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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