1-s2.0-S0165783696005243-main

ELSEVIER Fisheries Research 29 ( 1997) 1 - I2

Effects of hanging ratio and fishing depth on the catch rates of drifting tuna gillnets in Sri Lankan waters

Ariyapala Samaranayaka a, Arill EngAs bvl, Terje Jorgensen’ ‘** a Ministry of’ Fisheries and Aquutic Resources Development, Muliguwurtha, Colon&o 10, Sri Lanka

b Institute oj‘Murine Research, P.O. Box 1870, N-5024 Bergen, Norway ’ Department of Fisheries und Murine Biology, Universiry of’ Bergen, HIB, N-5020 Bergen, Norwuy

Accepted 11 June 1996

Abstract

Comparative fishing trials for tuna with drifting gillnets in Sri Lankan waters tested whether a reduction of the hanging ratio (E) from the currently used 0.6 to 0.5 would increase catch rates. The effect of varying fishing depth was also studied, using three different pendent line lengths (1, 6 and 8 m). Tuna (frigate, skipjack and yellowfin) and sharks made up approximately 95% by number and 80% by weight of the catches. Nets with E = 0.5 resulted in 40% higher overall catch by weight per unit netting area than nets with E = 0.6. The higher catch rates were mainly due to more large skipjack and

yellowfin tunas being caught by tangling. Even when catches per net were compared, the E = 0.5 nets gave 25% higher catches by weight, despite the net area being 10% less due to the reduced hanging ratio. The fishing trials further indicated that pendent lines exceeding 6m will lower catch rates compared with shorter pendent lines. This is caused by declining skipjack catches as fishing depth increases.

Keywords: Drifting gillnet; Hanging ratio; Tuna

1. Introduction

Resource surveys, present production levels and

fishing gear experiments in Sri Lankan waters indi-

cate the possibility of further development of the marine fishery, based mainly on offshore pelagic

resources (BOBP, 1988; IPTDMP, 1988; StCquert and Marsac, 1989; Joseph, 1990; Maldeniya and Suraweera, 1991). Skipjack tuna (Kutsuwonus

pelumis) and yellowfin tuna (Thunnus albacares) are

* Corresponding author. Tel: + 47 55 58 44 08; Fax: + 47 55 58 44 50; E-mail: [email protected].

’ Equal authorship.

the most important offshore pelagic resources, though other tuna species, sharks and bill-fish species are also available (Sivasubramanium, 1985a).

Gillnetting, pole-and-line fishing, trolling and

longlining are the current methods used to fish for tuna in Sri Lanka. Gillnets have several advantages

over other tuna-fishing methods since they do not depend on the availability of bait, and require mini-

mal specialisation of the fishing vessel. Gillnetting has thus become the most widely used fishing method

in both inshore and offshore fisheries. Catches by

gillnets account for approximately 70% of total fish landings. However, the cost of synthetic netting material has increased markedly in recent years and it is

0165-7836/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved PII SO165-7836(96)00524-3

2 A. Samaranayaku et al./ Fisheries Research 29 (1997) l-12

therefore essential to make the nets as efficient as possible.

Apart from the introduction of synthetic fibres (polyamide multifilament) in the late 195Os, until the end of the 1970s little effort was devoted to upgrad- ing the driftnet fishery, either by reducing cost or improving efficiency (Pajot, 198Oa). Pajot observed that thinner twine (210/21) is more efficient and cost-effective than the 210/27 and 210/30 grades which were then in use. Nets made of cheaper material (polyethylene) proved to be as effective as nets of polyamide multifilament, but were liable to suffer frequent damage (Pajot, 198Ob). Pajot suggested improving efficiency by reducing the hanging ratio (E) (the length of the mounted net as a fraction of the stretched length of the netting) from 0.60 to 0.50 or 0.55 to ensure better enmeshing, but did not carry out fishing trials to confirm his own suggestions. Comparative fishing trials with mesh sizes of 125, 150 and 180mm (commonly used by commercial fishermen) showed that 150 mm meshes gave the highest catch rates (Maldeniya and Suraweera, 1991). These authors also suggested further fishing-gear experiments in order to improve gear efficiency and to determine the optimum fishing depth (i.e. distance from surface to float line). At present, nets hung 0.5 to 7m below the surface (usually 0.5 to 2m) are used in the commercial fishery. However, no systematic study of the effect of fishing depth on catch rates has been made although the target species are dis- tributed over a wide depth range (BOBP, 1988).

Gillnets are highly selective with respect to the size of fish caught. According to Baranov (1976), only a few fish are caught whose length differs from the modal value by more than 20%. The size range of the target species in the Sri Lankan gillnet fishery is very wide, ranging from 35 to 110cm. Thus, ways of reducing size selectivity are important in this fishery. The way the net is constructed may have a significant effect on its selectivity (Hamley, 1975). Nets with a lower hanging ratio are expected to improve tangling considerably for most species.

However, reducing the hanging ratio increases the surface area of the net that is capable of reflecting light, thus increasing its visibility. More visible nets are more likely to be avoided by fish, since gillnet avoidance is primarily dependent on visibility (Hes- ter and Taylor, 1965; Hamley, 1975; Cui et al., 1991;

Wardle et al., 1991). Fishing trials in Sri Lanka with transparent monofilament nets have ended with dis- couraging results since the entangling is seriously affected (personal observation). These nets are also too bulky for the local vessels, they are difficult to repair, create handling problems, and easily damage the fish by meshes cutting into their flesh, thereby reducing their market value (personal communica- tion with fishermen).

This paper describes the results of comparative fishing trials using nets with hanging ratios of E = 0.5 and E = 0.6, respectively. The effect of varying the fishing depth was also studied, using three different pendent line lengths (1, 6 and 8 m).

2. Materials and methods

The experiments were conducted 40-1OOkm off the western and southern coasts of Sri Lanka from September 1993 to January 1994 (Fig. 1). Two vessels were used for the fishing trials. The first was a 17.5m gillnetter. This boat was not available in January 1994 and a 9.5m gillnetter was therefore used for the final nine operations. Since this boat was unable to hold the entire net fleet, the final nine

6

74 80’ 81‘ 82

Fig. 1. The two fishing areas (shaded) off Sri Lanka where the

comparative fishing trials were carried out.

A. Samaranaynka et d/Fisheries Research 29 (1997) 1-12 3

E=0.6 E=0.5 Ez0.5 E=0.6

Fig. 2. The arrangement of the nets in the experiments. Only the surface floats and sinkers at each end of the nets are shown. The drawing is

not to scale.

operations were done with only the first six nets of

the fleet, counting from the vessel (Fig. 2).

The fleet of nets consisted of two equal parts,

each of four nets with a hanging ratio of either 0.5 or

0.6. The nets were placed in the fleet alternately

(Fig. 2) and the individual nets were separated by

lines to minimise any guiding effect. All nets were identical except for hanging ratio, length, type of

floats and mesh depth. The E = 0.6 nets are similar

to the nets currently used in the commercial fishery (Table 1). Due to reduced length of the nets with the

lowest hanging ratio, they will have fewer floats and

higher weight per unit net area than the E = 0.6 nets. To compensate for this, the small floats in the E = 0.5 nets were increased in size to keep the total flotation

Table I Gear specifications for the experimental nets

per net the same for the two types of nets. The length

of the pendent lines was kept at 1 m and 8 m for the

first 2 1 operations, and then at 1 m and 6 m for the

remaining 28 operations.

The nets were shot around sunset, parallel to the

direction of the wind, and the end rope of the fleet

was tied to the vessel. The fleet of nets and the vessel were then allowed to drift for about 6-12 h,

and the nets were hauled in the opposite order of

shooting. Sixty-five days were spent at sea, and 49 fishing operations were carried out. Species, fork

length (to the nearest cm) and mode of capture

(wedged, gilled or tangled) were recorded for each fish caught. Weights of individual fish were mea-

sured on board when possible, or in certain cases

Hanging ratio

Parameter

Length

Depth Mesh size

Netting type

Twine thickness

Float line

E = 0.5

3000 meshes (225 m)

111 meshes (14.4m)

150mm

polyamide multifilament

2 1 O/24 (R6OOtex)

polypropylene 10 mm

E = 0.6

3000 meshes (270 m)

120 meshes ( 14.4 m)

150mm

polyamide multifilament

2 1 O/24 (R600tex)

polypropylene 10 mm

Sinker line

Floats at floatline

Surface floats

Sinkers

Rigging

not used

260 gf alternately

at each 5m and IOm

1600gf per 15m

together with IOOOOgf

at each end

concrete 600 gf per 45 m

rope reeved through meshes,

stapling at each 33rd or

Pendent line a

Netting colour

34th mesh at 2.5 m distance

polypropylene 8 mm

green

not used

200 gf alternately

at each 5m and 10m

16OOgf per 15m

together with IOOOOgf

at each end

concrete 600 gf per 45 m

rope reeved through meshes,

stapling at each 27th or 28th

mesh at 2.5 m distance

polypropylene 8 mm

green

a Lengths were adjustable (1 and 8m for the first 21 operations, and 1 and 6m for the remaining 28 operations were used).

4 A. Sumaranayuka et d/Fisheries Reseurch 29 (1997) 1-12

after landing, using a scale of 50 g resolution for fish up to 6 kg and 500 g resolution for larger fish.

3. Data analysis

The length and weight data were used to establish length-weight relationships for frigate, skipjack and yellowfin tunas (Table 2). These relationships were used to estimate the weights of fish which could not be recorded on board due to bad weather.

Preliminary analysis did not demonstrate any differences between the two fishing areas, with respect to either species or to size composition. Data from the two areas were therefore pooled. Nor did the analysis show significant dependence of catch rates on the relative position of the net in the fleet (or on the soak time of the nets) and the analyses were performed without regard to relative position and soak time.

Catch rates were calculated as catch per standard net (270 m long and 14.4m deep). Because nets of both hanging ratios had the same depth, a correction was needed for net length in order to compare the catch rates of the two net types. This was done by using a raising factor of 1.2 (ratio of the net lengths, see Table 1) to the catches in the E = 0.5 nets. The catch was found to cover a wide range of sizes (15 to 218 cm fork length). Catch rates in weight were therefore used to compare the overall catch efficiency of the different nets.

To test the effect of hanging ratio and pendent line length on the overall catch rates of the experimental nets, a three-factor ANOVA for a balanced

Table 2

Parameters of the length-weight relationship W = aL b, where W is round weight in gm and L is fork length in cm. The data were

log-transformed and the parameters estimated using linear regres-

sion (Sparre and Venema, 1989). Data collected at fish markets

and cold stores were used for skipjacks and yellowfins; for

frigates only measurements taken at sea were available. n is the

number of fish measured and r* the proportion of the log-weight

variability explained by the linear relation with log-length

Species n

Frigate 645

Skipjack 142

Yellowfin 117

Cl b i-2

0.131 2.318 0.972

0.008 3.214 0.946

0.025 2.932 0.994

experimental design was used (Milliken and John- son, 1992). The model used to describe the experimental catch data was:

Yijk/ = p + ai + bj + Yk + “Pij ’ aYik + Prjk

+ “PYijk + &ijkl (1)

where yijkr is the log transformed catch rate in weight in the Ith net of hanging ratio i and pendent line length j at station k; p is the overall mean effect; oi is the effect of ith hanging ratio; pj is the effect of jth pendent line length; yk is the effect of the kth station; apij, o-yik, Pyjk are the two-way interaction effects of the above main factors; aPyijk is the three-way interaction effect; and cijk, is the random error term for the Ith observation with the ith hanging ratio and the jth pendent line at the kth station.

The ANOVA was carried out using the ANOVA/MANOVA module of the “Statistica” statistical package (StatSoft, 1994). As only two different pendent line lengths were used simultane- ously, the catch data for stations with 1 and 6m, and 1 and 8m pendent lines were analysed separately. Only stations with at least 15 fish caught were included in the analysis. Stations with no or poor catches indicate low availability of fish and do not give useful information on the comparative fishing power of the nets. Moreover, including these stations in the analysis would have made it less likely to detect a difference in catching power even if it exists.

Due to the resulting large number of empty cells, an ANOVA based on Eq. (1) was inappropriate for the analysis for individual species. A paired-comparison r-test (Zar, 1984) was therefore used to test whether the hanging ratio and the fishing depth affected the catch rates of individual species. This test gives equal weight to all stations, irrespective of the size of the catch. The analyses were restricted to skipjack, yellowfin and frigate tunas because the catch of sharks and other species was too small to do a separate analysis for these species. For each species the minimum number of fish that had to be caught to include a station in the analyses was set at five. When testing the effect of hanging ratio, the by-station differences in mean catch rates (of all the nets of the given hanging ratio, irrespective of depth) were

A. Sumcrrctnuyuka et d/Fisheries Resrurch 29 (19971 I-12 5

used. To test the effect of fishing depth, the by-sta-

tion differences in mean catch rates of E = 0.5 nets

fished at the two depths were used.

The proportion of fishes caught by a given cap-

ture mode (i.e. wedged, gilled or tangled) was estimated using the ratio estimator (Cochran, 1977, p.

31):

CYi,

ri = &

i k

(2)

where rI is the proportion caught by the ith capture

mode, and yik is the number of fish caught by

capture mode i at the kth station.

Confidence intervals for the estimated proportions

were found by bootstrapping, using 2000 re-sam-

plings and with each sample the size of the original

data sample (Efron and Tibshirani, 1993).

4. Results

4.1. Overview of data

Table 3 shows the species caught while Table 4

summarises the catch and effort data for the fishing trials. Skipjacks, yellowfins, frigates and sharks together made up 96% of the catch by number and

79% by weight. None of the other species exceeded

1% by number or 7% by weight. No separate analy-

sis was therefore made for these species individually,

but rather collectively

species”.

4.2. Effect of hanging

overall catch rates

as a group termed “other

ratio and fishing depth on

Catch rates (catch per unit netting area) in weight

for all species pooled were approximately 40% higher with E = 0.5 nets than with E = 0.6 nets. The

ANOVA provided strong evidence for an effect of

hanging ratio on catch rates (Table 5). The analysis also provided strong evidence for a difference in

overall catch rates between nets with pendent lines

of 1 m and 8 m; higher catches being taken with 1 m pendent lines (Tables 4 and 5). However, no evi-

dence was found for a difference in total catch rates between nets with pendent line lengths of 1 m and

6m (Table 5).

4.3. Effect qf hanging ratio on catch rates of indiuid-

ml species

Catch rates of nets with hanging ratio E = 0.5

were significantly higher for skipjacks and yellowfins in terms of both weight (p < 0.001 and

p = 0.013, respectively) and number of fish ( p <

Table 3

Fish species caught during the fishing trials. The species are listed in decreasing order of importance in the catches

Taxonomic grouping Common name Local name

Kursuwonu.~ pelum7.s

Thunnus ctlhocure.~

Auis thazurd

Carcharhinidae and related families

Euthynnus @ini.\

Thunnus muccoyii

Thunnus ohesus

Other Scombermorids

Carangidae

Mokuiru nw~um

Mukuiru in&u

Tetrqmm.~ crur1o.r I.stiophorou.\ plrtypterus

Xiphius ~ladius

Trygenidae and related families

Skipjack tuna

Yellowfin tuna

Frigate tuna

Shark

Kawakawa/mackerel tuna

Bluefin tuna

Bigeye tuna

Spanish/king mackerel

Horse mackerel

Blue marlin

Black marlin

Stripped marlin

Sail fish

Sword fish

Ray/skate

Balaya

Kelawalla

Alagoduwa

Mora

Atawalla

Kelawalla

Kelawalla

Thora

Parawa/katta

Koppara

Koppara

Koppara

Thalapath

Sappara

Maduwa

6 A. Sumuranayaka et al./Fisheries Research 29 (1997) 1-12

0.001 and p = 0.011, respectively) compared with nets with E = 0.6 (Table 6). For frigates the catch rate in terms of weight was significantly lower in nets with E = 0.5 than in nets with E = 0.6 (p =

0.030), while there was no evidence of a difference in numbers of fish caught. The higher catch rates in E = 0.5 nets for yellowfins, and especially skipjacks, were mainly due to higher catches in the length ranges from approximately 60 to 75 cm and 50 to 65 cm, respectively (Fig. 3). The lower weight-based catch rates for frigates were due to reduced catches mainly in the length range between 25 and 4Ocm

(Fig. 3 and Table 6). The catch data for sharks indicated higher catch rates in E = 0.5 than E = 0.6 nets, while the “other species” group showed the opposite trend (Table 4). The catch of these species, however, was too small to allow a statistical analysis to be made. No difference in length range between fishes caught by nets of the two hanging ratios was seen for any of the main species (Fig. 3).

Tangling was the most important capture mode (Fig. 4), regardless of species and hanging ratio. More than 60% of the fish of each species were caught by tangling in nets hung at both ratios, except

Table 4 Overview of the catch and effort data for the fishing trials. The catch data include all stations and no correction has been made for the difference in net area for nets of the two hanging ratios

Hanging ratio Pendent line length (m) Region

0.5 0.6 1 6 8 West SOUth

No. of operations (stations) 49 49 49 No. of settings (nets) 184 187 196 No. of empty nets 21 19 23

Tom1 catch (numherl

Skipjack Yellowfin Frigate Shark Other species All species

930 144 1054 368 252 788 886 163 141 155 94 55 130 174 233 366 333 136 130 404 195 110 108 99 56 63 94 124 47 66 54 34 25 61 52

1483 1425 1695 688 525 1477 1431

Total catch (kg)


2775 2113 3102 968 817 2429 2459 769 630 686 438 215 611 789

74 133 114 41 46 136 71 945 661 609 459 539 762 843 969 1178 IO29 775 343 1103 1044

5532 4715 5540 2687 2020 5041 5206

Catch per net (number)


5.05 3.98 5.38 4.04 3.00 4.15 4.90 0.89 0.75 0.79 1.03 0.65 0.68 0.96 1.27 1.96 1.70 1.49 1.55 2.13 1.08 0.60 0.58 0.5 1 0.62 0.75 0.49 0.69 0.26 0.35 0.28 0.37 0.30 0.32 0.29 8.06 7.62 8.65 7.56 6.25 1.77 7.91

Catch per net (kg)


15.08 11.30 15.83 10.64 9.73 12.78 13.59 4.18 3.37 3.50 4.8 1 3.27 3.22 4.36 0.40 0.71 0.58 0.52 0.55 0.72 0.39 5.14 3.53 3.11 5.04 6.42 4.0 1 4.66 5.21 6.30 5.25 8.52 4.08 5.81 5.77

30.07 25.21 28.27 29.53 24.05 26.53 28.76

28 21 26 23 91 84 190 181 IO 7 16 24

A. Swnarunuyuka et d/Fisheries Research 29 (1997) I-12 7

Table 5

Analysis of variance table for total catch rates of gillnets with different hanging ratios and pendent line lengths

Source of variation

1 and 6m pendent lirw length

Hanging ratio

Fishing depth

Station

Hanging ratio * Fishing depth

Hanging ratio * Station

Fishing depth ’ Station

Hanging ratio ’ Fishing depth * Station

Error

dj

I I

14

I 14

14

14

60

1 and 8m pendent line length

Hanging ratio

Fishing depth

Station

Hanging ratio * Fishing depth

Hanging ratio l Station

Fishing depth * Station

Hanging ratio * Fishing depth * Error

SS MS F-value p-value

3.917 3.917

I .760 I.760

44.968 3.212

0.267 0.267

7.422 0.530

7.112 0.508

4.970 0.355

45.841 0.764

1 3.909 3.909 7.460 0.008

I 4.115 4.115 7.853 0.007

1.5 79.980 5.332 10.176 < 0.00 I

I 0.191 0.191 0.364 0.548

I5 5.265 0.35 I 0.670 0.804

I5 10.785 0.719 I .372 0.189

I5 12.705 0.847 I.616 0.094

64 3 I ,442 0.524

5.127 0.027

2.304 0.134

4.204 <o.cOt

0.349 0.556

0.694 0.772

0.665 0.799

0.465 0.943

df = degrees of freedom, SS = sum of squares, MS = mean squares.

SKIPJACK TUNA

1 E = 0.5 “=1116, I=529 7

0 20 40 60 60

SHARKS 25

n-132. I=923 20

0 0 50 100 ‘50 200

Length (an)

YELLOWFIN TUNA

0 20 40 60 60 100 120

OTHER SPECIES 14

0 E = 0.5

12 n=ee, Ii821

= E=06 10 n.55. I=593

0 50 100 150 200 250

Length (cm)

FRIGATE TUNA 50

ALL SPECIES POOLED 800

0 E=05. n=,,*, I=542

0 50 100 150 200 250

Length (cm)

-

Fig. 3. Length distribution of the catches in the nets with hanging ratios of E = 0.5 and E = 0.6. n is the number caught and 1 is the mean

length in cm. Numbers caught in the E = 0.5 nets are corrected (raising factor 1.2, see Table I) for the smaller net length of these nets so as

to make them comparable to the catch in the E = 0.6 nets.

8 A. Sumaranayaka et al./ Fisheries Research 29 (1997) l-12

Table 6 Comparison by species of catch rates (in weight and number) in nets with hanging ratios E = 0.5 and E = 0.6. T is the test statistic for a

paired-comparison r-test and the p-value refers to a one-sided test

Species Catch unit Mean catch rate Difference No. of stations T p-value

E = 0.5 E = 0.6

Skipjack Weight 22.76 14.67 8.08 36 5.80 < 0.001 Number 7.64 5.15 2.49 36 6.17 < 0.001

Yellowfin Weight 7.36 4.67 2.70 31 2.34 0.013 Number I .56 1.06 0.50 31 2.44 0.011

Frigate Weight 0.64 0.98 - 0.34 34 - 1.94 0.030 Number 2.05 2.73 - 0.68 34 - 1.30 0.102

for skipjack in nets with hanging ratio E = 0.6. All frigates were caught by tangling, regardless of hanging ratio. As Fig. 5 shows, the length composition of fish caught by different capture modes was different. The length range of wedged and gilled skipjacks and yellowfins is narrow (approximately 40 to 65cm),

while the length distribution of tangled fish covered a wide range, from 20 to 70 cm and 10 to 105 cm, respectively. The larger numbers of skipjacks taken by E = 0.5 nets compared with E = 0.6 nets was largely attributable to the significantly (p < 0.05) higher catch of tangled fishes (Fig. 5).

TANGLED

TANGLED GILLED

Skipjack Frigate Other Skipjack Frigate Olher

Yellowtin Sharks All Yellowfin Sharks All

GILLED

100 1

60.

IO-

6-

4-

WEDGED

Skipjack Frigate Other

Yellowfin Sharks All

Fig. 4. Upper panel: percentage of fish caught by tangling. a hilling and wedging for species groups and the overall catch. Lower panel: catch

rates in numbers per standard net of 270 m by tangling, gilling and wedging for species groups and the overall catch. For all estimates 95%

confidence limits are indicated.

A. Sumarunuyaka et al. / Fishcries Research 29 (19971 1 - I2 9

SKIPJACK TUNA 100 -

0 Tangled. n=601.1=53.1 ?

60 - . weded: :o

n=l92.1=53.1 Gb : b

$60

5 * 40-

YELLOWFIN TUNA 25 14

1

10 -

$ 6

E

2 4-

2.

20 40 60 60

OTHER SPECIES

Length (cm)

0 20 40 60 60 120 0 50 loo 150 200

Length (cm)

ALL SPECIES POOLED 350

i P

0 Tangled

300 .: ns1093.1=542

200

150

100

50

0

0 50 loo 150 200 250

Length (cm)

Fig. 5. Length distribution by capture mode for the catches in the E = 0.5 nets. n is the number caught and I is the mean length in cm.

Table 7 Comparison of catch rates (in weight and number) of E = 0.5 nets using pendent line lengths of I and 6m, and 1 and 8 m, respectively. 7’ is

the test statistic for a paired-comparison r-test and the p-value refers to a two-sided test

Species Unit Mean catch rate Mean catch rate No. of T p-value

Shortest line Longest line difference stations

I and 6 m pendent line length

Skipjack Weight 25.67 21.57 4.10 18 1.20 0.248 Number 7.39 6.47 0.92 18 I .04 0.314

Yellowfin Weight 7.76 8.21 - 0.45 16 -0.16 0.875 Number I.41 1.47 0.06 16 -0.14 0.888

Frigate Weight 0.48 1.01 -0.53 9 - 0.92 0.386 Number I .89 2.61 - 0.72 9 - 0.40 0.701

I and 8 m pendent line length

Skipjack Weight 28.99 15.10 13.89 18 5.33 < 0.001 Number 1.69 3.94 3.75 18 5.24 < 0.001

Yellowfin Weight 7.30 6.22 1.08 15 0.45 0.659 Number 1.40 0.90 0.50 15 1.41 0.181

Frigate Weight 1.28 0.92 0.36 13 0.58 0.576 Number 3.00 2.46 0.54 13 0.33 0.745

10 A. Samaranayaka et al. / Fisheries Research 29 (1997) l-12

100 1 0 lmh: 61

0 20 40 60 60

Length (cm)

Fig. 6. Length distribution for the skipjacks caught in nets fished with I and 8m-long pendant lines, respectively. n is the number caught and 1 is the mean length in cm.

4.4. Effect ofjshing depth on catch rates of individual species

Catch rates tended to decrease with increasing fishing depth for skipjacks (Table 4). The difference in catch rates was not significant for 1 and 6m-long pendent lines, but it was significant between 1 and 8 m (Table 7). The lower catch rates for 8 m-long pendent lines were due to reduced catches of all the size groups (Fig. 6). No significant differences in catch rates between the three pendent line lengths were found for yellowfin and frigate.

5. Discussion

5.1. Hanging ratio

This investigation showed that nets with a hanging ratio of E = 0.5 yielded 40% higher overall catch rates than nets with a hanging ratio of E = 0.6. This was due to catching more yellowfins and especially skipjacks by tangling, mostly fish in the length range 50-75cm. The rise in tangling as a result of lower hanging ratio is in line with the suggestions of Baranov (19481, Riedel (19631, Mohr (1965a) and Ishida (1969a), all cited by Hamley (1975). The netting becomes slacker and more easily forms pockets when the hanging ratio is reduced. Mesh bars will also be closer in the more loosely hung nets. This makes it more probable that a fish will become tangled in these nets.

Because mesh size is not as critical for tangling as

for wedging and gilling, a loosely hung net may be expected to catch a much wider range of sizes than a tightly hung net (Riedel, 1963, cited by Hamley, 1975). However, the results of this study did not demonstrate any difference in the length range of fishes caught in the nets of the two hanging ratios. This may be explained by the fact that the E = 0.6 nets are already loosely hung by the way the nets are rigged. The meshes are stapled at long intervals (Table l>, allowing the meshes to slide along the float line. The numbers of sinkers and floats are comparatively small and no sinker line is used. The nets thus already have good tangling properties (Fig. 5). Approximately 50% of the skipjacks and more than 60% of the other species caught in E = 0.6 nets were tangled. A further reduction of the hanging ratio increases the number of fish tangled but not the length range. The range of fishes caught also de- pends on the size range of the population being fished, which can vary seasonally.

The importance of skipjacks to the higher overall catch rates of E = 0.5 nets, is probably due to the higher abundance of this species in the waters being fished. Skipjacks were by far the most abundant species caught. In comparison with yellowfins, approximately five times as many skipjacks were caught (Table 4). When the catch rates of E = 0.5 and E = 0.6 nets were compared, the percentages were found to be similar for the two species.

The larger catches of skipjacks and yellowfins in E = 0.5 nets were mainly attributable to comparatively large fish, most of them in the size range 50-70cm. Due to their size and strength, large fish can also more easily push the loosely hung net in front of them (thereby forming pockets and increasing the probability of entangling) or wrap the net around them while they are struggling to free them- selves.

No satisfactory explanation was found for the lower catch rates of larger sized frigates in E = 0.5 nets as compared with E = 0.6 nets. The frigate is the smallest of the tuna species, and is caught en- tirely by tangling.

5.2. Fishing depth

The results indicated that pendent lines longer than 6m resulted in reduced catches, mainly due to

A. Sumarunuyrrka et d/Fisheries Reseurch 29 (19971 l-l.2 11

Table 8

Comparison by species of catch rates in nets with hanging ratios E = 0.5 and E = 0.6. Catch rates used are catch in weight per net as made

from the same amount of netting (i.e. the net area for an E = 0.5 net is 10% less than that of an E = 0.6 net). As no interaction was found

between hanging ratio and depth or between hanging ratio and station (Table 5), data were pooled irrespective of fishing depth. T is the test

statistic for a oaired comoarison t-test and the D-value refers to a one-sided test

Species Mean catch rate Difference No. stations T p-value

E = 0.5 E = 0.6

Skipjack 20.50 14.64 5.86 36 4.744 < 0.001

Yellowfin 6.63 4.65 1.98 31 1.823 0.039

Frigate 0.57 0.98 -0.40 34 - 2.408 0.01 1 All species 34.58 27.22 7.36 41 3.209 0.00 I

fewer skipjack being caught at greater depths. How- ever, most of the fishing trials using 1 and 8m pendent lines were done in the western fishing area, while those using pendent lines of 1 and 6m were done in the area off the south-east coast. The results must therefore be interpreted with care as the vertical distribution of fish may differ between the two areas.

5.3. Economic and practical aspects

The 40% higher overall catch rates in E = 0.5 as compared with E = 0.6 nets refer to catch per unit net area. However, if the same amount of netting is used for E = 0.5 and E = 0.6 hanging ratios, the E = 0.5 net will be 20% shorter but deeper. Altema- tively, netting can be made at the same cost so that the mounted nets have the same depths (i.e. the E = 0.5 net will be fewer meshes deep). In any case, reducing the hanging ratio from E = 0.6 to E = 0.5 reduces net area by 10%. Thus, to obtain the same net area, more netting is needed, implying an approx- imate 10% increase in net cost. Referring to the catch rates during the experimental fishing season, the catch rates of E = 0.5 nets were an average of 11 kg higher than those of E = 0.6 nets. The market value of these 11 kg is at least 300SLR (Sri Lankan rupees), while the additional investment for the E = 0.5 net is approximately 3500SLR. Thus the additional investment can be met in fewer than 15 fishing operations. This compares very favourably with the lifetime of the nets which is three to four years.

Even when catches per net (for nets made of the same amount of netting) were compared, a re-analysis of the data (pooled irrespective of fishing depth) still showed significantly higher catches in E = 0.5

than E = 0.6 nets (Table 8). The average catch rate of E = 0.5 nets was now approximately 25% higher than that of E = 0.6 nets. The cost of ropes for 0.5 hanging ratio nets is also slightly less than that of 0.6 hanging ratio nets. Shooting and hauling times will also be shorter, resulting in lower operational cost. The E = 0.5 nets are therefore preferable to E = 0.6 nets.

The experimental fishing was done during the fourth quarter of the year. Considerably higher catches are usually taken during the second and third quarters (Maldeniya and Suraweera, 1991). On a yearly basis therefore, the E = 0.5 nets can be expected to perform even better in financial terms, relative to the E = 0.6 nets, than is suggested by the above calculations.

The only handling difficulty observed with the lower hanging ratio nets was that meshes occasion- ally slipped over the smaller floats and this had to be cleared while hauling the nets. This can easily be remedied by placing the two staplings near the smaller floats at a greater distance than 45 cm.

A short pendent line is economically advanta- geous as it gives the highest catch rates and is preferable from the point of view of net handling. However, further trials are needed to see if this conclusion regarding catch rates is valid on an an- nual basis and for other fishing areas.

6. Conclusions and recommendations

The Sri Lankan drifting gillnet fishery targets a wide range of species and fish sizes. Tangling is therefore the most important capture mode in this

12 A. Samaranayaka et al./Fisheries Research 29 (1997) l-12

fishery. This study has clearly documented that a reduction in the hanging ratio from 0.6 to 0.5 will produce appreciably higher catches (25% by weight). This was due to the improved tangling properties of the E = 0.5 nets resulting in more fishes being caught, especially in the size range 50-70cm. Stud- ies should be carried out in order to determine whether a further reduction in the hanging ratio would increase catches even more.

A pendent line length of 8m reduced catches in comparison with lines of 1 and 6m. Since the verti- ca1 distribution of the target species seems to be linked to seasonal migration patterns (Sivasubra- manium, 1985b; Amarasiry and Joseph, 1988; Maldeniya and Joseph, 1988; Stiquert and Marsac, 1989), trials should cover at least one migrational cycle and a wider area before general recommendations are made. Pendent line lengths can very easily be changed if systematic variations on a seasonal or geographical basis are found.

Acknowledgements

The authors are indebted to NORAD (the Norwe- gian Agency for Development Cooperation) for financial support for this study. We also gratefully acknowledge the comments of two anonymous refer- ees and the assistance of the skippers and crews of the fishing vessels “Kayts Maru” and “Tangalle

I? 1 .

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