UNIVERSITI PUTRA MALAYSIA

SELECTIVITY STUDIES ON MALAYSIAN TRAWLS

MOHD. IBRAHIM BIN HAJI MOHAMED

FPSS 1987 2

It entitled Ibrahim

is hereby certified that we have read "Selectivity Studies on Malaysian Trawls"

bin Haji Mohamed, and in our opinion it is of scope, quality, and presentation

o f the requirements for the degree o f in terms fulfilment Philosophy

�� . .. . . . . . . . . n'UDDIK, Ph.D.

Assoc . Profess an of Graduate Studies Universiti Pertanian Malaysia (Chairman Board of Examiners)

TSUK , Ph.D. Professor/President

Shimonoseki University of Fisheries Yoshimi, Shimoniseki, JAPAK

(External Examiner)

AKDRES K BRAKDT, Ph.D . rofessor

University of Hamburg Wiesbaden, Fed. Rep . of Germany

(External Examiner)

� . . K.WJ..W� A � .M. MOHSIK, Ph.D .

Associate Professor Faculty of Fisheries and Marine Science

Universiti Pertanian Malaysia (Internal Examiner )

this thesis by Mohd.

satisfactory as partial

Doctor o f

... 111.!l1.�� GUNZO KAWAMURA, Ph.D.

Faculty of Fisheries and Marine Science

Universiti Pertanian Malaysia

(Internal Examiner/Supervisor)

This thesis was submitted to the Senate of Universiti Pertanian Malaysia and was accepted as partial fulfilment of the requirements for the degree of Doctor of Philosophy.

Date: t 5 JUl 1987 Alan Zainuddin, Ph. D. Associate Professor/ Dean of Graduate Studies

SELECTIVITY STUDIES ON MALAYSIAN TRAWLS

by

Mohd . Ibrahim Bin Haj i Mohamed

A thesis submi tted in partial fulf i l lment of the requirements for the degree of Doctor of Phi losophy

in the Faculty of Fisheries and Marine Science , Universiti Pertanian Malaysia .

June , 1 987

To my wife Aidah,

&

my son Adil Ridha ,

DEDICATION

Your sacrifices , undying support , and constant encouragement

wil l forever be remembered .

ACKNOWLEDGEMENT

I am truly indebted to my superv isor Assoc iate Professor

Dr . Gunzo Kawamura whose guidance , cr itical comments , constant

encouragement and intellec tual st imu lation has tremendousl y

assisted m e during this per iod o f candidature . Also t o m y co­

supervisor Associate Professor Dr . Abu Khai r Mohammad Mohsin ,

whose patience , encouragement , and c r itical comments have been

a source of inspiration for me during the l ong hours o f

preparation , fieldwork , analysis and thesis preparat ion .

My heartfelt grati tude are also extended to Mohamed Bin

Muda , Akesah Bin I smail , Sujak Bin Samad , Saat Bin Mat and

Mohd . Zin Bin Abu Bakar whom at one t ime or another assisted me

during the f ield work in Terengganu and Selangor and i n the

ardous task of measuring the f ish samples . A special mention is

reserved for Mahmud Bin Yusoh whose assi stance cont inued during

the data entry and thesi s preparation . I am also grateful to

Puan Badar iah Yusof who assisted me in typing the various

tables and i l lustrations quickl y and efficientl y .

I am very grateful to the Univers it i Pertanian Ma laysia

for providing the financ ial assistance necessary to carry out

th i s project . To the ma ny fr iends and col legues who ha ve been a

constant source of encouragement , I extend my sincere thanks .

To the f ishermen who were kind enough to accomodate me on their

f i shing trips I am true1 y grateful .

Finally my eternal gratitude goes to Professor Nelson

Marshall of the University of Rhode Island , USA , for his

encouragement and t rust .

i i i

TABLE OF CONTENTS

page

ACKNOWLEDGEMENT iii

TABLE OF CONTENTS iv

LIST OF TABLES viii

LIST OF FIGURES x

LIST OF PLATES xiii

LIST OF APPENDICES xiv

NOMENCLATURE xv

ABSTRAK xvi

ABSTRACT xviii

CHAPTER 1. INTRODUCTION

The Malaysian Fishery 1

The Developmen t of fishing Technology in Malaysia 5

Signs of Overfishing

Promise of The EEZ

Fishery Management in Malaysia

The Present Fishery Malaysia . . . .

Conservation Methods

Probl ems of Fishery Managemen t in Malaysia

How This Study Relates

CHAPTER 2 . LITERATURE REVIEW

Theory of Mesh Selection

Selection Factor

Length-Girth Relationship

iv

8

13

16

in 1 8

23

29

34

36

38

4 1

page

Depth Ratio 42

Selectivity Da ta on Shrimp 43

Optimum Mesh Size 44

Prac t ical Considerations in the Conduct of The Exper iment 45

Mesh Measurement 46

Design of Cod-end Covers 47

Randomization 48

Subsampl ing 48

Other Factors Affecting Cod-end Mesh Selection 49

Cod-end Material

Catch Size

Trawling Speed

Bottom t ime

CHAPTER 3 . MATERIALS AND METHODS

The Study Area

The Trawls

Cod-end Cover Design

The Boat s

Methodology

Mesh Measuremen t

Speed Determination

Fish Measurement

v

49

49

50

50

52

55

58

60

65

65

66

66

CHAPTER 4 . A MODEL FOR DETERMINATION OF SELECTION CURVE BY USING LENGTH-DEPTH DATA

INTRODUCTION

Theory

MATERIALS AND METHODS

RESULTS

Computing the Selection Curve

Masking

DISCUSSION

CHAPTER 5 . CATCH SELECTION

INTRODUCTION

MATERIALS AND METHODS

RESULTS

Fish Trawl

Shr imp Trawl

DISCUSSION

Mesh Selection in Fish and Shrimp Trawl

Trawling Speed

Bot t om Time

Catch Size

CHAPTER 6. THE EFFECT OF SPEED , BOTTOM TIME AND CATCH SIZE ON TRAWL SELECTION

INTRODUCTION

MATERIALS AND METHODS

RESULTS

v i

page

69

72

78

79

89

99

99

1 1 3

1 1 5

1 1 7

1 1 7

1 23

1 28

1 28

1 29

1 29

1 32

1 37

140

143

Effect of Trawling S peed

Effect of Bottom Time

Effect of Catch Size

DISCUSSION

CHAPTER 7 . THE APPLICATION OF MESH SELECTION IN THE MALAYSIAN TRAWL FISHERY

INTRODUCTION

MATERIALS AND METHODS

RESULTS

DISCUSSION

Implementation of Trawls

Strategy for Mesh Selec tion

CHAPTER 8 . CONCLUSION , SUMMARY AND RECOMMENDATIONS

CONCLUSION

SUMMARY

RECOMMENDATIONS

BIBLIOGRAPHY

APPENDICES

VITA

vii

page

143

149

155

158

1 64

1 66

1 68

1 74

1 76

180

187

193

1 95

202

2 18

Tables

1 . 1

1 . 2

3 . 1

3 . 2

4 . 1

4 . 2

4 . 3

4 . 4

4 . 5

4 . 6

5 . 1

5 . 2

5 . 3

LIST OF TABLES

Marine Fish Landings by Gear Groups in 1983 ( Peninsular Malaysia ) .

Potential Resources in the Malaysian EEZ ( in Tonnes) .

Trawl Boat Par t iculars .

Experimental Design .

Cod-end Mesh Size Measurement of Fish Trawl .

Cod-end Mesh Size Measurement of Shrimp Trawl .

Examples of Values of Standard Dev iation of Maximum Body Depth Distribu tion .

Tables of aD , p and q Values of Major Species in Fish Trawl .

Tables of aD , p and q Values of Major Species in Shr imp Trawl .

Pred icted and Exper imental Values of L50 , SR and E Values for Fish and Shrimp Trawls .

Catch Composi tion of Major Groups in the Fish Trawl for 25 mm and 51 mm Cod-end .

Mean Fish Trawl Catch by 25 mm and 5 1 mm Cod-end Mesh Size .

Mean Fish Trawl Catch by 2 and 3 Knots Trawling Speed .

5 . 4 Mean Fish Trawl Ca tch by Mesh S ize and Bottom Time .

5 . 5

5 . 6

Catch Composi tion of Major Groups in the Shrimp Trawl for 25 mm and 38 mm Cod-end .

Mean Shr imp Trawl Catch by 25 mm and 38 mm Cod-end .

v iii

Page

5

14

61

62

81

82

94

95

96

97

1 18

1 20

1 21

1 22

1 24

1 25

S . 7

S . 8

S . 9

5 . 10

6 . 1

6 . 2

6 . 3

6 . 4

6 . 5

6 . 6

6 . 7

7.1

Mean Shrimp Trawl Catch by Mesh Size and Trawling Speed .

Mean Shrimp Trawl Catch by Mesh Size and Bottom Time .

Coefficient of Determination ( R2 ) on the Correlations of Cover Ca tch , trash Fish Catch and Total Catch on Bot tom Time and Total Catch .

Mutiple Linear Regression of Cover Catch and Trash Fish Catch on Total Catch .

LSO Values of 2 and 3 Knots Trawling Speed for Fish Trawl .

LSO Values of 2 . S and 3 . S Knots Trawling Speed for Shrimp Trawl .

LSO Values of 1 , 2 and 3 Hours Bottom Time for Fish Trawl .

LSO Values of 2 . 5 and 3 . S Hours Bottom Time for Shrimp Trawl .

LSO Values for 3 Catagories of Catch Size of Fish Trawl .

LSO Values for 2 Catagor ies of Catch Size of Shrimp Trawl .

A Summary of Findings and Compar ison with Publ ished Data .

Selection Factors of Species for Fish Trawl .

7 . 2 Selection Fac tors of Species for Shr imp Trawl .

7 . 3 Optimum Mesh Size for Fish and Shrimp Trawl .

ix

page

1 26

1 2 7

1 3 2

1 33

146

1 47

150

IS3

IS6

158

1 60

169

170

1 7 4

Figure

1 . 1

1 . 2

2 . 1

3 . 1

'3 . 2

3 . 3

3 . 4

3 . 5

4 . 1

4 . 2

4 . 3

4 . 4

4 . 5

4 . 6

4 . 7

4 . 8

4 . 9

LIST OF FIGURES

Total Marine Fish Landings in Malaysia

Trawl Catch Composi t ion in Peninsular Malaysia

Length Selec t ion Curve showing L50

and SR

Fish Trawl Study Area .

Shrimp Trawl Study Area .

Plan of Fish Trawl .

Plan of Shrimp Trawl .

Cod-end Detachable Arrangement .

Relationship between Maximum Body Depth D and Vertical Mesh Opening MI

Distribut ion of Each Length ( L ) Various Length

Maximum Body Depth ( D) for when aD is Common for

Normal Distribution Curve Showing Retention a t Critical Depth Dc

Cummulative Distribut ion Curve of the Probability Density Function of the Standard Normal Distribut ion

Distribution of P for Various L

Standard Length - Maximum Dep th Relationship of Major Spec i es in Fish Trawl .

Standard Length/Total Length - Maximum Depth Relationship of Major Species in Shr imp Trawl .

Carapace Length-Total Length Relationship of Shrimp Species .

Example of Determination of Deviation of Maximum Bod y Depth Probabi lity Paper .

x

Standard by Using

Page

2

1 1

39

53

54

56

57

59

72

74

75

76

76

83

86

90

92

4 . 10

4 . 1 1

4 . 1 2

4 . 1 3

4 . 1 4

4 . 1 5

4 . 1 6

4 . 1 7

4 . 1 8

4 . 1 9

5 . 1

6 . 1

6 . 2

6 . 3

Pred icted and Exper imental Select ion Curves for Fish Trawl Species .

Predicted and Experimental Selection Curves for Fish Trawl Species .

Predicted and Experimental Selec tion Curves for Fish Trawl Spec ies .

Predi cted and Experimental Selection Curves for Shrimp Trawl Species .

Pred icted and Exper imen tal Selection Curves for Shrimp Trawl Species .

Pred icted and Experimental Selection Curves for Shrimp Trawl Species .

Length Composi t ion of Catches by 25 mm Cod­end Fish Trawl by Covered and Non-covered Cod-ends .

Length Composit ion of Catches by Covered and Non-covered Cod-ends .

Length Composi t ion of Catches by 38 mm Cod­end Shrimp Trawl by Covered and Non-covered Cod-ends .

S ize Relationships and General Body Forms of Fish and Shrimp Species Examined

Catch Componen ts of Trawl .

The Effect o f Trawling Speed on Select iv i t y of Fish Trawl .

The Effect of Trawling Speed on Selectivity of Fish Trawl .

The Effect of Trawling Speed Selectivity of Shrimp Trawl .

on

the

the

the

6 . 4 The Effect of Bot tom Time on the Selectivity of Fish Trawl .

6 . 5 The Effec t of Bot tom Time on the Selectivity of Fish Trawl .

x i

page

100

101

102

103

104

105

106

107

108

III

1 30

144

145

148

1 5 1

1 52

6 . 6

6 . 7

6 . 8

7 . 1

7 . 2

The Effect of Bottom Time on the Selectivity o f Shrimp Trawl .

The Effect of Catch Size on the Selectivity of Fish Trawl .

The Effect of Catch S ize on the Selectivity of Shrimp Trawl .

Scat tergram of Selec t ion Factor and Mesh S ize for Fish and Shrimp Trawl

Regression of LsO Against Mesh Size for Fish and Shrimp Trawl .

x i i

page

1 54

157

1 59

1 72

1 7 3

LIST OF PLATES

Plate page

1 . A Typical Shrimp Trawl Catch in the West Coast of Peninsular Malaysia 10

2 . Cod-end Cover During Operation 60

3 . Constant Tension Mesh Guage and Dutchman ' s Log 67

4 . Differences in the Size of Fish Caught by 25 mm and 5 1 mm Cod-end 1 34

xii i

LIST OF APPENDICES

Appendix

A . Table o f Catch Data

B . Worksheet Measurement o f Data

C . Fish Trawl Catch Data

D . Shrimp Trawl Catch Data

E . Results o f T- test Computat ions for Fish Trawl

F . Results o f T-test Computations for Shrimp Trawl

G .

H .

Analysis of Variance Tables for Fish Trawl Catch Components

Analysis of Var iance Tables for Shrimp Trawl Catch Components

xiv

page

202

203

204

205

208

2 1 0

2 1 2

2 1 5

NOMENCLATURE

D max imum body depth

Dc = critical body depth

DR = depth ratio

L25 = 25 percent retention length

L50 = 50 percent retention length

L75 = 75 percen t retention length

Lc = length of f irst capture

Loo = the maximum length of a species

Lo = opt imum length of first capture

L/G = l ength - gi rth

M' = vert ical mesh opening of a net

Ms = mesh size of a net

Mext . = mesh size extension after loading

Mo = optimum mesh size for a f ishery

Sc = selection curve

S f = selection factor

SR selection range

growth constant K

F rate of f ishing mortal i t y

M = rate of mortal ity

Z rate of natural mortality

E experimental L50 / Predicted L50

CL carapace length

TL = total length

xv

Abstrak tesis yang d ikemukakan kepada Senat Pertanian Malaysia sebagai memenuhi sebahagian keperluan untuk Ijazah Doktor Falsafah .

SELECTIVITY STUDIES ON MALAYSIAN TRAWLS

oleh

Mohd . Ibrahim Bin Haj i Mohamed

Jun , 1 987

Penyelia Profesor Madya Dr . Gunzo Kawamura

Uni versiti daripada

Penyelia Bersama Profesor Madya Dr . Abu Khair Mohammad Mohsin

Fakul t i Perikanan dan Sains Samudera

Suatu kaj ian yang menggunakan kaedah "Covered Cod-end" dan

70 tundaan puka t tunda ikan dan udang telah d i jalankan di Laut

China Selatan dan Selat Melaka untuk mengkaji corak pemilihan

pukat pukat tunda di Malay sia . Penyiasatan ini adalah bertujuan

un tuk mengurangkan tekanan yang hebat terhadap sumber-sumber

ikan terutamanya terhadap tangkapan anak-anak ikan komersial .

Suatu "model" untuk meramalkan geraf pemi lihan spesis ikan

dan udang berasaskan ukuran panjang dan lebar spesis tersebut

telah dicipta . "Model" ini akan menyenangkan kerja penyiasatan

pemil ihan pada pukat dengan hanya mendapatkan makluman tersebut

dari sampel ikan atau udang dari mana-mana tangkapan .

Hasil penyel id ikan ini menunjukkan bahawa saiz ma ta

keroncong 25 rom menangkap 98 . 20 peratus berbanding dengan saiz

xvi

mata keroncong 5 1 mm yang menghasi lkan 56 . 10 peratus dari ikan­

ikan yang memasuki pukat tunda ikan . Dengan pukat tunda udang

pula , sai z ma ta keroncong 25 mm menangkap 92 . 60 peratus

manakala saiz mata keroncong 38 mrn menangkap 5 1 . 35 peratus dari

jumlah ikan yang memasuki pukat tersebut .

Bersamaan ini , ikan baja merupakan 46 . 40 peratus dari

tangkapan pukat tunda ikan dan 68 . 20 peratus dari tangkapan

pukat tunda udang yang menggunakan saiz mata keroncong 25 mm .

Ini berbanding dengan 34 . 60 peratus bagi pukat tunda ikan yang

menggunakan saiz mata keroncong 51 mm dan 56 . 80 peratus untuk

pukat tunda udang yang menggunakan saiz mata keroncong 38 mm .

Walaupun kenaikan kelaj uan menunda mengurangkan pemilihan

dan tambahan masa menunda menambah pemilihan pada pukat saiz

mata keroncong yang besar , saiz mata keroncong yang keci l telah

menghasi lkan keputusan yang berlawanan . Kesan-kesan kelajuan

dan masa menunda hanya kecil dan d iatasi oleh kesan tangkapan

yang besar . Tangkapan besar mengurangkan pemilihan pad a semua

sai z mata keroncong dan pukat tunda ikan mahupun pukat tunda

udang .

biasa

Berbeza dari anggapan rama i , saiz mata pukat kecil yang

d igunakan dalam indust r i perikanan di Malaysia,

mengakibatkan faktor pemil ihan yang depend en kepada saiz mata

pukat . Berasaskan kepada faktor pemilihan yang d idapati, saiz

mata pukat yang optimum bagi industri pukat tunda negara adalah

55 mm bagi pukat tunda ikan dan 38 mm untuk pukat tunda udang .

xvii

An abstract of the thesis presented to the Senate of Universiti Pertanian Malaysia in partial fulfillment of the requirements for the Degree of Doc tor of Phi losophy .

SELECTIVITY STUDIES ON MALAYSIAN TRAWLS

by

Mohd . I brahim Bin Haj i Mohamed

June , 1987

Supervisor .: Associate Professor Dr . Gunzo Kawamura

Co-Supervisor Associate Professor Dr . Abu Khair Mohammad

Mohsin

Faculty Fisheries and Marine Science

A covered cod-end experiment was conducted employing 70

hauls of f ish and shrimp t rawls in the South China Sea and the

Malacca Stra its to determine the selectivity pattern of

Malaysian t rawl s . Thi s study was conducted i n order to

alleviate the intensive fishing effort on the f ishery stocks

especia l ly towards the juveni les of commerciall y important

species .

A mathematical model was developed to predict the

selec tion curve based on the standard length and maximum body

depth of fish and shrimp species retained by the net . Data

xviii

col lec tion in a selec tivit y experiment is now simplified to

obtaining the two parameters f rom f ish or shrimp sampled from

any catch .

Results of the study revealed that the 25 mm cod-end f ish

trawl retained 98 . 2 percent as compared to the 5 1 mm cod-end

which retained 56 . 1 percent of the catch . Wi th the shrimp

trawl , the 25 mm cod-end retained 92 . 6 percent while the 38 mm

cod-end retained 5 1 . 35 percen t of the catch .

Correspondingl y , the trash f i sh component of the 25 mm

mesh size cod-end was high , registering 46 . 4 percent for the

fish trawl and 68 . 2 percent for the shrimp trawl . This is

compared to that of 34 . 6 percent for the 5 1 mm cod-end f ish

trawl and 56 . 8 percent for the 38 mm cod-end shrimp trawl .

While increased trawling speed decreased the escapement

and increased bot tom t ime inc reased escapement in the larger

mesh size cod-ends , similar t reatments on the small mesh size

cod-ends produced the opposite resul ts . The effect of trawling

speed and bottom t ime on trawl selectivity is small and is

overcomed by the effect of catch size . A large catch size

decreased escapement in all mesh sizes and trawl s .

Contrary to normal assumption , small mesh sizes normally

operated in the Malaysian fishery produced selection factors

which were mesh size dependent . Based on the new selection

factors obtained , the optimum mesh size for the Malaysian trawl

f ishery was determined to be 55 mm for f i sh t rawl and 38 mm

for shrimp trawl .

xix

The Malaysian Fishery

CHAPTER 1

INTRODUCTION

The Malaysian f i shing industry recorded a spectacular

growth in marine f ish landings in the last 20 years . Total

land ings have increased almost threefold f rom 243 , 000 tonnes in

1963 ( FAO Yearbook of Fishery Statistic s, 1964 ) to

tonnes in 1 983 (Ministry of Agriculture Malaysia,

727 , 493

1 984 ) .

The mean growth rate of the fishery over the years from 1 963

was approximately 10 percent .

Figure 1 . 1 i l lustrates the total marine fi sh landings in

Malay sia f rom 1971 to 1 984 . Two d istinct peaks in the total

catch can be detected. The first is the result of the rapid

rise in catch in 1972 and 1 973 culminating in the peak in 1974 .

A further period of rapid rise in catch can be seen in 1 976

followed by a more sustained growth culminating in 1 98 1 with a

total catch of 757 , 974 . 1 8 tonnes . Since then the total catch

has remained at about 700 , 000 tonnes decreasing to 603 , 272 . 96

tonnes i n 1 984 (Ministry of Agriculture Malaysia, 1 986 ) .

The FAO Yearbook of Fishery Statistics 1 982 , places

Malaysia twenty-third among the top 80 f ishing nations of the

world . Compared to her ASEAN neighbours however, her annual

f ish landings fell far short of Indonesia with 2 , 020 , 000

1

FIGURE 1 . 1

000 IOMes TOTAL MARINE FISH LANDINGS IN MALAYSIA 800

700

600

500

�"'i /�� 300

200

,/ /.

100

0/ /

./ /°_-_0'

0----,

....-..-

....-0" ........... '-.. _0 ___ 0-"

-

'o ___ o __ �_�

0-

--" ...

___ 0- __ - _0-----<) _____ 0_ --- _0- - - - -0---- _0- __ - -0-_ - - -0- __ - -0 - ___ -<> _____ -<>

71 72 13 74 75 76

(Source: Ministry of Agriculture Malaysia, 1972-1986)

77 YEAR

78 19 80 81 82

LEGEND

+---+ TDTAl LANDINGS

El- - � TOTAL TRAWL LANDINGS

00---__4_ TOTAL TRAWL LANDINGS PENINSULAR MALAYSIA

0- - - -0 TOTAL TRAWL LANDINGS SARAWAK

83 84

N