rate E for different values of Lc/Loo

RELATIVE YIELD/RECRUIT : Knife-edge Parameters : Lc/Loo = .46 M/K = 1.51

Opt irna : Emax = 0.669 E-.1 = 0.616 E-.5 : 0.351

[FiSAT Output :11-29-1997 (15:46:06) 1

RELATIVE YIELD/RECRUIT : Knife-edge Parameters : ~c/Loo = .68 M/K = 1.51

Optima : Emax = 1.000 E-.l = 1.000 E-.5 : 0.416

[FiSAT Output :12-01.1997 (14:32:00) ]

T a b l e l 2 . a . Estinated total annual wr ta l i ty coefficient 12) using different nethods and annual averaqe I of Encrasicholina punctifer in tram1 fishery

____I_________-__-_ ----- -----

Methods 1993 1994 Rveraqe

Lenqth converted catch curve 6.47 4.89 5.69

Iones andVan Zalinge plot 4.02 4.58 4.30

Yetherall e t a. from P ~ l l - b thera l l plot

Beverton k Halt Z fm mean length 6.92 6 . m 6.57

Z f r m Rult k Ehrhastdt 6.91 6.21 6.56 ___________________-----------------------_I__---

Table 4 12.b Estiaated total annual wr ta l i ty coefficient 121 usinq different nethods and annual average 2 of Encrasicholina punctifer ma rinq seine fishery

Methads 1993 1994 Rveraqe

lcnqth ccoverted catch curve 13.9 9.65 11.77

Jmes and Van Zalinqe plot 7.37 8.33 7.85

Yetherall et a. frcu Parell- Yetherall plot

Bcvertm & Holt 1 fm nean lenqth 7.6 5.32 6.46

Z f r m Rult & Ehrhastdt 7.59 1.75 4.67

Table 4.13. The c a l c u l a t i o n procedure f o r y i e l d and average biomass i n Jones' Length Cohort Analysis o f Enrrasicholins punctifer a t Cochin.

Length-structured VPA results for

LENGTH CLASS CATCHES (N) (c*1oA 2 )

40.00- 45.00 57.61 45.00- 50.00 0.00 50.00- 55.00 19,982.36 55.00- 60.00 153,140.42 60.00- 65.00 434,227.15 65.00- 70.00 240,105.67 70.00- 75.00 338,605.97 75.00- 80:OO 357,149.82 80.00- 85.00 162,254.92 85.00- 90.00 80,053.35 90.00- 95.00 22,215.71 95.00-100.00 87,437.25 100.00-105.00 0.00 105.00-110.00 0.00 110.00-115.00 0.00 (Ct)

Total catch : 189,523,008.00 Mean E : 0.300 (fromLrnin Mean F : 0.874 to Lmax)

POPULATION F. MORTALITY (~*10- 2 )

Natural mort. : 2.04 Loo : 120 mm K : 1.4

[ FisAT Output :11-26-1997(16:15:23) I

VPA I1 results for (a= 3.32E-06 ; b= 3.15277

--- ............................................................

ML DELTA T MEAN N CATCH STEADY-STATE (mm) (years) (tonnes) BIOMASS

--- ............................................................

42.50 0.046 27809350.00 0.00 12.62 47.50 0.049 26974008.00 0.00 17.36 52,50 0.053 26054936.00 1.76 22.98 57.50 0.057 24670296.00 17.99 28.98 62.50 0.062 21973652.00 66.32 33.56 67,50 0.068 18975354.00 46.73 36.93 72.50 0.075 16066935.00 82.53 39.16 77.50 0.084 12548380.00 107.41 37.74 82.50 0.095 9684789.00 59.42 35.47 87.50 0.110 7912381.00 35.29 34.88 92.50 0.130 6775648.00 11.67 35.58 97.50 0.159 17487450.00 54.20 108.41 102.50 0.205 0.00 0.00 0.00 107.50 0.290 0.00 0.00 0.00 112.50 0.495 0.00 0.00 0.00 --- ............................................................

TOTAL 216933184.00 483.32 443.67 --- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Table. 4.14. Thompson and Bell long term forecast of Encrasicholina punrt i fer at Cachin. (Trawl 1993- 1994). USY = Maximum Sustaianble Yield. W E = Maximum Sustainable E C M ~ i c Yield. (Value).

Combinations:

E punctifer and Trawl (f-factor varies from 0

Biomass ( l o A 7 )

Value (10- 61

Table. 4.15. Thompson and Bell long term forecast of Encrasicholina punctifer at Cochin (Ring seine 1993-1994). MSY = Maximum Sustainable Yield. MSE = Maximum Sustainable Economic Yield (Value).

Combinations:

E punctifer and Ring Seine (f-factor varies from 0 to 4 )

f-factor Yield Biomass Value (loA 6 ) (10" 71 (loA 61

Table 4.16. Relatlvr Yield per Hprvult < V / R ) n relative Bl-nss I - Racru~t <R/R) n F fn~rasichol in* n t , n r - t i f e r a k f,oci~rn as a fu~~s.llon s>f ex(11oikaL~on - ~. ~-~

r a t e E for d ~f ferer~t values af Lc/i.oo

RELATIVE YIELD/RECRUIT : Knife-edge Parameters : Lc/Loo = .52 M/K = 1.46

Optima: Emax = 0.738 E-.1 = 0.650 E-.5 : 0.369

[FiSAT Output :12-01-1997 (14:21:16) 1

RELATIVE YIELD/RECRUIT : Knife-edge Parameters : Lc/Loo = .68 M/K = 1.46

Optima : Emax = 1.000 E-.l = 1.000 E-.5 : 0.416

SUMMARY