- Email: [email protected]
Selectivity of diamond, hexagonal and square mesh codends for three commercial cephalopods in the Mediterranean
Fisheries Research 97 (2009) 95–102
Contents lists available at ScienceDirect
Fisheries Research journal homepage: www.elsevier.com/locate/fishres
Research article
Selectivity of diamond, hexagonal and square mesh codends for three commercial cephalopods in the Mediterranean Zafer Tosuno˘glu a,∗ , Celalettin Aydın a , Alp Salman a , Paulo Fonseca b a b
Ege University, Faculty of Fisheries, 35100 Bornova, Izmir, Turkey INRB/L-IPIMAR, National Institute of Biological Resources/Laboratory of Fisheries and Sea Research, Ave Brasilia, P-1449006 Lisbon, Portugal
a r t i c l e
i n f o
Article history: Received 4 June 2008 Received in revised form 14 January 2009 Accepted 19 January 2009 Keywords: Codend selectivity Mesh shape Size-selection Bycatch Loligo vulgaris Illex coindetii Sepia orbignyana Aegean Sea Eastern Mediterranean
a b s t r a c t Undersized and immature commercially important cephalopods are often inadvertently caught by trawlers in the eastern Mediterranean. To evaluate the effectiveness of different mesh codends (diamond, hexagonal and square) to reduce bycatch of juvenile commercially important Mediterranean cephalopods (European squid Loligo vulgaris, broadtail shortfin squid Illex coindetii and pink cuttlefish Sepia orbignyana), a series of selectivity experiments was undertaken with a modified bottom trawl. The covered codend technique was used to capture escapees. For the broadtail shortfin squid and the pink cuttlefish, the square-shaped mesh displayed the highest 50% retention lengths (L50 ) compared to diamond and hexagonal mesh. For the European squid, selectivity could only be measured by pooling the data from all hauls for each mesh shape. For all three species, the L50 values of square and hexagonal mesh codends were significantly different (p < 0.01). Beyond the mesh variability, species catch was found to have a significant impact in the selection range of the broadtail shortfin squid. Furthermore, for the cuttlefish, total catch and haul duration likely account for variability of L50 attributed to mesh configuration. For all species, regardless of the mesh shape, L50 values were substantially lower than the minimum landing size or length at first maturity. Therefore, the current legal minimum mesh size and codend configurations for demersal trawling are not suitable for the management of these species. As such, sustainability in Mediterranean cephalopod fisheries would profit from more selective gears. This could be achieved both by an increase in codend mesh size and change in codend shape; however, being part of a mixed fishery, when these changes are being practiced, the impact on the fish catches of target species will have to be taken into consideration. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Cephalopods comprise approximately 700 species worldwide and many ecologically important cephalopods have Atlantic origins (Mangold and Boletzky, 1987). Both as direct human consumption and as a principal food for top predators (Clarke, 1983), these cephalopods constitute one of the most important demersal fishery resources in the Mediterranean (Worms, 1983). Cephalopod fisheries production in Mediterranean was 53800 t in 2001, constituting 1.6% of the total world cephalopod catch (Jereb and Roper, 2005). Because of the economic and ecological importance of cephalopods, the factors affecting the sustainability of the cephalopod fisheries of the Mediterranean must be understood. In addition to catches by trammel, fyke nets, seiners, specially designed longlines and hand-jigging, cephalopods also constitute a valuable by-catch in the trawl fishery (Jereb and Roper, 2005).
∗ Corresponding author. Tel.: +90 535 5927692; fax: +90 232 3883685. E-mail address: [email protected] (Z. Tosuno˘glu). 0165-7836/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.fishres.2009.01.006
However, different Mediterranean nations deal with the by-catch differently. In Turkish waters, codends with a minimum diamond mesh size of 44 mm are required for demersal fish trawling (Anonymous, 2006). In contrast, European Union Mediterranean countries are enforcing the use of 40 mm square mesh codends as of 2008 (E.C., 2006). Although a considerable amount of trawl selectivity research has been carried out for finfish and crustacean species in the Mediterranean, selectivity data for commercial cephalopods are only available for the European squid Loligo vulgaris (Ordines et al., 2006) and the broadtail shortfin squid Illex coindetii (Sala et al., 2008). Understanding the size ranges and maturation stages of these species will reveal if mesh shape (and size) are capturing these species at important life stages. Considering the commercial relevance of cephalopods and the need for sustainable management, we conducted a preliminary investigation on the codend selectivity of different mesh shape (diamond, hexagonal and square) for the three most important species, the European squid, the broadtail shortfin squid and the pink cuttlefish Sepia orbignyana. Potential benefits from an improved codend were matched against the length at first maturity (LFM), i.e., the length corresponding to a 50% pro-
96
Z. Tosuno˘glu et al. / Fisheries Research 97 (2009) 95–102
portion of mature individuals, for the squid species (the maturation cycle for the pink cuttlefish is unknown). 2. Materials and methods Fishing was conducted in the commercial fishing grounds of the Aegean Sea (eastern Mediterranean) between 30 September and 25 October 2007 on board a commercial stern trawler. Technical details of the sampling gear and procedure were given in Aydın and Tosuno˘glu (2009). Characteristics of the experimental codends are as follows: (1) diamond mesh (DMC, a commercially used standard codend): PE 210 d/24 knotless, nominal 44 mm (44.7 mm ± 0.08), 400 meshes on its circumference and 5 m in length. (2) hexagonal mesh (HMC, a combination PE + PA 210 d/27 knotless, nominal 40 mm (42.6 ± 0.11 mm), 200 meshes on its circumference and 5 m in length. (3) square mesh (SMC, a PE 0.40*10 knotted, nominal 40 mm (42.4 ± 0.19 mm), 200 bars in circumference and 5 m in length). Selectivity data were collected using the covered codend method and selection curves of the individual hauls were obtained by fitting a logistic function: r(l) = exp(v1 + v2 l)/[1 + exp(v1 + v2 l)] (Wileman et al., 1996), where the parameters v1 (the intercept) and v2 (the slope) were estimated by maximum likelihood using the software CC2000 (ConStat, 1995). When the numbers of individuals retained and escaped were insufficient for individual haul estimation, data were pooled over all hauls to estimate the selection curve. The statistical significance among pooled selection curves was evaluated with a likelihood ratio test (e.g., Campos et al., 2003). Fryer (1991) model was used to test the between-haul variation of the selectivity parameters v1 and v2 by mesh configuration, allowing the estimation of mean curves for the three different mesh codends by using the software ECModeller (ConStat, 1995) which utilizes the REML (REsidual Maximum Likelihood) approach. The same approach was also used to test the statistical significance of additional explanatory variables (total, tot, codend, cod, and species, sp, catch weight, and haul duration, dur.) on the selectivity parameter estimates. In this case, a general design matrix Xi was assumed for the departure model, where i = 1 . . . H, the number of hauls,
Xi =
˛1 0
0 ˛2
˛3 mi 0
˛4 toti 0
˛5 codi 0
˛6 spi 0
˛7 duri 0
0 ˛8 mi
0 ˛9 toti
Fig. 2. Size structure of European squids that entered the different codends. Thin line corresponds to length frequency in codend, dashed line in cover and thick line to total numbers. Percentages of marketed (MRK), discarded (DSC), and escaped (ESC) in diamond, hexagonal and square mesh codends.
0 ˛10 codi
0 ˛11 spi
0 ˛12 duri
The choice of the model which best describes the data was based on the lowest value for Akaike’s Information Criterion-AIC (Fryer and Shepherd, 1996). 3. Results 3.1. Species catch from hauls
Fig. 1. Species composition by weight in codend + cover, corresponding to a total catch of 6109 kg in 33 hauls.
Thirty-three hauls yielded a total catch biomass of 6.1 t, 2.1 t with DMC (11 hauls), 2.0 t with HMC (12 hauls), and 2.0 t with SMC (10 hauls), during 112 trawling hours. Deep-water rose shrimp (Parapenaeus longirostris), horse mackerel (Trachurus trachurus) and hake (Merluccius merluccius) accounted for the majority of the weight, followed by broadtail shortfin squid, pink cuttlefish, European squid and greater forkbeard (Phycis blennoides) (Fig. 1). Other commercial species were John Dory (Zeus faber), angler fish (Lophius piscatorius), tub gurnard (Chelidonichthys lucernus), piper gurnard (Trigla lyra), red mullet (Mullus barbatus), blackspot seabream (Pagellus bogaraveo), blackbelly rosefish (Helicolenus dactylopterus dactylopterus), silver scabbardfish (Lepidopus caudatus), chub mackerel (Scomber japonicus), common octopus (Octopus vulgaris), musky octopus (Eledone moschata), and large thornback ray (Raja clavata) and European
SR, selection range (L75 -L25 ); SF, selection factor (L50 /m) with m equal to mesh size; d.f., degrees of freedom; vi1 , vi2 , parameters of the logistic curve; Ri11 , Ri12 , Ri22 , elements of variance–covariance matrix.
58 198 61 3 7 3 1412 1410 1173 2146 2013 1950 0.9654 0.9941 0.9739 11 16 10 1.97 5.29 3.29 2.299 1.223 1.446 9 12 10 DM HM SM
4.6 4.2 6.0
4.0–5.2 4.0–4.5 5.5–6.5
1.0 1.8 1.5
0.4–1.7 1.5–2.1 0.7–2.3
1.0 1.0 1.5
−10.469 −5.162 −8.717
Codend
vi2 SF 95%CI for L50 L50 Number of hauls Codend
Table 1 Selectivity estimates for European squid.
SR
95%CI for SR
3.3. Broadtail shortfin squid The length-distributions for total, codend and cover catches, and percentages of marketable, discarded and escaped broadtail shortfin squid in all codends are shown in Fig. 4. The catch is composed of a high percentage of immature individuals (males below 10–12 cm and females below 15 cm). As the codend mesh shape changes from DMC to HMC and SMC, there is an increasing ability to reduce the capture of the smaller individuals of the population. The selection curve parameters for DMC were estimated from the pooled data over all hauls, while, for HMC and SMC, the mean curves were derived from Fryer’s approach (Fryer, 1991), which takes inter-haul variability into account. The estimated selectivity parameters are given in Table 2. The lowest L50 , 4.2 cm, is found in DMC, increasing to 5.2 cm for HMC (24% greater than DMC) and 7.8 cm for the SMC (86% greater than DMC). Using Fryer’s approach, a statistically significant difference (p < 0.01) is found between the L50 values of SMC and HMC (Table 3). The use of the square mesh codends seems to positively affect the L50 (˛3 = 2.528), but the difference in the SR was not statistically significant (p > 0.05). A direct comparison with DMC is not possible due to the different estimation approach. However, since the HMC and SMC confidence intervals do not overlap with those of DMC, the DMC likely has a different selectivity from the other codends for this species. SR is very close for the three
vi1
Ri11
Ri12
Ri22
Fig. 2 shows the length-distributions for total, codend and cover catches, and percentages of marketable, discarded and escaped European squid based on data pooled for each codend type over the different codend mesh sizes and shapes. Because the catches in DMC and SMC were small, these results must be conservatively interpreted. Furthermore, the length distribution for the HMC is different from both the DMC and SMC, being biased towards small squids. Overall, juvenile individuals dominate the catch, with very few squids caught above the 50% maturation length for males (11 cm), the smaller of the two sexes. The small catches in both codends and covers were insufficient for single haul analysis, so to estimate selection curves, the data were pooled across codend shape. The estimated selectivity parameters are given in Table 1 and selection curves displayed in Fig. 3. The highest 50% retention lengths (L50 ) are estimated as 6.0 cm in SMC, while the DMC and HMC yielded very similar L50 values of 4.6 and 4.2 cm, respectively. With respect to selection range (SR), the estimated values vary between 1.0 and 1.8 cm.
0.4302 0.0108 0.1220
Deviance
3.2. European squid
−1.8690 −0.0355 −0.7588
Codend
d.f.
p-Value
conger (Conger conger). In addition, the cephalopod species; Sepia elegans, *S. orbignyana, Sepietta oweniana, Rondeletiola minor, *Loligo vulgaris, Alloteuthis media, *Illex coindetii, *Octopus vulgaris, Scaeurgus unicirrhus, *Eledone moschata and *E. cirrhosa were caught in the trawled area (asterisks identify commercial cephalopod species).
8.4864 0.1278 4.8221
Number in
Total
Species Catch weight (kg)
Fig. 3. Selection curves for European squid based on pooled data with observed retention in DMC (— ♦), HMC (- - - ) and SMC (– – – ).
24 572 26
97 Cover
Z. Tosuno˘glu et al. / Fisheries Research 97 (2009) 95–102
98
Table 2 Selectivity estimates for broadtail shortfin squid. Codend (Hauls)
SM 1 2 4 5 7 8 10 Pooled (10) Fryer (7) D
95%CI for L50
SR
95%CI for SR
SF
vi1
vi2
Ri11
Ri12
Ri22
Deviance
d.f.
p-Value
Catch weight (kg)
Number in
Dur. (min)
Total
Codend
Species
Codend
Cover
4.2
3.7–4.6
2.9
2.4–3.5
1.0
−3.098
0.746
0.1762
−0.0267
0.0044
26.54
17
0.0651
2146
1412
59
1312
114
5.1 5.5 5.0 4.4 4.7 5.4 5.7 5.2 5.3 5.2 5.2
4.0–6.3 4.9–6.1 4.1–6.0 3.7–5.2 4.2–5.2 5.0–5.8 5.0–6.4 4.5–5.9 4.3–6.4 5.0–5.4 5.1–5.3
3.1 2.0 2.2 2.7 2.4 2.1 3.4 3.7 3.3 2.9 2.8
0.9–5.3 1.1–2.9 0.4–4.0 1.6–3.7 1.4–3.3 1.4–2.7 2.3–4.4 2.5–4.9 1.8–4.9 2.5–3.2 2.6–3.0
1.3 1.4 1.3 1.1 1.2 1.4 1.4 1.3 1.3 1.3 1.3
−3.654 −6.077 −5.025 −3.675 −4.411 −5.803 −3.733 −3.136 −3.540 −3.946 −4.001
0.711 1.102 0.998 0.828 0.934 1.069 0.655 0.600 0.662 0.764 0.777
1.7995 2.0579 3.5501 0.7738 0.8340 0.7504 0.4325 0.3668 0.8932 0.0752 0.1220 0.2513
−0.3081 −0.3389 −0.6707 −0.1283 −0.1501 −0.1287 −0.0596 −0.0538 −0.1276 −0.0119 −0.0213 −0.0534
0.0570 0.0585 0.1337 0.0227 0.0282 0.0234 0.0088 0.0085 0.0197 0.0020 0.0043 0.0139
5.41 17.08 5.50 7.26 4.84 18.77 9.59 19.31 6.67 11.47
13 14 11 17 13 15 13 15 11 18
0.9650 0.2517 0.9047 0.9800 0.9786 0.2242 0.7272 0.2000 0.8251 0.8732
222 175 131 114 206 216 156 156 190
145 139 95 82 142 168 80 92 130
3 4 2 7 9 6 14 10 16
45 86 31 206 204 143 260 216 109
16 28 15 34 51 72 62 66 27
240 210 240 220 240 200 190 220 260
8.2 7.4 6.8 7.8 7.3 8.2 7.7 7.7 7.8
7.6–8.8 6.4–8.4 5.3–8.3 7.2–8.4 6.7–8.0 7.6–8.7 6.4–8.9 7.5–8.0 7.7–7.9
2.4 3.4 2.4 1.6 3.9 2.9 2.7 3.0 3.2
1.3–3.4 1.6–5.1 0.4–4.4 0.8–2.4 2.8–5.0 1.9–4.0 0.8–4.6 2.6–3.5 2.9–3.5
2.0 1.8 1.7 2.0 1.8 2.0 1.9 1.9 2.0
−7.630 −4.810 −6.180 −10.752 −4.174 −6.092 −6.362 −5.596 −5.350
0.932 0.650 0.909 1.377 0.568 0.747 0.827 0.725 0.696
2.8713 1.7879 7.5388 7.5244 0.4289 1.1183 5.2639 0.2018 0.4094 0.9414
−0.3275 −0.2046 −0.8995 −0.9122 −0.0482 −0.1297 −0.6088 −0.0231 −0.0438 −0.0909
0.0382 0.0243 0.1094 0.1123 0.0057 0.0155 0.0730 0.0027 0.0048 0.0088
10.60 8.48 1.17 2.54 18.86 6.05 2.71 15.26
15 13 9 12 15 11 10 15
0.7802 0.8112 0.9989 0.9980 0.2203 0.8702 0.9874 0.4331
178 208 265 247 240 271 136
126 120 151 171 117 135 71
4 4 3 4 14 7 3
82 78 64 65 243 100 537
38 30 10 26 91 71 12
190 190 145 210 195 205 195
SR, selection range (L75 –L25 ); SF, selection factor (L50 /m) with m equal to mesh size; d.f., degrees of freedom; vi1 , vi2 , parameters of the logistic curve; Ri11 , Ri12 , Ri22 , elements of variance–covariance matrix; D, (inter-haul) variance matrix.
Z. Tosuno˘glu et al. / Fisheries Research 97 (2009) 95–102
DM (11) HM 3 4 5 7 8 9 10 11 12 Pooled (12) Fryer (9) D
L50
Z. Tosuno˘glu et al. / Fisheries Research 97 (2009) 95–102
99
Table 3 Estimates of the contribution of additional variables to the selectivity of the broadtail shortfin squid. Significance level: 0.01 (data concerns only hexagonal and square mesh codends where individual haul data was available). Illex coindetii
Parameters
Estimate
S.D.
t-Value
d.f.
p-Value
HX & SQ only mesh
˛1 (L50 , constant) ˛2 (SR, constant) ˛3 (L50 , mesh)
5.223 2.598 2.528
0.148 0.191 0.228
35.353 13.633 11.071
26 26 26
0.000 0.000 0.000
HX & SQ Mesh plus additional variables
˛1 (L50 , constant) ˛2 (SR, constant) ˛3 (L50 , mesh) ˛11 (SR, species catch)
5.205 1.515 2.587 0.138
0.144 0.273 0.221 0.037
36.299 5.542 11.690 4.102
25 25 25 25
0.000 0.000 0.000 0.000
codends, varying from 2.8 to 3.2 cm (Fig. 5), while the selection factors (SF = L50 /mesh size) increase from 1.0 in DMC to 1.3 and 2.0 for HMC and SMC, respectively. A further analysis was carried out to investigate the possibility of other variables in addition to mesh configuration, i.e., haul duration, total catch weight, codend catch weight, and species weight (Table 3). Only species weight has a significant effect (p < 0.01), but it only affects SR. 3.4. Pink cuttlefish The length-distributions for total, codend and cover catches, and percentages of marketable, discarded and escaped pink cuttlefish for all codends are given in Fig. 6, showing a clear bi-modal pattern (the smaller at about 3–4 cm and the larger at 7–8 cm, DML). The length ranges are similar between the different codend shapes, although individuals smaller than 5 cm were less common in SMC. As inferred from the percentage of escaped and discarded indi-
Fig. 4. Size structure of the broadtail shortfin squids that entered the different codends. Thin line corresponds to length frequency in codend, dashed line in cover and thick line to total numbers. Percentages of marketed (MRK), discarded (DSC), and escaped (ESC) in diamond, hexagonal and square mesh codends.
Fig. 5. Selection curves for individual broadtail shortfin squid hauls (thin gray straight and dashed lines for HMC and SMC, respectively), mean curves according to Fryer (1991) for HMC (thick straight line) and SMC (thick dashed line), and pooled data for DMC (thick dotted line).
Fig. 6. Size structure of the pink cuttlefish that entered the different codends. Thin line corresponds to length frequency in codend, dashed line in cover and thick line to total numbers. Percentages of marketed (MRK), discarded (DSC), and escaped (ESC) in diamond, hexagonal and square mesh codends.
100
Table 4 Selectivity estimates for pink cuttlefish. Codend (Hauls)
SM 1 5 6 7 8 Pooled (10) Fryer (5) D
95%CI for L50
SR
95%CI for SR
SF
vi1
vi2
Ri11
Ri12
Ri22
Deviance
d.f.
p-Value
Catch weight (kg)
Number in
Dur. (min)
Total
Codend
Species
Codend
Cover
3.0
2.8–3.1
1.2
0.9–1.5
0.7
−5.290
1.796
0.3927
−0.1152
0.0353
2.63
8
0.9552
2146
1412
23
741
117
2.6 3.1 3.2 2.9 3.4 3.1 3.1 3.3 3.2 2.5 2.7 3.0 3.1
1.0–3.4 2.6–3.6 2.9–3.6 2.5–3.4 2.9–3.9 2.7–3.6 2.9–3.3 2.8–3.8 2.7–3.8 2.1–2.9 2.3–3.0 2.8–3.2 3.0–3.2
1.8 1.6 0.7 1.1 1.4 1.7 1.0 1.5 1.5 1.2 1.1 1.5 1.4
0.9–2.8 0.7–2.5 0.2–1.2 0.4–1.8 0.6–2.1 0.9–2.4 0.6–1.3 0.8–2.3 0.7–2.3 0.7–1.7 0.4–1.8 1.1–1.8 1.3–1.5
0.7 0.8 0.8 0.7 0.9 0.8 0.8 0.8 0.8 0.6 0.7 0.8 0.8
−3.121 −4.307 −10.698 −5.934 −5.550 −4.180 −7.159 −4.771 −4.825 −4.562 −5.492 −4.543 −4.606
1.190 1.387 3.295 2.024 1.625 1.328 2.309 1.434 1.489 1.813 2.072 1.520 1.537
0.9054 1.3348 11.5231 2.9765 2.0020 0.7668 1.4664 1.2215 1.4977 1.0239 2.9703 0.2589 0.1287 0.0251
−0.2249 −0.3604 −3.5117 −0.9094 −0.5389 −0.2104 −0.4314 −0.3068 −0.3825 −0.3176 −0.9672 −0.0716 −0.0341 0.0136
0.0592 0.1013 1.0979 0.2902 0.1531 0.0625 0.1312 0.0818 0.1026 0.1025 0.3194 0.0209 0.0113 0.0091
12.20 13.16 0.46 2.44 15.76 3.37 1.98 15.52 10.50 1.39 4.98 27.48
6 6 7 7 6 7 6 6 5 6 6 8
0.0576 0.0406 0.9996 0.9314 0.0151 0.8485 0.9214 0.0166 0.0623 0.9666 0.5465 0.0006
145 222 175 131 152 114 206 216 156 156 190
111 145 139 95 118 82 142 168 80 92 130
6 5 3 3 5 4 8 10 4 6 4
232 221 103 110 184 127 339 334 128 269 191
27 75 17 19 64 40 48 78 25 40 39
230 240 210 240 215 220 240 200 190 220 260
3.7 4.2 3.4 3.7 4.1 3.6 3.8
3.2–4.1 3.5–4.9 3.1–3.7 3.3–4.1 3.5–4.6 3.4–3.8 3.6–4.0
0.9 1.5 1.3 0.9 1.3 1.3 1.3
0.3–1.5 0.7–2.3 0.7–1.8 0.4–1.4 0.6–2.0 1.0–1.5 1.2–1.4
0.9 1.0 0.8 0.9 1.0 0.9 1.0
−9.061 −6.269 −5.840 −8.671 −6.836 −6.219 −6.336
2.463 1.494 1.727 2.335 1.686 1.729 1.699
7.6824 3.0749 1.2737 3.9092 2.7834 0.2869 0.5011 0.0026
−1.8935 −0.6033 −0.3447 −0.9841 −0.6046 −0.0703 −0.1175 0.0054
0.4743 0.1244 0.0965 0.2562 0.1383 0.0181 0.0324 0.0155
2.66 6.03 4.57 3.84 9.25 7.03
5 7 6 6 6 7
0.7517 0.5358 0.6002 0.6983 0.1601 0.4256
178 247 137 240 271
126 171 108 117 135
5 4 3 6 3
156 97 146 156 90
16 15 52 25 20
190 210 195 195 205
SR, selection range (L75 –L25 ); SF, selection factor (L50 /m) with m equal to mesh size; d.f., degrees of freedom; vi1 , vi2 , parameters of the logistic curve; Ri11 , Ri12 , Ri22 , elements of variance–covariance matrix; D, (inter-haul) variance matrix.
Z. Tosuno˘glu et al. / Fisheries Research 97 (2009) 95–102
DM (11) HM 2 3 4 5 6 7 8 9 10 11 12 Pooled (12) Fryer (11) D
L50
Z. Tosuno˘glu et al. / Fisheries Research 97 (2009) 95–102
101
Table 5 Estimates of the contribution of additional variables to the selectivity of the pink cuttlefish. Significance level: 0.01 (data concerns only hexagonal and square mesh codends where individual haul data was available). Sepia orbignyana
Parameters
HX & SQ only mesh
˛1 (L50 , constant) ˛2 (SR, constant) ˛3 (L50 , mesh)
HX & SQ Mesh plus additional variables
˛1 ˛2 ˛4 ˛7
(L50 , constant) (SR, constant) (L50 , total catch) (L50 , duration)
Estimate
S.D.
t-Value
d.f.
p-Value
3.067 1.190 0.692
0.095 0.082 0.172
32.372 14.442 4.025
26 26 26
0.000 0.000 0.000
5.028 1.181 0.005 −0.749
0.736 0.078 0.002 0.185
6.828 15.200 3.386 −4.055
25 25 25 25
0.000 0.000 0.002 0.000
Fig. 7. Selection curves for individual pink cuttlefish hauls (thin gray straight and dashed lines for HMC and SMC, respectively), mean curves according to Fryer (1991) for HMC (thick straight line) and SMC (thick dashed line), and pooled data for DMC (thick dotted line).
viduals, the selection properties of DMC and HMC are statistically indistinguishable. While the percentage of escaping individuals is also the same for SMC, the discarded fraction is smaller compared to DMC and HMC. This difference is likely a consequence of the smaller catch size since there is a slight tendency to release larger individuals. The selectivity parameters based on the pooled data for DMC and mean curves (according to Fryer) for HMC and SMC are shown in Table 4. The L50 s for DMC and HMC are similar, 3.0 and 3.1 cm, respectively, showing no difference in selection between either mesh configurations. Use of the SMC results in a small but significant (˛3 = 0.692; p < 0.01) increase up to 3.8 cm compared to HMC, whereas there was no statistically significant difference between the SR values (p > 0.05) (Table 5, Fig. 7). The SR similarity (around 1.3 cm) suggests that there are no changes in the patterns of selection (i.e., the curves have the same shape) (Fig. 7). However, when other variables (haul duration and catch weight) are added to the model (see Table 5), mesh configuration is no longer significant (p < 0.05), while both total catch weight and haul duration come up as the main components influencing selectivity (p < 0.01). 4. Discussion To evaluate the effect of codend mesh shape on commercially important Mediterranean cephalopods, we conducted selectivity experiments for three different mesh shapes on three species. For the European squid the data structure did not allow for a haulby-haul analysis, but selection curves could be estimated for all codends based on pooled data. The likelihood ratio test results showed a statistically significant difference (p < 0.01) between the HMC and SMC (L50 = 4.2 and 6.0 cm, respectively), but not between DMC and HMC (L50 of 4.6 and 4.2 cm). However, the very small sampling for both DMC and SMC suggests that caution is needed in drawing conclusions. Selectivity parameters from our estimates are similar to previous estimates for this species for DMC and SMC (Ordines et al., 2006), as evidenced by the range of selection factors (from 0.9 to 1.5). Further data are available from Atlantic waters
(Fonseca et al., 2002), where estimated L50 s of 9.7 and 11.4 cm, were obtained for 80 and 90 mm diamond mesh size codends, respectively. These results correspond to a common selection factor of 1.3, which is within the range of values found in the present study. However, the Atlantic curves shapes as given by the selection ranges are different from those in both Mediterranean studies (1.0–2.1 cm). We found the largest SR to be 1.8 cm compared to 5.1 and 6.2 cm in Portuguese waters. The former leads to curves with higher slope suggesting a sharper selectivity. This may facilitate the matching of selectivity parameters such as L50 to a reference length (MLS or LFM), if knife-edge selection is sought. Despite the possible adoption of SMC, given the size structure of the fished population, there will always be a high catch of immature European squids, as the LFMs (18.5 cm in females and 11 cm in males, Roper et al., 1984) are much higher than the estimated L50 s. The broadtail shortfin squid size range was similar to that of the European squid, and likewise if the species LFM (females, 15 cm; males, 10–12 cm, Jereb and Ragonese, 1995) is considered a potential MLS, then a high proportion of immature individuals will always be retained. Nevertheless, there is an increase in selectivity when the HMC and particularly the SMC are compared to the currently enforced 44 mm DMC. The estimated selectivity parameters, L50 of 4.2, 5.2 and 7.8 cm, corresponding to selection factors of 1.0, 1.3 and 2.0, for the DMC, HMC and SMC, respectively, are similar to previous selectivity studies for broadtail shortfin squid in both Mediterranean and Atlantic waters (SF = 1.3–2.2, Sala et al., 2008; SF = 1.5, Fonseca et al., 2002). Similar to the observations on the European squid, the SR for the broadtail shortfin squid in the Mediterranean is considerably smaller than the estimates for Portuguese waters. As the first report on selectivity for pink cuttlefish, this study indicates that the 44 mm DMC used in the multi-species fisheries of the Aegean Sea is rather unselective, capturing a high proportion of immature individuals. Furthermore, all three codend configurations resulted in poor selectivity for the three cephalopod species, with the SMC displaying the highest L50 values. L50 values of the 40 mm SMC were 27, 30 and 86% higher than the 44 mm DMC for the pink cuttlefish, European squid and broadtail shortfin squid, respectively. However, only for the two squid species is this difference significant. Although the extra mesh opening obtained from the use of HMC was expected to increase selectivity, the broadtail shortfin squid was the only species for which a sizeable increase in the codend selection properties from DMC to HMC was found. However, the mesh size for both HMC and SMC (40 mm) was smaller than that of DMC (44 mm), so an increase in their selective properties might be expected if mesh sizes had been constant. Similar to round fishes, Loliginids selection is favored by the use of SMC. However, due to its more compact body shape and the calcareous internal skeletal structure of sepiids, cuttlefish do not show the same trend. Hence, there is a very small increase in selectivity with the use of a square-shaped mesh codend by contrast with loliginids, where the non-calcareous raquis allows for a considerably more flexible body. The morphological and behavioural differences between both groups are reflected in the modeling of additional variables other than mesh configuration. While the broadtail short-
102
Z. Tosuno˘glu et al. / Fisheries Research 97 (2009) 95–102
fin squid modeling confirmed mesh shape as the key factor in determining L50 (species weight was shown to influence SR), for the pink cuttlefish, mesh shape does not apparently have a primary role in determining selectivity. Instead, for this species, total catch weight and haul duration, were shown to be the main variables affecting selectivity. Although such outcome may be attributable to a lower swimming ability and higher body stiffness that prevents the species from benefiting from an increase in mesh opening, care must be exercised. In fact, there is evidence of an improvement derived from the use of square mesh shaped-codends which must not be discarded. Further data on this species selectivity is needed to resolve the cross effects of the different variables. For all cephalopod species, regardless of the mesh shape, L50 values were substantially lower than the minimum landing size or length at first maturity. Therefore, the current legal minimum mesh sizes and configuration codend for demersal trawling are not suitable for the management of these species. Overall, the current results would point to the need of adopting a more suitable mesh size, combined with a change in codend design, to match either their legal MLS or LFM. However, catches mostly consist of immature individuals, and consequently any adjustment would result in the end of the cephalopod trawl fishery. Additionally cephalopods are caught within the scope of a multispecies fishery, where fish and crustaceans constitute the main catch. As such, the most sensible option will be to determine the optimal mesh size and configuration for the management of both fishes and crustaceans, and then evaluate its impact on the cephalopods fraction. Similarly to what happens for most mixed trawl fisheries it will be most difficult to manage the different target-species based only on technical measures. Apart from a compromise that includes the requirements of all target species involved based on the existing MLS restrictions, biological parameters and known selectivity data, complementary measures, such as effort control, will certainly be necessary for resource sustainability. Acknowledgements We would like to thank all individuals involved in collecting the data on board the commercial trawler Hapulo˘glu and Prof. Dr. John Dean (University of South Carolina, USA) for his assistance in reviewing the text. The present study was funded by the Science and Technology Centre of Ege University (Project no 2007/BIL/004).
References Anon., 2006. The commercial fish catching regulations in seas and inland waters in 2006–2008 fishing period: circular No. 37/1 (in Turkish). Republic of Turkey, Minister of Agriculture and Rural Affair, General Directorate of Conservation and Inspection, Ankara, 108 pp. Aydın, C., Tosuno˘glu, Z., 2009. Selectivity of square and hexagonal mesh codends for the deep water rose shrimp, Parapenaeus longirostris (Lucas, 1846) (Decapoda, Penaeidae) in the Aegean Sea. Crustaceana 82, 89–98. Campos, A., Fonseca, P., Henriques, V., 2003. Size selectivity for four fish species of the deep groundfish assemblage off the Portuguese southwest coast: evidence of mesh size, mesh configuration and cod end catch effects. Fish. Res. 63, 213–233. Clarke, M.R., 1983. Cephalopod biomass estimation from predation. Mem. Nal Museum Vic. 44, 95–107. ConStat, 1995. CC selectivity program. Gronspaettevej 10, DK-9800 Hjorring, Denmark. E.C., 2006. Council Regulation (EC 1967/2006) concerning management measures for the sustainable exploitation of fishery resources in the Mediterranean Sea, amending Regulation (EEC) No 2847/93 and repealing Regulation (EC) No 1626/94. Off. J.E.U. 409, 75 p. Fonseca, P., Campos, A., Garcia, A., 2002. Bottom trawl codend selectivity for cephalopods in Portuguese continental waters. Fish. Res. 59, 263–271. Fryer, R.J., 1991. A model of between-haul variation in selectivity. ICES J. Mar. Sci. 48, 281–290. Fryer, R.J., Shepherd, J.G., 1996. Models of codend size selection. J. North. Atl. Fish. Sci. 19, 51–58. Jereb, P., Ragonese, S., 1995. An outline of the biology of the squid Illex coindetii in the Sicilian channel (Central Mediterranean). J. Mar. Biol. Assoc. UK 75, 373– 390. Jereb, P., Roper, C.F.E., 2005. Cephalopod of the world. An annotated and illustrated catalogue of cephalopod species known to date. Volume 1. Chambered nautiluses and sepioids (Nautilidae, Sepiidae, Sepiolidae, Sepiadariidae and Spirulidae). FAO Species catalogue for Fishery Purposes. No. 4, vol. 1, 262 p. Mangold, K., Boletzky, S.V., 1987. Céphalopodes. Fiches FAO d’identification des espèces pour les besoins de la pêche. (Révision 1) Méditerranée et Mer Noire. Zone de pêche 37 1, 633–714. Ordines, F., Massutí, E., Guijarro, B., Mas, R., 2006. Diamond vs. square mesh codend in a multi-species trawl fishery of the western Mediterranean: effects on catch composition, yield, size selectivity and discards. Aquat. Living Resour. 19, 329–338. Roper, C.F.E., Sweeney, M.J., Nauen, C.E., 1984. Cephalopods of the world. An annotated and illustrated catalogue of species of interest to fisheries. FAO Fish. Synop. 3 (125), 277. Sala, A., Lucchetti, A., Piccinetti, C., Ferretti, M., 2008. Size selection by diamond- and square-mesh codends in multi-species Mediterranean demersal trawl fisheries. Fish. Res. 93, 8–21. Wileman, D.A., Ferro, R.S.T., Fonteyne, R., Millar, R.B. (Eds.), 1996. Manual of Methods of Measuring the Selectivity of Towed Fishing Gears. Copenhagen, ICES Cooperative Research Report No. 215, 126 pp. Worms, J., 1983. World fisheries for Cephalopods: a synoptic overview. In: Caddy, J.F. (Ed.), Advances in Assessment of World Cephalopods Resources, 231. FAO Fish. Tech. Rep., pp. 1–19.
Contents lists available at ScienceDirect
Fisheries Research journal homepage: www.elsevier.com/locate/fishres
Research article
Selectivity of diamond, hexagonal and square mesh codends for three commercial cephalopods in the Mediterranean Zafer Tosuno˘glu a,∗ , Celalettin Aydın a , Alp Salman a , Paulo Fonseca b a b
Ege University, Faculty of Fisheries, 35100 Bornova, Izmir, Turkey INRB/L-IPIMAR, National Institute of Biological Resources/Laboratory of Fisheries and Sea Research, Ave Brasilia, P-1449006 Lisbon, Portugal
a r t i c l e
i n f o
Article history: Received 4 June 2008 Received in revised form 14 January 2009 Accepted 19 January 2009 Keywords: Codend selectivity Mesh shape Size-selection Bycatch Loligo vulgaris Illex coindetii Sepia orbignyana Aegean Sea Eastern Mediterranean
a b s t r a c t Undersized and immature commercially important cephalopods are often inadvertently caught by trawlers in the eastern Mediterranean. To evaluate the effectiveness of different mesh codends (diamond, hexagonal and square) to reduce bycatch of juvenile commercially important Mediterranean cephalopods (European squid Loligo vulgaris, broadtail shortfin squid Illex coindetii and pink cuttlefish Sepia orbignyana), a series of selectivity experiments was undertaken with a modified bottom trawl. The covered codend technique was used to capture escapees. For the broadtail shortfin squid and the pink cuttlefish, the square-shaped mesh displayed the highest 50% retention lengths (L50 ) compared to diamond and hexagonal mesh. For the European squid, selectivity could only be measured by pooling the data from all hauls for each mesh shape. For all three species, the L50 values of square and hexagonal mesh codends were significantly different (p < 0.01). Beyond the mesh variability, species catch was found to have a significant impact in the selection range of the broadtail shortfin squid. Furthermore, for the cuttlefish, total catch and haul duration likely account for variability of L50 attributed to mesh configuration. For all species, regardless of the mesh shape, L50 values were substantially lower than the minimum landing size or length at first maturity. Therefore, the current legal minimum mesh size and codend configurations for demersal trawling are not suitable for the management of these species. As such, sustainability in Mediterranean cephalopod fisheries would profit from more selective gears. This could be achieved both by an increase in codend mesh size and change in codend shape; however, being part of a mixed fishery, when these changes are being practiced, the impact on the fish catches of target species will have to be taken into consideration. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Cephalopods comprise approximately 700 species worldwide and many ecologically important cephalopods have Atlantic origins (Mangold and Boletzky, 1987). Both as direct human consumption and as a principal food for top predators (Clarke, 1983), these cephalopods constitute one of the most important demersal fishery resources in the Mediterranean (Worms, 1983). Cephalopod fisheries production in Mediterranean was 53800 t in 2001, constituting 1.6% of the total world cephalopod catch (Jereb and Roper, 2005). Because of the economic and ecological importance of cephalopods, the factors affecting the sustainability of the cephalopod fisheries of the Mediterranean must be understood. In addition to catches by trammel, fyke nets, seiners, specially designed longlines and hand-jigging, cephalopods also constitute a valuable by-catch in the trawl fishery (Jereb and Roper, 2005).
∗ Corresponding author. Tel.: +90 535 5927692; fax: +90 232 3883685. E-mail address: [email protected] (Z. Tosuno˘glu). 0165-7836/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.fishres.2009.01.006
However, different Mediterranean nations deal with the by-catch differently. In Turkish waters, codends with a minimum diamond mesh size of 44 mm are required for demersal fish trawling (Anonymous, 2006). In contrast, European Union Mediterranean countries are enforcing the use of 40 mm square mesh codends as of 2008 (E.C., 2006). Although a considerable amount of trawl selectivity research has been carried out for finfish and crustacean species in the Mediterranean, selectivity data for commercial cephalopods are only available for the European squid Loligo vulgaris (Ordines et al., 2006) and the broadtail shortfin squid Illex coindetii (Sala et al., 2008). Understanding the size ranges and maturation stages of these species will reveal if mesh shape (and size) are capturing these species at important life stages. Considering the commercial relevance of cephalopods and the need for sustainable management, we conducted a preliminary investigation on the codend selectivity of different mesh shape (diamond, hexagonal and square) for the three most important species, the European squid, the broadtail shortfin squid and the pink cuttlefish Sepia orbignyana. Potential benefits from an improved codend were matched against the length at first maturity (LFM), i.e., the length corresponding to a 50% pro-
96
Z. Tosuno˘glu et al. / Fisheries Research 97 (2009) 95–102
portion of mature individuals, for the squid species (the maturation cycle for the pink cuttlefish is unknown). 2. Materials and methods Fishing was conducted in the commercial fishing grounds of the Aegean Sea (eastern Mediterranean) between 30 September and 25 October 2007 on board a commercial stern trawler. Technical details of the sampling gear and procedure were given in Aydın and Tosuno˘glu (2009). Characteristics of the experimental codends are as follows: (1) diamond mesh (DMC, a commercially used standard codend): PE 210 d/24 knotless, nominal 44 mm (44.7 mm ± 0.08), 400 meshes on its circumference and 5 m in length. (2) hexagonal mesh (HMC, a combination PE + PA 210 d/27 knotless, nominal 40 mm (42.6 ± 0.11 mm), 200 meshes on its circumference and 5 m in length. (3) square mesh (SMC, a PE 0.40*10 knotted, nominal 40 mm (42.4 ± 0.19 mm), 200 bars in circumference and 5 m in length). Selectivity data were collected using the covered codend method and selection curves of the individual hauls were obtained by fitting a logistic function: r(l) = exp(v1 + v2 l)/[1 + exp(v1 + v2 l)] (Wileman et al., 1996), where the parameters v1 (the intercept) and v2 (the slope) were estimated by maximum likelihood using the software CC2000 (ConStat, 1995). When the numbers of individuals retained and escaped were insufficient for individual haul estimation, data were pooled over all hauls to estimate the selection curve. The statistical significance among pooled selection curves was evaluated with a likelihood ratio test (e.g., Campos et al., 2003). Fryer (1991) model was used to test the between-haul variation of the selectivity parameters v1 and v2 by mesh configuration, allowing the estimation of mean curves for the three different mesh codends by using the software ECModeller (ConStat, 1995) which utilizes the REML (REsidual Maximum Likelihood) approach. The same approach was also used to test the statistical significance of additional explanatory variables (total, tot, codend, cod, and species, sp, catch weight, and haul duration, dur.) on the selectivity parameter estimates. In this case, a general design matrix Xi was assumed for the departure model, where i = 1 . . . H, the number of hauls,
Xi =
˛1 0
0 ˛2
˛3 mi 0
˛4 toti 0
˛5 codi 0
˛6 spi 0
˛7 duri 0
0 ˛8 mi
0 ˛9 toti
Fig. 2. Size structure of European squids that entered the different codends. Thin line corresponds to length frequency in codend, dashed line in cover and thick line to total numbers. Percentages of marketed (MRK), discarded (DSC), and escaped (ESC) in diamond, hexagonal and square mesh codends.
0 ˛10 codi
0 ˛11 spi
0 ˛12 duri
The choice of the model which best describes the data was based on the lowest value for Akaike’s Information Criterion-AIC (Fryer and Shepherd, 1996). 3. Results 3.1. Species catch from hauls
Fig. 1. Species composition by weight in codend + cover, corresponding to a total catch of 6109 kg in 33 hauls.
Thirty-three hauls yielded a total catch biomass of 6.1 t, 2.1 t with DMC (11 hauls), 2.0 t with HMC (12 hauls), and 2.0 t with SMC (10 hauls), during 112 trawling hours. Deep-water rose shrimp (Parapenaeus longirostris), horse mackerel (Trachurus trachurus) and hake (Merluccius merluccius) accounted for the majority of the weight, followed by broadtail shortfin squid, pink cuttlefish, European squid and greater forkbeard (Phycis blennoides) (Fig. 1). Other commercial species were John Dory (Zeus faber), angler fish (Lophius piscatorius), tub gurnard (Chelidonichthys lucernus), piper gurnard (Trigla lyra), red mullet (Mullus barbatus), blackspot seabream (Pagellus bogaraveo), blackbelly rosefish (Helicolenus dactylopterus dactylopterus), silver scabbardfish (Lepidopus caudatus), chub mackerel (Scomber japonicus), common octopus (Octopus vulgaris), musky octopus (Eledone moschata), and large thornback ray (Raja clavata) and European
SR, selection range (L75 -L25 ); SF, selection factor (L50 /m) with m equal to mesh size; d.f., degrees of freedom; vi1 , vi2 , parameters of the logistic curve; Ri11 , Ri12 , Ri22 , elements of variance–covariance matrix.
58 198 61 3 7 3 1412 1410 1173 2146 2013 1950 0.9654 0.9941 0.9739 11 16 10 1.97 5.29 3.29 2.299 1.223 1.446 9 12 10 DM HM SM
4.6 4.2 6.0
4.0–5.2 4.0–4.5 5.5–6.5
1.0 1.8 1.5
0.4–1.7 1.5–2.1 0.7–2.3
1.0 1.0 1.5
−10.469 −5.162 −8.717
Codend
vi2 SF 95%CI for L50 L50 Number of hauls Codend
Table 1 Selectivity estimates for European squid.
SR
95%CI for SR
3.3. Broadtail shortfin squid The length-distributions for total, codend and cover catches, and percentages of marketable, discarded and escaped broadtail shortfin squid in all codends are shown in Fig. 4. The catch is composed of a high percentage of immature individuals (males below 10–12 cm and females below 15 cm). As the codend mesh shape changes from DMC to HMC and SMC, there is an increasing ability to reduce the capture of the smaller individuals of the population. The selection curve parameters for DMC were estimated from the pooled data over all hauls, while, for HMC and SMC, the mean curves were derived from Fryer’s approach (Fryer, 1991), which takes inter-haul variability into account. The estimated selectivity parameters are given in Table 2. The lowest L50 , 4.2 cm, is found in DMC, increasing to 5.2 cm for HMC (24% greater than DMC) and 7.8 cm for the SMC (86% greater than DMC). Using Fryer’s approach, a statistically significant difference (p < 0.01) is found between the L50 values of SMC and HMC (Table 3). The use of the square mesh codends seems to positively affect the L50 (˛3 = 2.528), but the difference in the SR was not statistically significant (p > 0.05). A direct comparison with DMC is not possible due to the different estimation approach. However, since the HMC and SMC confidence intervals do not overlap with those of DMC, the DMC likely has a different selectivity from the other codends for this species. SR is very close for the three
vi1
Ri11
Ri12
Ri22
Fig. 2 shows the length-distributions for total, codend and cover catches, and percentages of marketable, discarded and escaped European squid based on data pooled for each codend type over the different codend mesh sizes and shapes. Because the catches in DMC and SMC were small, these results must be conservatively interpreted. Furthermore, the length distribution for the HMC is different from both the DMC and SMC, being biased towards small squids. Overall, juvenile individuals dominate the catch, with very few squids caught above the 50% maturation length for males (11 cm), the smaller of the two sexes. The small catches in both codends and covers were insufficient for single haul analysis, so to estimate selection curves, the data were pooled across codend shape. The estimated selectivity parameters are given in Table 1 and selection curves displayed in Fig. 3. The highest 50% retention lengths (L50 ) are estimated as 6.0 cm in SMC, while the DMC and HMC yielded very similar L50 values of 4.6 and 4.2 cm, respectively. With respect to selection range (SR), the estimated values vary between 1.0 and 1.8 cm.
0.4302 0.0108 0.1220
Deviance
3.2. European squid
−1.8690 −0.0355 −0.7588
Codend
d.f.
p-Value
conger (Conger conger). In addition, the cephalopod species; Sepia elegans, *S. orbignyana, Sepietta oweniana, Rondeletiola minor, *Loligo vulgaris, Alloteuthis media, *Illex coindetii, *Octopus vulgaris, Scaeurgus unicirrhus, *Eledone moschata and *E. cirrhosa were caught in the trawled area (asterisks identify commercial cephalopod species).
8.4864 0.1278 4.8221
Number in
Total
Species Catch weight (kg)
Fig. 3. Selection curves for European squid based on pooled data with observed retention in DMC (— ♦), HMC (- - - ) and SMC (– – – ).
24 572 26
97 Cover
Z. Tosuno˘glu et al. / Fisheries Research 97 (2009) 95–102
98
Table 2 Selectivity estimates for broadtail shortfin squid. Codend (Hauls)
SM 1 2 4 5 7 8 10 Pooled (10) Fryer (7) D
95%CI for L50
SR
95%CI for SR
SF
vi1
vi2
Ri11
Ri12
Ri22
Deviance
d.f.
p-Value
Catch weight (kg)
Number in
Dur. (min)
Total
Codend
Species
Codend
Cover
4.2
3.7–4.6
2.9
2.4–3.5
1.0
−3.098
0.746
0.1762
−0.0267
0.0044
26.54
17
0.0651
2146
1412
59
1312
114
5.1 5.5 5.0 4.4 4.7 5.4 5.7 5.2 5.3 5.2 5.2
4.0–6.3 4.9–6.1 4.1–6.0 3.7–5.2 4.2–5.2 5.0–5.8 5.0–6.4 4.5–5.9 4.3–6.4 5.0–5.4 5.1–5.3
3.1 2.0 2.2 2.7 2.4 2.1 3.4 3.7 3.3 2.9 2.8
0.9–5.3 1.1–2.9 0.4–4.0 1.6–3.7 1.4–3.3 1.4–2.7 2.3–4.4 2.5–4.9 1.8–4.9 2.5–3.2 2.6–3.0
1.3 1.4 1.3 1.1 1.2 1.4 1.4 1.3 1.3 1.3 1.3
−3.654 −6.077 −5.025 −3.675 −4.411 −5.803 −3.733 −3.136 −3.540 −3.946 −4.001
0.711 1.102 0.998 0.828 0.934 1.069 0.655 0.600 0.662 0.764 0.777
1.7995 2.0579 3.5501 0.7738 0.8340 0.7504 0.4325 0.3668 0.8932 0.0752 0.1220 0.2513
−0.3081 −0.3389 −0.6707 −0.1283 −0.1501 −0.1287 −0.0596 −0.0538 −0.1276 −0.0119 −0.0213 −0.0534
0.0570 0.0585 0.1337 0.0227 0.0282 0.0234 0.0088 0.0085 0.0197 0.0020 0.0043 0.0139
5.41 17.08 5.50 7.26 4.84 18.77 9.59 19.31 6.67 11.47
13 14 11 17 13 15 13 15 11 18
0.9650 0.2517 0.9047 0.9800 0.9786 0.2242 0.7272 0.2000 0.8251 0.8732
222 175 131 114 206 216 156 156 190
145 139 95 82 142 168 80 92 130
3 4 2 7 9 6 14 10 16
45 86 31 206 204 143 260 216 109
16 28 15 34 51 72 62 66 27
240 210 240 220 240 200 190 220 260
8.2 7.4 6.8 7.8 7.3 8.2 7.7 7.7 7.8
7.6–8.8 6.4–8.4 5.3–8.3 7.2–8.4 6.7–8.0 7.6–8.7 6.4–8.9 7.5–8.0 7.7–7.9
2.4 3.4 2.4 1.6 3.9 2.9 2.7 3.0 3.2
1.3–3.4 1.6–5.1 0.4–4.4 0.8–2.4 2.8–5.0 1.9–4.0 0.8–4.6 2.6–3.5 2.9–3.5
2.0 1.8 1.7 2.0 1.8 2.0 1.9 1.9 2.0
−7.630 −4.810 −6.180 −10.752 −4.174 −6.092 −6.362 −5.596 −5.350
0.932 0.650 0.909 1.377 0.568 0.747 0.827 0.725 0.696
2.8713 1.7879 7.5388 7.5244 0.4289 1.1183 5.2639 0.2018 0.4094 0.9414
−0.3275 −0.2046 −0.8995 −0.9122 −0.0482 −0.1297 −0.6088 −0.0231 −0.0438 −0.0909
0.0382 0.0243 0.1094 0.1123 0.0057 0.0155 0.0730 0.0027 0.0048 0.0088
10.60 8.48 1.17 2.54 18.86 6.05 2.71 15.26
15 13 9 12 15 11 10 15
0.7802 0.8112 0.9989 0.9980 0.2203 0.8702 0.9874 0.4331
178 208 265 247 240 271 136
126 120 151 171 117 135 71
4 4 3 4 14 7 3
82 78 64 65 243 100 537
38 30 10 26 91 71 12
190 190 145 210 195 205 195
SR, selection range (L75 –L25 ); SF, selection factor (L50 /m) with m equal to mesh size; d.f., degrees of freedom; vi1 , vi2 , parameters of the logistic curve; Ri11 , Ri12 , Ri22 , elements of variance–covariance matrix; D, (inter-haul) variance matrix.
Z. Tosuno˘glu et al. / Fisheries Research 97 (2009) 95–102
DM (11) HM 3 4 5 7 8 9 10 11 12 Pooled (12) Fryer (9) D
L50
Z. Tosuno˘glu et al. / Fisheries Research 97 (2009) 95–102
99
Table 3 Estimates of the contribution of additional variables to the selectivity of the broadtail shortfin squid. Significance level: 0.01 (data concerns only hexagonal and square mesh codends where individual haul data was available). Illex coindetii
Parameters
Estimate
S.D.
t-Value
d.f.
p-Value
HX & SQ only mesh
˛1 (L50 , constant) ˛2 (SR, constant) ˛3 (L50 , mesh)
5.223 2.598 2.528
0.148 0.191 0.228
35.353 13.633 11.071
26 26 26
0.000 0.000 0.000
HX & SQ Mesh plus additional variables
˛1 (L50 , constant) ˛2 (SR, constant) ˛3 (L50 , mesh) ˛11 (SR, species catch)
5.205 1.515 2.587 0.138
0.144 0.273 0.221 0.037
36.299 5.542 11.690 4.102
25 25 25 25
0.000 0.000 0.000 0.000
codends, varying from 2.8 to 3.2 cm (Fig. 5), while the selection factors (SF = L50 /mesh size) increase from 1.0 in DMC to 1.3 and 2.0 for HMC and SMC, respectively. A further analysis was carried out to investigate the possibility of other variables in addition to mesh configuration, i.e., haul duration, total catch weight, codend catch weight, and species weight (Table 3). Only species weight has a significant effect (p < 0.01), but it only affects SR. 3.4. Pink cuttlefish The length-distributions for total, codend and cover catches, and percentages of marketable, discarded and escaped pink cuttlefish for all codends are given in Fig. 6, showing a clear bi-modal pattern (the smaller at about 3–4 cm and the larger at 7–8 cm, DML). The length ranges are similar between the different codend shapes, although individuals smaller than 5 cm were less common in SMC. As inferred from the percentage of escaped and discarded indi-
Fig. 4. Size structure of the broadtail shortfin squids that entered the different codends. Thin line corresponds to length frequency in codend, dashed line in cover and thick line to total numbers. Percentages of marketed (MRK), discarded (DSC), and escaped (ESC) in diamond, hexagonal and square mesh codends.
Fig. 5. Selection curves for individual broadtail shortfin squid hauls (thin gray straight and dashed lines for HMC and SMC, respectively), mean curves according to Fryer (1991) for HMC (thick straight line) and SMC (thick dashed line), and pooled data for DMC (thick dotted line).
Fig. 6. Size structure of the pink cuttlefish that entered the different codends. Thin line corresponds to length frequency in codend, dashed line in cover and thick line to total numbers. Percentages of marketed (MRK), discarded (DSC), and escaped (ESC) in diamond, hexagonal and square mesh codends.
100
Table 4 Selectivity estimates for pink cuttlefish. Codend (Hauls)
SM 1 5 6 7 8 Pooled (10) Fryer (5) D
95%CI for L50
SR
95%CI for SR
SF
vi1
vi2
Ri11
Ri12
Ri22
Deviance
d.f.
p-Value
Catch weight (kg)
Number in
Dur. (min)
Total
Codend
Species
Codend
Cover
3.0
2.8–3.1
1.2
0.9–1.5
0.7
−5.290
1.796
0.3927
−0.1152
0.0353
2.63
8
0.9552
2146
1412
23
741
117
2.6 3.1 3.2 2.9 3.4 3.1 3.1 3.3 3.2 2.5 2.7 3.0 3.1
1.0–3.4 2.6–3.6 2.9–3.6 2.5–3.4 2.9–3.9 2.7–3.6 2.9–3.3 2.8–3.8 2.7–3.8 2.1–2.9 2.3–3.0 2.8–3.2 3.0–3.2
1.8 1.6 0.7 1.1 1.4 1.7 1.0 1.5 1.5 1.2 1.1 1.5 1.4
0.9–2.8 0.7–2.5 0.2–1.2 0.4–1.8 0.6–2.1 0.9–2.4 0.6–1.3 0.8–2.3 0.7–2.3 0.7–1.7 0.4–1.8 1.1–1.8 1.3–1.5
0.7 0.8 0.8 0.7 0.9 0.8 0.8 0.8 0.8 0.6 0.7 0.8 0.8
−3.121 −4.307 −10.698 −5.934 −5.550 −4.180 −7.159 −4.771 −4.825 −4.562 −5.492 −4.543 −4.606
1.190 1.387 3.295 2.024 1.625 1.328 2.309 1.434 1.489 1.813 2.072 1.520 1.537
0.9054 1.3348 11.5231 2.9765 2.0020 0.7668 1.4664 1.2215 1.4977 1.0239 2.9703 0.2589 0.1287 0.0251
−0.2249 −0.3604 −3.5117 −0.9094 −0.5389 −0.2104 −0.4314 −0.3068 −0.3825 −0.3176 −0.9672 −0.0716 −0.0341 0.0136
0.0592 0.1013 1.0979 0.2902 0.1531 0.0625 0.1312 0.0818 0.1026 0.1025 0.3194 0.0209 0.0113 0.0091
12.20 13.16 0.46 2.44 15.76 3.37 1.98 15.52 10.50 1.39 4.98 27.48
6 6 7 7 6 7 6 6 5 6 6 8
0.0576 0.0406 0.9996 0.9314 0.0151 0.8485 0.9214 0.0166 0.0623 0.9666 0.5465 0.0006
145 222 175 131 152 114 206 216 156 156 190
111 145 139 95 118 82 142 168 80 92 130
6 5 3 3 5 4 8 10 4 6 4
232 221 103 110 184 127 339 334 128 269 191
27 75 17 19 64 40 48 78 25 40 39
230 240 210 240 215 220 240 200 190 220 260
3.7 4.2 3.4 3.7 4.1 3.6 3.8
3.2–4.1 3.5–4.9 3.1–3.7 3.3–4.1 3.5–4.6 3.4–3.8 3.6–4.0
0.9 1.5 1.3 0.9 1.3 1.3 1.3
0.3–1.5 0.7–2.3 0.7–1.8 0.4–1.4 0.6–2.0 1.0–1.5 1.2–1.4
0.9 1.0 0.8 0.9 1.0 0.9 1.0
−9.061 −6.269 −5.840 −8.671 −6.836 −6.219 −6.336
2.463 1.494 1.727 2.335 1.686 1.729 1.699
7.6824 3.0749 1.2737 3.9092 2.7834 0.2869 0.5011 0.0026
−1.8935 −0.6033 −0.3447 −0.9841 −0.6046 −0.0703 −0.1175 0.0054
0.4743 0.1244 0.0965 0.2562 0.1383 0.0181 0.0324 0.0155
2.66 6.03 4.57 3.84 9.25 7.03
5 7 6 6 6 7
0.7517 0.5358 0.6002 0.6983 0.1601 0.4256
178 247 137 240 271
126 171 108 117 135
5 4 3 6 3
156 97 146 156 90
16 15 52 25 20
190 210 195 195 205
SR, selection range (L75 –L25 ); SF, selection factor (L50 /m) with m equal to mesh size; d.f., degrees of freedom; vi1 , vi2 , parameters of the logistic curve; Ri11 , Ri12 , Ri22 , elements of variance–covariance matrix; D, (inter-haul) variance matrix.
Z. Tosuno˘glu et al. / Fisheries Research 97 (2009) 95–102
DM (11) HM 2 3 4 5 6 7 8 9 10 11 12 Pooled (12) Fryer (11) D
L50
Z. Tosuno˘glu et al. / Fisheries Research 97 (2009) 95–102
101
Table 5 Estimates of the contribution of additional variables to the selectivity of the pink cuttlefish. Significance level: 0.01 (data concerns only hexagonal and square mesh codends where individual haul data was available). Sepia orbignyana
Parameters
HX & SQ only mesh
˛1 (L50 , constant) ˛2 (SR, constant) ˛3 (L50 , mesh)
HX & SQ Mesh plus additional variables
˛1 ˛2 ˛4 ˛7
(L50 , constant) (SR, constant) (L50 , total catch) (L50 , duration)
Estimate
S.D.
t-Value
d.f.
p-Value
3.067 1.190 0.692
0.095 0.082 0.172
32.372 14.442 4.025
26 26 26
0.000 0.000 0.000
5.028 1.181 0.005 −0.749
0.736 0.078 0.002 0.185
6.828 15.200 3.386 −4.055
25 25 25 25
0.000 0.000 0.002 0.000
Fig. 7. Selection curves for individual pink cuttlefish hauls (thin gray straight and dashed lines for HMC and SMC, respectively), mean curves according to Fryer (1991) for HMC (thick straight line) and SMC (thick dashed line), and pooled data for DMC (thick dotted line).
viduals, the selection properties of DMC and HMC are statistically indistinguishable. While the percentage of escaping individuals is also the same for SMC, the discarded fraction is smaller compared to DMC and HMC. This difference is likely a consequence of the smaller catch size since there is a slight tendency to release larger individuals. The selectivity parameters based on the pooled data for DMC and mean curves (according to Fryer) for HMC and SMC are shown in Table 4. The L50 s for DMC and HMC are similar, 3.0 and 3.1 cm, respectively, showing no difference in selection between either mesh configurations. Use of the SMC results in a small but significant (˛3 = 0.692; p < 0.01) increase up to 3.8 cm compared to HMC, whereas there was no statistically significant difference between the SR values (p > 0.05) (Table 5, Fig. 7). The SR similarity (around 1.3 cm) suggests that there are no changes in the patterns of selection (i.e., the curves have the same shape) (Fig. 7). However, when other variables (haul duration and catch weight) are added to the model (see Table 5), mesh configuration is no longer significant (p < 0.05), while both total catch weight and haul duration come up as the main components influencing selectivity (p < 0.01). 4. Discussion To evaluate the effect of codend mesh shape on commercially important Mediterranean cephalopods, we conducted selectivity experiments for three different mesh shapes on three species. For the European squid the data structure did not allow for a haulby-haul analysis, but selection curves could be estimated for all codends based on pooled data. The likelihood ratio test results showed a statistically significant difference (p < 0.01) between the HMC and SMC (L50 = 4.2 and 6.0 cm, respectively), but not between DMC and HMC (L50 of 4.6 and 4.2 cm). However, the very small sampling for both DMC and SMC suggests that caution is needed in drawing conclusions. Selectivity parameters from our estimates are similar to previous estimates for this species for DMC and SMC (Ordines et al., 2006), as evidenced by the range of selection factors (from 0.9 to 1.5). Further data are available from Atlantic waters
(Fonseca et al., 2002), where estimated L50 s of 9.7 and 11.4 cm, were obtained for 80 and 90 mm diamond mesh size codends, respectively. These results correspond to a common selection factor of 1.3, which is within the range of values found in the present study. However, the Atlantic curves shapes as given by the selection ranges are different from those in both Mediterranean studies (1.0–2.1 cm). We found the largest SR to be 1.8 cm compared to 5.1 and 6.2 cm in Portuguese waters. The former leads to curves with higher slope suggesting a sharper selectivity. This may facilitate the matching of selectivity parameters such as L50 to a reference length (MLS or LFM), if knife-edge selection is sought. Despite the possible adoption of SMC, given the size structure of the fished population, there will always be a high catch of immature European squids, as the LFMs (18.5 cm in females and 11 cm in males, Roper et al., 1984) are much higher than the estimated L50 s. The broadtail shortfin squid size range was similar to that of the European squid, and likewise if the species LFM (females, 15 cm; males, 10–12 cm, Jereb and Ragonese, 1995) is considered a potential MLS, then a high proportion of immature individuals will always be retained. Nevertheless, there is an increase in selectivity when the HMC and particularly the SMC are compared to the currently enforced 44 mm DMC. The estimated selectivity parameters, L50 of 4.2, 5.2 and 7.8 cm, corresponding to selection factors of 1.0, 1.3 and 2.0, for the DMC, HMC and SMC, respectively, are similar to previous selectivity studies for broadtail shortfin squid in both Mediterranean and Atlantic waters (SF = 1.3–2.2, Sala et al., 2008; SF = 1.5, Fonseca et al., 2002). Similar to the observations on the European squid, the SR for the broadtail shortfin squid in the Mediterranean is considerably smaller than the estimates for Portuguese waters. As the first report on selectivity for pink cuttlefish, this study indicates that the 44 mm DMC used in the multi-species fisheries of the Aegean Sea is rather unselective, capturing a high proportion of immature individuals. Furthermore, all three codend configurations resulted in poor selectivity for the three cephalopod species, with the SMC displaying the highest L50 values. L50 values of the 40 mm SMC were 27, 30 and 86% higher than the 44 mm DMC for the pink cuttlefish, European squid and broadtail shortfin squid, respectively. However, only for the two squid species is this difference significant. Although the extra mesh opening obtained from the use of HMC was expected to increase selectivity, the broadtail shortfin squid was the only species for which a sizeable increase in the codend selection properties from DMC to HMC was found. However, the mesh size for both HMC and SMC (40 mm) was smaller than that of DMC (44 mm), so an increase in their selective properties might be expected if mesh sizes had been constant. Similar to round fishes, Loliginids selection is favored by the use of SMC. However, due to its more compact body shape and the calcareous internal skeletal structure of sepiids, cuttlefish do not show the same trend. Hence, there is a very small increase in selectivity with the use of a square-shaped mesh codend by contrast with loliginids, where the non-calcareous raquis allows for a considerably more flexible body. The morphological and behavioural differences between both groups are reflected in the modeling of additional variables other than mesh configuration. While the broadtail short-
102
Z. Tosuno˘glu et al. / Fisheries Research 97 (2009) 95–102
fin squid modeling confirmed mesh shape as the key factor in determining L50 (species weight was shown to influence SR), for the pink cuttlefish, mesh shape does not apparently have a primary role in determining selectivity. Instead, for this species, total catch weight and haul duration, were shown to be the main variables affecting selectivity. Although such outcome may be attributable to a lower swimming ability and higher body stiffness that prevents the species from benefiting from an increase in mesh opening, care must be exercised. In fact, there is evidence of an improvement derived from the use of square mesh shaped-codends which must not be discarded. Further data on this species selectivity is needed to resolve the cross effects of the different variables. For all cephalopod species, regardless of the mesh shape, L50 values were substantially lower than the minimum landing size or length at first maturity. Therefore, the current legal minimum mesh sizes and configuration codend for demersal trawling are not suitable for the management of these species. Overall, the current results would point to the need of adopting a more suitable mesh size, combined with a change in codend design, to match either their legal MLS or LFM. However, catches mostly consist of immature individuals, and consequently any adjustment would result in the end of the cephalopod trawl fishery. Additionally cephalopods are caught within the scope of a multispecies fishery, where fish and crustaceans constitute the main catch. As such, the most sensible option will be to determine the optimal mesh size and configuration for the management of both fishes and crustaceans, and then evaluate its impact on the cephalopods fraction. Similarly to what happens for most mixed trawl fisheries it will be most difficult to manage the different target-species based only on technical measures. Apart from a compromise that includes the requirements of all target species involved based on the existing MLS restrictions, biological parameters and known selectivity data, complementary measures, such as effort control, will certainly be necessary for resource sustainability. Acknowledgements We would like to thank all individuals involved in collecting the data on board the commercial trawler Hapulo˘glu and Prof. Dr. John Dean (University of South Carolina, USA) for his assistance in reviewing the text. The present study was funded by the Science and Technology Centre of Ege University (Project no 2007/BIL/004).
References Anon., 2006. The commercial fish catching regulations in seas and inland waters in 2006–2008 fishing period: circular No. 37/1 (in Turkish). Republic of Turkey, Minister of Agriculture and Rural Affair, General Directorate of Conservation and Inspection, Ankara, 108 pp. Aydın, C., Tosuno˘glu, Z., 2009. Selectivity of square and hexagonal mesh codends for the deep water rose shrimp, Parapenaeus longirostris (Lucas, 1846) (Decapoda, Penaeidae) in the Aegean Sea. Crustaceana 82, 89–98. Campos, A., Fonseca, P., Henriques, V., 2003. Size selectivity for four fish species of the deep groundfish assemblage off the Portuguese southwest coast: evidence of mesh size, mesh configuration and cod end catch effects. Fish. Res. 63, 213–233. Clarke, M.R., 1983. Cephalopod biomass estimation from predation. Mem. Nal Museum Vic. 44, 95–107. ConStat, 1995. CC selectivity program. Gronspaettevej 10, DK-9800 Hjorring, Denmark. E.C., 2006. Council Regulation (EC 1967/2006) concerning management measures for the sustainable exploitation of fishery resources in the Mediterranean Sea, amending Regulation (EEC) No 2847/93 and repealing Regulation (EC) No 1626/94. Off. J.E.U. 409, 75 p. Fonseca, P., Campos, A., Garcia, A., 2002. Bottom trawl codend selectivity for cephalopods in Portuguese continental waters. Fish. Res. 59, 263–271. Fryer, R.J., 1991. A model of between-haul variation in selectivity. ICES J. Mar. Sci. 48, 281–290. Fryer, R.J., Shepherd, J.G., 1996. Models of codend size selection. J. North. Atl. Fish. Sci. 19, 51–58. Jereb, P., Ragonese, S., 1995. An outline of the biology of the squid Illex coindetii in the Sicilian channel (Central Mediterranean). J. Mar. Biol. Assoc. UK 75, 373– 390. Jereb, P., Roper, C.F.E., 2005. Cephalopod of the world. An annotated and illustrated catalogue of cephalopod species known to date. Volume 1. Chambered nautiluses and sepioids (Nautilidae, Sepiidae, Sepiolidae, Sepiadariidae and Spirulidae). FAO Species catalogue for Fishery Purposes. No. 4, vol. 1, 262 p. Mangold, K., Boletzky, S.V., 1987. Céphalopodes. Fiches FAO d’identification des espèces pour les besoins de la pêche. (Révision 1) Méditerranée et Mer Noire. Zone de pêche 37 1, 633–714. Ordines, F., Massutí, E., Guijarro, B., Mas, R., 2006. Diamond vs. square mesh codend in a multi-species trawl fishery of the western Mediterranean: effects on catch composition, yield, size selectivity and discards. Aquat. Living Resour. 19, 329–338. Roper, C.F.E., Sweeney, M.J., Nauen, C.E., 1984. Cephalopods of the world. An annotated and illustrated catalogue of species of interest to fisheries. FAO Fish. Synop. 3 (125), 277. Sala, A., Lucchetti, A., Piccinetti, C., Ferretti, M., 2008. Size selection by diamond- and square-mesh codends in multi-species Mediterranean demersal trawl fisheries. Fish. Res. 93, 8–21. Wileman, D.A., Ferro, R.S.T., Fonteyne, R., Millar, R.B. (Eds.), 1996. Manual of Methods of Measuring the Selectivity of Towed Fishing Gears. Copenhagen, ICES Cooperative Research Report No. 215, 126 pp. Worms, J., 1983. World fisheries for Cephalopods: a synoptic overview. In: Caddy, J.F. (Ed.), Advances in Assessment of World Cephalopods Resources, 231. FAO Fish. Tech. Rep., pp. 1–19.