FFB-311-L1&2-(Introd)POPULATION DYNAMICS

8/7/2019 FFB-311-L1&2-(Introd)POPULATION DYNAMICS

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FFB-311Population

Dynamics&

Stock Management


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Carrying Capacity

Migration

(emigration / immigration)

Density Dependent

Mortality

Density DependentNatility

Density Dependent

Population Growth

Ecological Conditions

Density Independent

Exploitation

Density Independent

Sustainability of Stock


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The Komering River in Indonesia now dries up almostcompletely in the dry season due to irrigation works

upstream


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Barrier traps such as corong flume traps and tuguk suspendedtrawls in Indonesia are expensive to construct, but highly effective

at catching migrating fish


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In Bangladesh, dry season floodplain water-bodies may even be pumped dry to extract the

last remaining fish


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Even small floodplain water-bodies may enable manybrood fish to survive the dry season, if protected as

reserves (River Ganges, India)


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Fish StockFish stocks are subpopulations of a particular species of

fish, for which intrinsic parameters (growth, recruitment,mortality and fishing mortality) are the only significant factors

in determining population dynamics, while extrinsic factors

(immigration and emigration) are considered to be

insignificant.

The currently accepted definition of a stock in fisheries

science, is A stock describes characteristics of semi-

discrete groups of fish with some definable attributes

which are of interest to fishery managers (Begg et

al.,1999).

In fisheries management, stock refers to a harvested or

managed unit of a fish.


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Concept of Stock In general, stock is the genetically differentiated and

geographically isolated populations of a species. In fishes, there is a tendency to form a structured series

of discrete populations which have a degree ofreproductive isolation from each other in space, in time,or in both.

This isolation resulted into the genetic differences,morphological variations and variable exposure todifferent chemical regimes and parasitic species amongthe sub-populations.

Sub-populations also respond to fishing in such a waythat fishing on one population appears to have no effecton the population dynamics of a neighbouringpopulation.


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Stocks are not always composed of a single

species. Stocks can be composed of multiple

species due to their being harvested together. An example of a multispecies stock is river

herring. Alewives and blueback herring are

labeled as river herring for management

purposes due to their similar physicalappearances and being harvested together.

Individuals within a stock are subdivided into

cohorts. A cohort is a group of fish born inthe same year within a population or stock.


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A stock can be both transboundary and straddling stock

Straddling stock stocks of fish which migrate between, or

occur in both, the economic exclusion zone (EEZ) of one or

more states and the high seas"

Transboundary stock range in the EEZs of at least two

countries.

Straddling stock include highly migratory fish stock.

Straddling stocks are usually pelagic, rather than demersal

because demersal species move less than pelagic species,

since they tend to relate to bottom topography.

Pelagic species are more mobile as their movementsinfluenced by ocean temperatures and the availability of

zooplankton as food.


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Example of straddling pelagic fish are

capelin, pollock, herring, whiting, mackereland redfish etc.

There are, however, a few demersal

species that are straddling, such as the

Greenland halibut migrates in

feeding/spawning migrations to Greenlandin the west and to the Faeroes in the east.


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What is Population Dynamics(subject of moving bodies)?

The size of population at any given time may be taken to be a convenient endindex of many forces that act upon the population.

Ultimate size of the population is determined by the forces acting on growth of thepopulation which is influenced by natality, mortalityand dispersion.

Natality has the +ve influence on population growth and mortality has veinfluence where as dispersion may be positive or negative depending uponwhether it is one of emigration or immigration. All these forces are controlledby various ecological forces which are density dependent and independentfactors.

Independent factors given are optimum ecological condition of a population by theintervention of human beings, and grows its maximum size in right proportion withthe carrying capacity of its ecosystem but exploited population never attain this levelas the actual size being the function of intensity of exploitation.

The dynamics could be allowed if the short term benefits are worth for more than thelong term benefits normally fails to receive support from conservatives and

ecologist.It is always good if the reflection are made for optimizing yield so as the sustain the

benefits for long time.

Dynamics of fish population seek to provide the basis frame work for scientificadvised in respect of optimization which is the central time of the fisheries resourcesmanagement.

F P t t t t


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F s Popu at on tructure tsUnitsPopulation (Collection of units, which is having some common

characteristics): A fish population can be defined as aggregates ofindividuals of the same species comprising possibly in a number ofschools inhabiting in a well defined geographical area and cutoff fromanother discrete population of the same species through a discontinuitybetween their distributional ranges.

Unit Stock: A population can be considered as a unit stock if its membersexhibit sufficiently uniform characterstics in respect of the growth,spawning, natural mortality, responses of fishing, morphometric and

meristic characters and serological properties etc.Multiple Stock: In a multiple stock population signifies differences of their

in the characters. However, these multiple stock may tend toamalgamate (unite) into a unit stock through gene flow unless theconstituents stock separate junction spatially at time of spawning.

Exception:For the purpose of fishing yield models males and females of a

population are sometimes treated as separate units if they differ fromeach other in characters such as growth mortality etc.

Population of different species co-existing in a particular area andexhibiting similar characters is some times treated as a single stock (unitstock) for modeling purposes.


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Loss of Fisheries:Due to irrational exploitation of FisheryResources

1- North sea or Norwegian Herring (Clupea harengus) : Stocks was

fluctuated with a regular periodicity since the 19th century (1950) due

over exploitation.2-Hakkaido Herring (Clupea pallasi): Peak catch in1897; it subsequently

declined and disappeared in 1960.

3- Japanese Sardine (Sardinops melanosticta): Peak catch 2 million

tones in 1936 and catch was not recovered upto ofter 30 years and it

replaced by Anchovies during 1970s.

4-Californian Sardine: Fishery flourished between 1920-1951; peak catch

in 1936 and remained high until 1947 and after the year 1949-50, it

failed completely and fishery becomes extinct.

In the Country: In our Indian water the spotted sear fish(Scomberomorus guttatus), which supported commercial fishery in

Palk Bay for over three decades and it collapsed in the early 60s dueto un-control fishery .

In the case of prawn fishery: Although absolute catch is not declinedsignificantly but exact signs of stock depletion are apparent from the

general declining trend in CPUE and the decreasing size of the

prawns in catch.


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HISTORY:

The first principle of population dynamics is

widely regarded as the exponential law ofMalthus as modelled by the Malthusian growth

model.

A more general model formulation was

proposed by F.J. Richards in 1959, by which the

models of Gompertz, Verhulst and also Ludwig

von Bertalanffy are covered as special cases ofthe general formulation.


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A fishery is an area with an associated fish or aquaticpopulation which is harvested for its commercial or

recreational value.

Population dynamics describes the ways in which a givenpopulation grows and shrinks over time, as controlled bybirth, death, and emigration or immigration.

It is the basis for understanding changing fishery patternsand issues such as habitat destruction, predation andoptimal harvesting rates.

The population dynamics of fisheries is used by fisheriesscientists to determine sustainable yields.


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The basic accounting relation for

population dynamics is the BIDE

model:

N1 = N0 + B D + I E

where N1 is the number of individuals at time 1,

N0 is the number of individuals at time 0, B is

the number of individuals born, D the number

that died, I the number that immigrated, and Ethe number that emigrated between time 0 and

time 1. While immigration and emigration can be

present in wild fisheries, they are usually not

measured.
http://en.wikipedia.org/wiki/Matrix_population_modelshttp://en.wikipedia.org/wiki/Wild_fisheries_of_the_worldhttp://en.wikipedia.org/wiki/Wild_fisheries_of_the_worldhttp://en.wikipedia.org/wiki/Matrix_population_models


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A fishery population is affected by three dynamic ratefunctions:

Birth rate orrecruitment: Recruitment means numberof new youmg fish that enters in a population in a given

year.

Growth rate: This measures the growth of individuals insize and length. This is important in fisheries where the

population is often measured in terms of biomass.

Mortality: This includes harvest mortality and naturalmortality. Natural mortality includes non-human

predation, disease and old age


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Terms associated with Population Dynamics

Virtual population analysis:

Virtual population analysis (VPA) is a modelling

technique commonly used in fisheries science for

reconstructing historical fish numbers using information

on death of individuals each year. This death is usually

partitioned into catch by fisheries and natural mortality.

Minimum viable population:

MVP is the smallest possible size at which a biologicalpopulation can exist without facing extinction from naturaldisasters or demographic, environmental, or genetic

stochasticity


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Maximum sustainable yield:In population ecology and economics, the maximum

sustainable yield or MSY is, theoretically, the largest catchthat can be taken from a fishery stock over an indefinite

period.

Meta-population: A metapopulation is a group of spatiallyseparated populations of the same species which interact

at some level. The term was coined by Richard Levins in

1969.

A metapopulation generally consists of several distinctpopulations together with areas of suitable habitat which

are currently unoccupied.

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FFB-311-L1&2-(Introd)POPULATION DYNAMICS