Cod-end selectivities of a modified bottom trawl for three fish species in the Aegean Sea

Cod-end selectivities of a modi®ed bottom trawl

for three ®sh species in the Aegean Sea

A. Tokac,a,*, A. LoÈka, Z. TosunogÆlua, C. Metina, R.S.T. Ferrob

aDepartment of Fishing and Processing Technology, Faculty of Fisheries, Aegean University, 35100 Bornova, IÇzmir, TurkeybFRS Marine Laboratory, PO Box 101, Victoria Road, Aberdeen AB 11 9DB, UK

Received 14 October 1997; accepted 16 July 1998

Abstract

Recently, the catch per unit effort of conventional bottom otter trawl nets used in the Aegean Sea has decreased and the mean

body length of the major species has become smaller. The reason may be partly the poor selectivity of conventional trawl nets.

It may be possible to modify trawl net design to improve selection and protect demersal ®sh stocks. This study presents

selectivity results for the modi®ed design. Cod-end selectivity of red mullet (Mullus barbatus L.), annular sea bream

(Diplodus annularis L.) and axillary sea bream (Pagellus acarne Risso) were measured using the hooped covered cod-end

method. Cod-end mesh selectivity experiments were carried out using 36, 40, 44 and 48 mm diamond and square mesh cod-

ends both in Turkish and international waters of the Aegean Sea in 1995 on board R/V `̀ EgesuÈf''. The cover mesh size was

24 mm. A total of 85 successful tows were made, the raw data were assessed haul by haul and the valid hauls for each cod-end

were combined. The selection of cod-ends was found from the linear logistic function and the selection curves and parameters

were derived from these combined data by the maximum likelihood method. The square mesh cod-ends (36, 40 and 44 mm)

improved selectivity for red mullet. Fifty percent retention length and selection factors were higher for square mesh cod-ends

than diamond mesh cod-ends. However, this did not apply to annular sea bream or axillary sea bream. For these species, it was

found that there was no signi®cant difference between diamond and square mesh cod-ends and the diamond mesh cod-ends

even gave a slightly higher 50% retention length and selection factor than square mesh cod-ends for some mesh sizes. This

result may be explained by the different body shapes and behaviour of the individual species. # 1998 Elsevier Science B.V.

All rights reserved.

Keywords: Aegean Sea; Mesh selectivity; Covered cod-end; Mullus barbatus; Diplodus annularis; Pagellus acarne

1. Introduction

Although the Aegean Sea has many bays and gulfs,

the trawl ®shery is limited because of steep and broken

topography and a narrow continental shelf. However,

bottom otter trawls are widely used in the Aegean Sea

and about 15 000 mt annual total demersal ®sh catch is

taken from Turkish territorial waters of the Aegean

Sea by about 60 trawlers ranging in length from 15 to

25 m and in engine power from 150 to 500 hp. FAO

Yearbook, 1994 of Fishery Statistics, (1994), the

Statistical Bulletins of the General Fisheries Council

for the Mediterranean (GFCM, 1993) and the (State

Fisheries Research 39 (1998) 17±31

*Corresponding author. Tel.: +90 232 388 3225; fax: +90 232

388 3685; e-mail: [email protected]

0165-7836/98/$ ± see front matter # 1998 Elsevier Science B.V. All rights reserved.

P I I : S 0 1 6 5 - 7 8 3 6 ( 9 8 ) 0 0 1 7 2 - 6

Institute of Statistics, Prime Ministry, Republic of

Turkey, 1996) have usually indicated an increase in

the catches and landings of demersal ®sh species year

by year, but these increases can be explained by

increased ®shing effort, new ®shing grounds and

improvement of old ®shing gear. The vessels that

are being used for trawling have modern ®shing,

navigating and acoustic ®sh ®nder equipment and

these vessels may ®sh heavily on the demersal stocks.

While use of these vessels has increased the yield of

®sheries, the catch per tonnage (GRT) of trawlers has

tended to decline. Recently, there has been a consider-

able decrease in valuable demersal ®sh stocks because

of over®shing (Tokac,, 1993). Some measures have

already been taken to protect demersal ®sh stocks,

such as to limit the number and capacity of vessels and

the ®shing ground and period or to introduce a mini-

mum mesh size and minimum landing size for some

species.

In the Aegean Sea, many different species of ®sh are

caught simultaneously by bottom otter trawl. There-

fore, it is not possible to regulate selectivity for all

these species just by determining a minimum mesh

size because each species has different growth char-

acteristics and body shape. Northridge and Di Natale

(1991) report that 45% and 48%, respectively, of the

catch consisted of non-commercial species. Demersal

®sh which have high economic value are caught

generally by bottom trawl and the dominant species

in the trawl catch composition in the Aegean Sea are

red mullet (Mullus barbatus Lin., 1758), annular sea

bream (Diplodus annularis Lin., 1758), axillary sea

bream (Pagellus acarne Risso., 1826), common pan-

dora (Pagellus erythrinus Lin., 1758), poor cod (Tri-

sopterus minutus capelanus Lace., 1800) and hake

(Merluccius merluccius Lin., 1758).

Another very important problem concerns the trawl

designs used in the Aegean Sea. The great majority of

the trawls are of the conventional Mediterranean type.

These have very low selectivity and indiscriminately

catch undersize ®sh and by-catch on the ®shing

grounds of the Aegean Sea (Tokac,, 1993). The main

reason for the low selectivity of conventional trawls is

the net design. These nets have a low net opening and

high hydrodynamic resistance per unit mouth area

under the same towing conditions (Tokac,, 1989).

These features tend to close the meshes of the net

along its whole length since there is no systematic

tapering of the netting along selvedges which might

maintain the mesh opening. Because of this, improve-

ment in the design and hence selectivity of conven-

tional trawl nets would be useful.

Good ®sheries management requires that ®shing

gear should be designed to be selective (Armstrong et

al., 1990). The present selectivity experiments indi-

cate that the conventional Mediterranean type bottom

otter trawl has a poor selectivity for demersal ®sh

species in the Aegean Sea. It follows from the results

that an improvement in the design of conventional

trawl nets is of great importance. Therefore, a mod-

i®ed trawl net has been specially designed to replace

the conventional Mediterranean type trawl nets in this

study. In order to compare different mesh shape and

size selectivity of this modi®ed bottom otter trawl net

for three ®sh species, experiments have been made on

the selectivity of 36, 40, 44 and 48 mm diamond and

square mesh cod-ends. This paper presents the ®rst

selectivity results made in the region with the modi®ed

trawl net using the hooped covered cod-end method.

2. Materials and methods

Red mullet, annular sea bream and axillary sea

bream were sampled from a total of 85 experimental

hauls to study cod-end selection using nominal mesh

size of 36, 40, 44 and 48 mm in diamond and square

mesh cod-ends both in the Turkish and international

waters of the Aegean Sea during one year from

October 1995. All experimental trawl hauls were

carried out at a speed of 2±2.5 knots with a ®shing

time of 60 min at depths ranging between 30 and

110 m during daylight hours. The total time taken

to shoot, tow for 60 min and haul the gear was between

2 and 2.5 h. The method of measuring selectivity was

based on the hooped covered cod-end technique

(Wileman et al., 1996). A new modi®ed bottom trawl

net was designed for this study because there was clear

evidence of very low selectivity in the conventional

trawl used in the Aegean Sea (Tokac,, 1993). The

technical speci®cations of the conventional and mod-

i®ed trawls used in this study are given in Fig. 1. A

total of eight different cod-ends made from braided

mono®lament polyamide (PA) yarn R 620 tex in

both diamond and square mesh netting in 36, 40,

44, and 48 mm mesh size, respectively, were tested

18 A. Tokac, et al. / Fisheries Research 39 (1998) 17±31

Fig

.1.

Tec

hnic

aldra

win

gs

of

conv

enti

on

and

modif

ied

traw

ls.

A. Tokac, et al. / Fisheries Research 39 (1998) 17±31 19

to determine an appropriate mesh shape and size for

optimum selectivity in the cod-ends of the modi®ed

trawl net.

Sea trials were carried out on the research vessel

`̀ EgesuÈf'' owned by the Fisheries Faculty of the

Aegean University. She is 27.5 m in length (LOA),

has a gross tonnage of 100 m, a 500 hp main engine

and an 130 hp auxiliary engine with a hydraulic trawl

winch system.

The trawl operations were carried out mainly in the

eastern Aegean Sea between 398300 and 378N, and 268and 278E in both Turkish territorial and international

waters (Fig. 2). This study area is limited by Bay of

Edremit in the north and Bay of GuÈlluÈk in the south.

The Aegean Sea, as a part of the Mediterranean

ecosystem, has a special importance for the bottom

trawl ®shery because of its diversity of valuable

demersal ®sh species and the productive capacity of

Fig. 2. Chart of Turkey's west coast showing the north and south Aegean Sea experimental site (hatched) for the mesh selectivity studies.


living resources. However, there are also rich shrimp,

Norway lobster and squid stocks in the deep waters of

the Aegean Sea. For this reason, some trawl ®shermen

have recently started trawling at depths from 400 to

600 m.

The hooped covered cod-end method was used. The

cover net was 1.5 times larger in length and width than

the cod-end (Holden, 1971). (Stewart and Robertson,

1985; Reeves and Stewart, 1988; Robertson and Ferro,

1988; Reeves et al., 1992) have reported that the

masking effect of the cover may in¯uence the selec-

tivity of a test cod-end. This effect has been reduced

by the use of hoops to hold the cover netting away

from the cod-end meshes (Main et al., 1992). The

cover was made from multi®lament polyamide

diamond mesh netting of 24 mm mesh size.

After each haul, the trawl net was taken on board the

vessel and each part (cod-end and cover) was emptied

on the deck. The catch was sorted by species and all

individuals of red mullet, annular sea bream and

axillary sea bream were measured. In addition, the

catch composition and the amount of unmarketable

®sh (bycatch) were also determined. The ®sh species

in question were measured to the nearest 0.5 cm

fork length and also classi®ed by 0.5 cm length

intervals for selectivity analysis. Fork length was

preferred in order to take more sensitive length

measurements.

The raw data were evaluated on a haul by haul basis

and the valid hauls for each cod-end were combined

and selection curves and parameters were calculated

using a logistic model (Pope et al., 1975; Wileman et

al., 1996). In this study, the hauls without any problem

such as damage on the net, blocking of cod-end

meshes by unwanted objects like macro algae and

which can be obtained necessary data for calculation

of selection parameters have been accepted as valid

hauls. Combinations of valid hauls were analysed by

variance component analysis using the software

statistical package MINITAB (Ryan et al., 1985). This

statistical programme was also used for normal linear

regression and tests of signi®cance. The selection

curves were drawn using the software packages HAR-

VARD GRAPH, leading to maximum likelihood esti-

mates of the parameters of the curve.

The signi®cance of differences in selectivity

between different mesh sizes in both diamond and

square mesh has been tested at the 95% level (p<0.05).

3. Results

A total of 43142 red mullet, 16881 annular sea

bream and 6049 axillary sea bream were caught in 61

valid hauls for red mullet, 72 valid hauls for annular

sea bream and 24 valid hauls for axillary sea bream

from 85 hauls using the modi®ed bottom otter trawl

with 36, 40, 44 and 48 mm mesh cod-ends in both

diamond and square mesh netting. The number of

individuals caught in cod-end and cover according to

®sh species, mesh size and shapes are given in Table 1.

3.1. Selectivity of red mullet

The estimated selectivity parameters are shown in

Table 2. The 50% retention length of red mullet

increases consistently with mesh size (Table 2). There

are however, too few red mullet retained in the 48 mm

mesh cod-ends and they are not analysed further. If the

mesh size is increased too much, most of the market-

able ®sh escaped from the cod-end. This is shown in

the case of the 48 mm mesh which is not appropriate

for this size range of ®sh.

Selectivity is clearly controlled by mesh size and

mesh shape. The values of 50% retention length and

selection range both increase with mesh size for

diamond and square mesh sizes from 36 to 44 mm

(Fig. 3). These 50% lengths are signi®cantly different

(p<0.05) in all cases except the 40 and 44 mm square

mesh cases, using a t-test (Mann Whitney U test). The

selection ranges for diamond and square mesh cod-

ends for a given mesh sizes were not signi®cantly

different. The selection factors are approximately 3.06

for the diamond mesh cod-ends (Table 2) and vary

around 3.3 for the square mesh cod-ends. On this basis

a 36 mm square mesh cod-end and a 40 mm diamond

mesh cod-end have similar selectivities and similarly a

40 mm square mesh cod-end and a 44 mm diamond

mesh cod-end (Fig. 4).

3.2. Selectivity of annular sea bream

Few annular sea bream above 10 cm fork length

escaped from cod-ends of 36 and 40 mm diamond

mesh size and only about 50% of 10 cm ®sh escape

from the 44 mm diamond mesh cod-end (Fig. 5).

However, the 48 mm diamond mesh cod-end allows

much higher proportions of small ®sh to escape. The


Table 1

The number of individuals caught in cod-ends and covers according to fish species, mesh size and shapes (N: number of fish, x: sample mean,

SD: standard deviation, CI: confidence interval)

Shape Size (mm) Condition N x SD CI (95%)

Red mullet Diamond 36 Cod-end 1766 12.26 1.34 12.20±12.31

Cover 1101 10.67 0.94 10.62±10.71

40 Cod-end 2624 12.92 1.73 12.86±12.97

Cover 4989 10.80 1.18 10.77±10.82

44 Cod-end 2555 13.49 2.28 13.41±13.56

Cover 11 246 10.51 1.27 10.49±10.52

48 Cod-end 16 15.00 1.80 14.21±15.78

Cover 615 12.96 1.22 12.87±13.04

Square 36 Cod-end 1577 12.73 1.19 12.68±12.77

Cover 1886 10.84 1.14 10.79±10.88

40 Cod-end 1408 13.73 1.70 13.65±13.80

Cover 6345 10.60 1.27 10.57±10.62

44 Cod-end 880 14.18 1.98 14.07±14.28

Cover 4357 11.12 1.67 11.07±11.16

48 Cod-end 116 10.39 3.04 14.40±15.33

Cover 1661 9.89 1.81 9.81±9.96

Annular sea bream Diamond 36 Cod-end 5135 10.37 1.43 10.33±10.40

Cover 424 8.25 0.82 8.18±8.31

40 Cod-end 1600 10.21 0.96 10.17±10.24

Cover 286 8.84 0.75 8.76±8.91

44 Cod-end 1414 10.65 0.90 10.61±10.68

Cover 1367 9.16 0.88 9.12±9.19

48 Cod-end 260 13.12 0.82 13.03±13.20

Cover 924 10.35 1.40 10.27±10.42

Square 36 Cod-end 773 10.25 1.01 10.19±10.30

Cover 67 9.22 0.89 9.03±9.40

40 Cod-end 1255 10.43 1.17 10.37±10.48

Cover 380 8.91 0.84 8.83±8.98

44 Cod-end 584 10.04 0.96 9.97±10.10

Cover 213 8.77 0.69 8.69±8.84

48 Cod-end 695 12.08 1.32 11.99±12.16

Cover 1504 10.55 1.12 10.50±10.59

Axillary sea bream Diamond 36 Cod-end 259 13.10 1.29 13.04±13.15

Cover 65 10.72 1.47 10.41±11.02

40 Cod-end 733 13.50 1.08 13.43±13.56

Cover 398 10.78 1.30 10.67±10.88

44 Cod-end 31 13.70 0.85 13.44±13.95

Cover 165 12.57 0.80 12.46±12.67

Square 36 Cod-end 1301 13.64 1.61 13.56±13.71

Cover 162 11.25 1.24 11.08±11.41

40 Cod-end 717 14.06 0.97 14.00±14.11

Cover 187 12.72 1.13 12.58±12.85

44 Cod-end 659 13.66 1.03 13.59±13.72

Cover 543 12.79 0.84 12.73±12.84


50% retention length increases with mesh size for

diamond and square mesh (Table 3, Fig. 6). For the

diamond mesh the differences are signi®cant for 36,

40, 44 and 48 mm mesh (t-test, Mann Whitney U test

at p<0.05). For square mesh on the other hand, only

the 44 and 48 mm mesh size cod-ends have signi®-

cantly different 50% retention lengths. The selection

ranges seem to reduce with mesh sizes from 36 to

44 mm mesh size for both diamond and square mesh

before increasing again for 48 mm mesh size but the

differences are not signi®cant. The selection factors

vary from 2.1 to 2.2 for diamond mesh and from 2.0 to

2.2 for square mesh up to 44 mm mesh sizes. For the

larger mesh size of 48 mm the selection factor is

considerably increased.

There is no clear difference in selection for annular

sea bream between square and diamond mesh cod-

ends, unlike the results for red mullet.

3.3. Selectivity of axillary sea bream

The length-frequency distributions (Fig. 7) for axil-

lary sea bream show that increasing numbers of small

®sh escape from the cod-ends as the mesh size is

increased. The 50% retention lengths also increase

(Table 4, Fig. 8) but the values for the 36 and 40 mm

Table 2

Estimated selectivity parameters for red mullet

Shape Size (mm) l25 l50 l75 SF SR

Diamond 36 10.14 11.02�0.035 11.90 3.06 1.76

40 11.12 12.19�0.027 13.27 3.05 2.15

44 12.18 13.50�0.034 14.83 3.07 2.65

Square 36 11.03 11.82�0.600 12.62 3.28 1.58

40 12.27 13.20�0.021 14.13 3.30 1.85

44 13.22 14.67�0.060 16.11 3.33 2.89

Table 3

Estimated selectivity parameters for annular sea bream


Diamond 36 6.93 7.61�0.032 8.28 2.11 1.35

40 7.96 8.58�0.040 9.19 2.14 1.22

44 9.30 9.87�0.024 10.44 2.24 1.13

48 12.05 12.68�0.036 13.31 2.64 1.26

Square 36 6.44 7.47�0.120 8.50 2.08 2.06

40 8.04 8.79�0.045 9.55 2.20 1.51

44 8.27 8.82�0.050 9.37 2.00 1.10

48 10.92 12.03�0.053 13.41 2.51 2.22

Table 4

Estimated selectivity parameters for axillary sea bream


Diamond 36 9.50 10.61�0.073 11.72 2.95 2.21

40 10.99 11.80�0.027 12.61 2.95 1.61

44 13.47 14.16�0.011 14.85 3.22 1.38

Square 36 9.24 10.38�0.100 11.52 2.88 2.27

40 11.48 12.36�2.586 13.25 3.09 1.77

44 12.03 13.03�0.086 14.03 2.96 1.99


Fig. 3. Length±frequency distributions of red mullet caught in the cod-end according to total numbers of fish entered into trawl net.


mesh were not signi®cantly different at the 95% level.

The value for 44 mm mesh however, was signi®cantly

higher than that for the 40 mm mesh. There are no

results for the 48 mm diamond mesh cod-end for this

species. The selection factors vary from 3.0 to 3.2 for

diamond mesh cod-ends and from 2.9 to 3.1 for square

mesh. The selection range reduced as mesh size

increased for diamond mesh but showed no pattern

for square mesh.

There is no clear difference between the 50%

retention lengths of square and diamond mesh cod-

ends at the same mesh sizes. The square mesh has a

higher 50% retention length in one case but lower in

the other two relative to diamond mesh.

It was observed during the trials that especially

individual axillary sea bream struggled in the cod-

end meshes in order to escape, suggesting that for this

species escape is an active process.

4. Discussion

It is important that the trawl net and codend mesh

sizes tested in a selectivity experiment should be

choosen to cover a range of selectivity characteristics.

If trawl gear with very poor selectivity, e.g., the

traditional Mediterranean bottom trawl, or cod-ends

with relatively small mesh sizes are used only, then the

experiments may not give information on appropriate

selectivity parameters to achieve conservation of

demersal ®sh stocks.

It has been con®rmed as a general result for all three

®sh species investigated in this study that the 50%

retention lengths increased with mesh size for dia-

mond and square mesh codends. Pope et al. (1975)

reported that an increase of mesh size increases 50%

retention length while the selection factor decreases.

However, the values of selection factors for three ®sh

species with 36, 40 and 44 mm mesh sizes of diamond

and square meshes are very close to each other. It is

seen that from selectivity parameters in Tables 2±5

that the selection factor tends to remain constant with

increasing mesh size from 36 to 44 mm in diamond

and square mesh cod-ends. However, the selection

factor shows a considerable increase when the mesh

size is increased to the 48 mm mesh size and this result

agree with that reported by Robertson and Stewart

(1988); Isaksen and Valdemarsen (1986). There was

no consistent variation of selection range with mesh

Fig. 4. Selectivity curves for red mullet in the 36, 40 and 44 mm

diamond and square mesh cod-ends.

Table 5

A summary of the selectivity for each cod-end according to fish species based on the criteria that 50% retention length equals minimum

landing size

Mesh size (mm) Red mullet Annular sea bream Axillary sea bream

Diamond Square Diamond Square Diamond Square

36 ± � ± ± ± ±

40 � � ± ± � �44 � * ± ± � �48 * * � � � *

± Selectivity too low.

� Acceptable selectivity.

* Selectivity too high.


Fig. 5. Length±frequency distributions of annular sea bream caught in the cod-end according to total numbers of fish entered into trawl net.


size in either diamond and square mesh cod-end as in

previous studies by Pope et al. (1975); Robertson and

Stewart (1988).

Different results were found in the comparison of

diamond and square mesh selection parameters for red

mullet, annular sea bream and axillary sea bream.

Square mesh cod-ends gave higher 50% retention

length than diamond mesh cod-ends at the same mesh

size for red mullet. On the other hand, it was found that

for annular sea bream and axillary sea bream there was

no signi®cant difference between diamond and square

mesh cod-ends and even the diamond mesh cod-ends

gave a little higher 50% retention lengths than square

mesh cod-ends in some cases. This result may be due

to the different body shape and behaviour of each

species. It has been reported that the body shape of ®sh

(Efanov et al., 1987) and behaviour of ®sh Wardle

(1989); Watson (1988) also have an effect on selec-

tivity.

The most common measure taken to protect demer-

sal ®sh stocks in Turkey as in other countries in the

world is the minimum legal mesh size applied in the

cod-ends. Minimum legal mesh size in the cod-end of

trawl nets used in Aegean and Mediterranean Seas is

44 mm in Turkish regulations. But it is impossible to

provide suf®ciently good selectivity by the use of a

single appropriate mesh size for a trawl ®shery in

which the catch is composed of mixed species with

different body shapes and sizes. Cod-ends of different

mesh size and shape have different selectivity para-

meters for different ®sh species (Table 5). Therefore,

the choice of a particular species as target is essential

in order to determine an appropriate mesh size. A

reasonable compromise is to choose a mesh size which

has a 50% retention length equal to the minimum

landing size. A summary of the selection positions for

each cod-end related to red mullet, annular sea bream

and axillary sea bream is given in Table 5. Minimum

legal landing size is 13 cm (total length) for red mullet

in Turkish regulation. The 50% retention length values

for diamond mesh cod-end studies were estimated to

be 11.02 cm (36 mm), 12.19 cm (40 mm) and

13.50 cm (44 mm) (Table 3). The difference between

total length and fork length has been taken into

account in preparing Table 5 (see Appendix A). As

a result of the relevant analysis 40±44 mm diamond

and 36±40 mm square mesh cod-ends were taken as

appropriate mesh sizes, while 36±48 mm diamond and

44±48 mm square mesh cod-ends were not appropri-

ate for selection of red mullet when the values of 50%

retention length were compared to the minimum legal

landing size (Table 5). Although there are some pre-

vious studies in the Aegean and Mediterranean Sea,

they tested smaller mesh size cod-ends for various ®sh

species and cannot be compared with our results.

However, the work of Livadas (1988) is partly appro-

priate for comparing with ours. For cod-ends of

40 mm diamond mesh size, the results for red mullet

were very close to the values of 50% retention length,

selection factor and selection range derived by him.

It has been reported that annular sea bream reach

maturity between 11.3 and 15.0 cm fork length at the

age of IV in IÇzmir Bay (Mater, 1968). According to

this information, results from selectivity experiments

using both diamond and square mesh cod-ends indi-

cate that size selection for annular sea bream took

place only in the 48 mm diamond (12.68 cm) and

square (12.03 cm) mesh cod-end (Tables 3 and 5).

Larger 50% retention lengths in the 44 mm (9.87 cm)

and 48 mm (12.68 cm) diamond mesh compared to the

square mesh cod-ends (8.82 and 12.03 cm, respec-

tively) and the selection factors were also higher for

diamond mesh cod-ends in the 44 and 48 mm

(Table 3). Similar selectivity results have been

reported for winter ¯ounder (Pseudopleuronectes

Fig. 6. Selectivity curves for annular sea bream in the 36, 40, 44

and 48 mm diamond and square mesh cod-ends.


americanus) by Simpson (1989) and for American

plaice ¯ounder (Hippoglossoides platessoides) by

Walsh et al. (1989). The fact that annular sea bream

has a lateral compressed body shape gives rise to more

small ®sh escaping through elongated diamond

meshes in comparison to round ®sh such as red mullet.

It has been reported that axillary sea bream reached

to their ®rst maturity between 13 and 18 cm total

length at the age of I±II (Whitehead et al., 1986). It

was found that the cod-ends in the 40 and 44 mm

diamond and square mesh size are appropriate for size

selection of axillary sea bream (Table 5). It was

observed that axillary sea bream are heavily meshed

in the cod-ends. The heavy meshing of axillary sea

bream in the cod-ends may be due to the fact that the

individuals are exceedingly active and use suf®cient

force to deform the mesh when trying to escape.

5. Conclusion

It is evident that the use of a single mesh size

is insuf®cient for the selectivity of all ®sh species

existing in the Aegean Sea, because a mixed-species

Fig. 7. Length±frequency distributions of axillary sea bream caught in the cod-end according to total numbers of fish entered into trawl net.


®shery is invalued. The square mesh cod-ends showed

higher selectivity characteristics than diamond mesh

cod-ends in the 36, 40 and 44 mm mesh size for red

mullet. However, this result is not valid for either

annular or axillary sea bream because the 50% reten-

tion lengths are similar for diamond and square mesh

cod-ends for both ®sh species. This is probably due to

the fact that each ®sh species has a different body form

and behaviour. There is a need for further selectivity

experiments to study square mesh selectivity, shor-

tened selvedge ropes and cod-ends with fewer meshes

round the circumference to improve the selective

properties of a modi®ed trawl cod-ends in the Aegean

Sea. Cod-end and trawl designs should be studied

which are likely to sort species within a trawl e.g.,

by horizontal separating panels or sorting grids. Selec-

tivity experiments are very important for ®sheries

management. Therefore, these studies should be sup-

ported by basic biological studies related to spawning

period, growth, recruitment, mortality of some impor-Fig. 8. Selectivity curves for axillary sea bream in the 36, 40 and

44 mm diamond and square mesh cod-ends.

Table 6

Conversion table between the values of total length and fork length belong to red mullet in each group (from 7.5 to 19.5 cm by 0.5 cm interval)

N Total length Fork length Difference

x�Sx SD CI (%95) x�Sx SD CI (%95) Difference CI (%95)

7 7.55�0.02 0.07 7.48±7.62 6.64�0.08 0.22 6.43±6.84 0.914 0.65±1.73

21 8.01�0.02 1.27 7.95±8.07 7.16�0.03 0.15 7.09±7.23 0.847 0.77±0.91

34 8.53�0.02 0.14 8.48±8.58 7.60�0.03 0.17 7.54±7.66 0.932 0.88±0.97

37 9.03�0.02 0.13 8.99±9.07 8.07�0.03 0.20 8.01±8.14 0.956 0.91±1.00

37 9.47�0.01 0.12 9.43±9.51 8.46�0.03 0.19 8.39±8.53 1.010 0.94±1.07

36 9.99�0.02 0.13 9.94±10.03 8.91�0.03 0.21 8.83±8.98 1.080 1.02±1.13

32 10.52�0.02 0.13 10.47±10.57 9.41�0.03 0.18 9.35±9.48 1.109 1.05±1.16

38 11.01�0.01 0.12 11.00±11.08 9.90�0.03 0.21 9.83±9.97 1.139 1.08±1.19

34 11.52�0.01 0.11 11.48±11.56 10.28�0.03 0.19 10.22±10.35 1.238 1.16±1.31

35 11.99�0.01 0.09 11.95±12.02 10.56�0.02 0.12 10.52±10.60 1.425 1.37±1.47

28 12.52�0.02 0.14 12.47±12.58 11.15�0.03 0.17 11.08±11.21 1.375 1.29-1.45

33 13.03�0.02 0.14 12.98±13.08 11.48�0.04 0.25 11.38±11.57 1.551 1.47±1.63

33 13.53�0.01 0.09 13.50±13.57 11.95�0.03 0.21 11.87±12.02 1.581 1.51±1.64

38 14.06�0.01 0.12 14.02±14.10 12.40�0.04 0.24 12.32±12.48 1.657 1.58±1.72

33 14.53�0.01 0.11 14.49±14.57 12.80�0.04 0.22 12.72±12.89 1.730 1.65±1.80

37 15.05�0.02 0.13 15.00±15.09 13.21�0.03 0.23 13.13±13.28 1.840 1.78-1.89

32 15.51�0.02 0.11 15.46±15.55 13.64�0.04 0.23 13.55±13.73 1.868 1.79±1.93

36 16.03�0.02 0.12 15.99±16.07 14.15�0.04 0.25 14.06±14.24 1.880 1.79±1.96

32 16.50�0.02 0.12 16.46±16.55 14.42�0.04 0.22 14.34±14.51 2.081 2.00±2.15

35 17.14�0.04 0.28 17.04±17.24 15.01�0.03 0.20 14.94±15.08 2.128 2.02±2.23

31 17.52�0.02 0.11 17.48±17.56 15.4�0.02 0.16 15.34±15.46 2.116 2.06±2.17

34 17.98�0.02 0.13 17.93±18.02 15.97�0.04 0.28 15.87±16.07 2.005 1.92±2.09

19 18.43�0.02 0.12 18.37±18.49 16.25�0.04 0.21 16.15±16.36 2.178 2.07±2.28

9 19.02�0.04 0.13 18.92±19.12 16.83�0.06 0.20 16.67±16.99 2.188 2.07±2.30

5 19.48�0.08 0.17 19.25±19.70 17.20�0.01 0.23 16.90±17.49 2.280 2.14±2.41


tant demersal ®sh species and also studies on ®sh

behaviour observations with underwater television.

Acknowledgements

The authors thank The Scienti®c and Technical

Research Council of Turkey for ®nancial support.

We also thank Ali Ulas, and Celal GuÈven skippers

of R/V `̀ EgesuÈf'' and their crews for their help, and

our colleagues who assisted us in preparing this paper,

Celalettin Aydin, Hakan Kaykac, and Akin T. IÇlkyaz.

Appendix A

Conversion tables between the values of total length

and fork length in the each length group 7.5±19.5 cm

by 0.5 cm, 8±17 cm by 0.5 cm and 11±14 cm by

0.5 cm interval belonging to red mullet, annular sea

bream and axillary sea bream, respectively, are given

in Tables 6±8, respectively. In these tables N repre-

sents the number of ®sh, x the sample mean, SE the

standard error mean, SD the standard deviation and CI

represents con®dence interval.

Table 7

Conversion table between the values of total length and fork length belong to annular sea bream in each group (from 8 to 17 cm by 0.5 cm

interval)


x�Sx SD CI (%95) x�Sx SD CI (%95) Difference CI (95%)

5 8.10�0.04 0.10 7.97±8.22 7.48�0.05 0.13 7.31±7.64 0.620 0.45±0.78

18 8.56�0.03 0.12 8.50±8.63 7.85�0.04 0.18 7.76±7.94 0.711 0.64±0.77

27 9.10�0.02 0.11 9.05±9.14 8.35�0.03 0.16 8.28±8.41 0.748 0.67±0.81

32 9.52�0.02 0.14 9.47±9.57 8.74�0.03 0.21 8.66±8.81 0.784 0.73±0.83

34 10.00�0.02 0.16 9.95±10.06 9.14�0.03 0.21 9.06±9.21 0.864 0.79±0.93

31 10.49�0.02 0.16 10.43±10.55 9.55�0.03 0.20 9.47±9.62 0.938 0.88±0.99

33 10.99�0.02 0.15 10.93±11.04 9.94�0.04 0.28 9.84±10.04 1.048 0.96±1.13

26 11.48�0.02 0.12 11.42±11.53 10.49�0.04 0.22 10.40±10.58 0.984 0.90±1.06

19 12.05�0.02 0.12 11.99±12.11 10.99�0.03 0.13 10.92±11.06 1.063 1.00±1.12

26 12.48�0.02 0.12 12.43±12.53 11.43�0.02 0.14 11.37±11.48 1.057 1.00±1.10

25 13.05�0.02 0.11 13.00±13.09 11.90�0.03 0.15 11.83±11.96 1.152 1.10±1.99

19 13.51�0.02 0.12 13.45±13.57 12.35�0.03 0.16 12.27±12.43 1.570 1.08±1.22

13 14.03�0.03 0.13 13.94±14.11 12.77�0.05 0.21 12.64±12.90 1.253 1.16±1.33

21 14.51�0.02 0.13 14.45±14.57 13.27�0.03 0.17 13.19±13.35 1.238 1.15±1.32

12 15.03�0.03 0.10 14.96±15.10 12.77�0.05 0.18 13.65±13.89 1.258 1.17±1.34

9 15.54�0.05 0.15 15.42±15.66 14.42�0.14 0.44 14.07±14.76 1.222 0.84±1.40

4 16.05�0.05 0.10 15.89±16.20 14.60�0.05 0.11 14.41±14.78 1.450 1.29±1.60

3 16.53�0.13 0.23 15.96±17.10 15.16�0.16 0.28 14.45±15.88 1.366 1.22±1.51

2 17.10�0.10 0.14 15.82±18.37 15.40�0.40 0.56 10.31±20.48 1.710 2.11±5.51

Table 8

Conversion table between the values of total length and fork length belong to axillary sea bream in each group (from 11 to 14 cm by 0.5 cm

interval)


x�Sx SD CI (%95) x�Sx SD CI (%95) Difference CI (95%)

3 11.10�0.05 0.10 10.85±11.34 10.03�0.13 0.23 9.46±10.60 1.066 0.68±1.44

13 11.53�0.04 0.15 11.43±11.62 10.30�0.06 0.21 10.17±10.43 1.223 1.15±1.29

29 13.04�0.02 0.13 11.99±12.09 10.87�0.04 0.24 10.78±10.96 1.169 1.09±1.24

27 12.51�0.02 0.13 12.46±12.57 11.31�0.03 0.20 11.12±11.39 1.203 1.12±1.28

31 12.96�0.02 0.13 12.91±13.01 11.62�0.02 0.15 11.56±11.68 1.335 1.27±1.39

26 13.45�0.02 0.13 13.40±13.51 12.09�0.04 0.23 12.00±12.18 1.361 1.27±1.45

6 14.05�0.05 0.13 13.90±14.19 12.43�0.13 0.33 12.08±12.78 1.617 1.32±1.90


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Cod-end selectivities of a modified bottom trawl for three fish species in the Aegean Sea

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