Selectivity of diamond, square and hexagonal mesh codends for Atlantic horse mackerel Trachurus trachurus, European hake Merluccius merluccius, and greater forkbeard Phycis blennoides in the eastern Mediterranean

Knowledge on selectivity is considered essential for an adequate management of the trawl fishery. This paper compares the selectivity of conventionally used codends of 44 mm diamond mesh, 40 mm square, and hexagonal mesh for Atlantic horse mackerel, European hake and greater forkbeard in the eastern Mediterranean. Data were collected using the covered codend method and analyzed using the logistic equation with the maximum likelihood method. Mean selection curves were analyzed and compared using the between-haul variations model. The mean L ₅₀ v alues of diamond, square and hexagonal mesh codends are 14.7, 15.9 and 17.1 cm total length (TL) for horse mackerel, 10.4, 14.4 and 11.0 cm TL for hake and 12.2, 15.8 and 12.7 cm TL for greater forkbeard, respectively. For horse mackerel, the differences among the codends are statistically significant (P   <   0.05); however, the L ₅₀ values for all three meshes are higher than the minimum landing size (13 cm TL). For hake and greater forkbeard, the L ₅₀ value for the square mesh codend is significantly different from the values for diamond and hexagonal meshes (P   <   0.01), whereas there is no significant difference between diamond and hexagonal meshes (P   >   0.05). Regardless of the mesh shape, L ₅₀ values were substantially lower than the minimum landing size or size at first maturity for hake and greater forkbeard. To enable more effective solutions, research on species-selective trawls is required for the Mediterranean demersal trawl fishery.     

Selectivity of a 50-mm diamond mesh knotless polyethylene codend for commercially important fish species in the Aegean Sea

This research investigated effects of changing the diamond mesh size on codend selectivity in Mediterranean fisheries. The selectivity of a typical 50‐mm diamond knotless polyethylene (PE) codend used in the Turkish fishery in the Aegean Sea was measured for commercially important species, in particular hake (Merluccius merluccius), horse mackerel (Trachurus trachurus), anglerfish (Lophius piscatorius) and John Dory (Zeus faber). Fishing trials were carried out on the commercial trawler ‘Hapulo?lu’ between 9 and 12 December 2006 using a modified trawl net. Selectivity data were collected by the covered codend method and analysed by means of a logistic equation (Maximum Likelihood Method). The mean selectivity curve was estimated from individual hauls, taking between‐haul variations into account. Mean mesh size of the codend was 49.4 mm as measured by digital calliper. Mean values for 50% retention length of hake and horse mackerel were estimated to be 11.4 and 15.6 cm total length; corresponding selection ranges were 4.1 and 5.5 cm respectively. The 50‐mm diamond mesh codend showed adequate selectivity compared to the minimum landing size (MLS) for horse mackerel, while for hake it selected specimens in a size range far lower than the MLS. No selectivity values could be determined for anglerfish or John Dory. To design a more selective codend for the Turkish demersal trawl fishery, not only mesh size regulations but also other codend characteristics and netting material properties must be urgently considered.     

Size selectivity of three diamond mesh codends for the European hake (Merluccius merluccius) and the tub gurnard (Trigla lucerna) in the Sea of Marmara, Turkey

The effects of an increase in the codend mesh size from 44 to 56 and 80 mm on size selectivity of the two by‐catch species (the European hake Merluccius merlucius and the tub gurnard Trigla lucerna) commonly captured in the shrimp fishery in the Sea of Marmara, were evaluated. The parameters of the logistic selectivity model were estimated by maximum likelihood. Length at 50% retention, L₅₀, was found to increase with mesh size for both species. The L₅₀ values of 44, 56 and 80 mm were, respectively, 19.8, 21.5 and 33.4 cm for European hake, 17.9, 17.7 and 25.6 cm for tub gurnard. The main conclusions are that 44 mm and 56 mm diamond mesh codends retain a considerable amount of undersized hake and tub gurnard as by‐catch in the shrimp fishery. With 80 mm mesh size codend, most juvenile European hake are released. For the tub gurnard, however, even this mesh size does not provide a good enough selection. From the biological point of view, it is concluded that the legal 44 mm diamond mesh codend does not provide satisfactory selection for European hake and the tub‐gurnard in trawl fisheries of the Sea of Marmara.     

Seasonal variation in the trawl codend selectivity of common pandora (Pagellus erythrinus)

Seasonal changes in the selectivity of a 40 mm polyethylene trawl codend for common pandora (Pagellus erythrinus) were tested under similar fishing conditions in spring, summer and autumn 2002, and winter 2003. Data were obtained using the covered codend technique and analyzed using a logistic model fitted with the maximum likelihood method. Parameters of mean curves for each trip were estimated by using Fryer’s between‐haul variation model. To determine seasonal changes in the condition of the fish, samples were taken to cover the entire length range of the catch along with length measurements to the nearest mm and weights to the nearest 0.1 g. Results showed a tendency for decrease in selectivity from summer (L₅₀ = 11.3 cm, SR = 2.2 cm) and autumn (L₅₀ = 11.2 cm, SR = 1.7 cm) to winter (L₅₀ = 10.7 cm, SR = 2.4 cm) and spring (L₅₀ = 10.2 cm, SR = 2.9 cm). Potential effects of water temperature, fish condition, population size structure, and gonad developments on selectivity are discussed to explain the seasonal variation in trawl codend selectivity.     

Selectivity of commercial compared to larger mesh and square mesh trawl codends for four fish species in the Aegean Sea

Differences in size selectivity of commercial (40 mm diamond mesh, 40D), larger mesh (48 mm diamond mesh, 48D), and square mesh codends (40 mm square mesh, 40S) for hake (Merluccius merluccius), greater forkbeard (Phycis blennoides), blackbelly rosefish (Helicolenus dactylopterus dactylopterus) and four‐spot megrim (Lepidorhombus boscii) were investigated in the Aegean Sea. The study was conducted using the covered codend method. Data were analysed taking between‐haul variations into account. Results showed that changing from a 40D to 48D codend significantly improved mean L₅₀ values, with increases of about 22% for hake, 8% for greater forkbeard, 20% for blackbelly rosefish, and 11% for four‐spot megrim (P = 0.000). A change from diamond to square mesh configuration with the same 40 mm netting also significantly increased the mean L₅₀ values, with 45% for hake, 36% for greater forkbeard, and 25% for blackbelly rosefish (P = 0.000). For four‐spot megrim the mean L₅₀ value was about 10% lower, but this difference was statistically insignificant (P > 0.05). Despite the increases in L₅₀ values, this study concludes that the selectivity of the 48D and 40S codends is still not sufficient to release fish smaller than length at first maturity for these four species.     

Selectivity of conventional and alternative codend design for five fish species in the Aegean Sea

Currently, traditional Mediterranean trawls are generally made with non‐selective netting and the fishing boats are involved in multi‐species fisheries. As a result, most near‐shore stocks are over‐exploited. Weather permitting, the demersal trawl fleet tends to fish in relatively deeper, international waters of the Aegean Sea, where the catch is usually higher. Therefore, the need for evaluation of the codends used in this fishery and the potential improvements to their selectivity are of prime importance. In the present study, selectivity data were collected for hake (Merluccius merluccius), blue whiting (Micromesistius poutassou), greater forkbeard (Phycis blennoides), blackbelly rosefish (Helicolenus dactylopterus dactylopterus) and fourspotted megrim (Lepidorhombus boscii) in commercial (300 MC) and square mesh top panel (SMTPC) codends. Trawling was carried out at depths of 274–426 m onboard a commercial vessel chartered for a 15‐day sea trial in August 2004. Selection parameters were obtained by fitting a logistic equation using a maximum likelihood method. Results of the selectivity analysis indicated that the commercially used 40 mm nominal mesh size PE codend was rather unselective for the species investigated in this study. In general, the square mesh top panel codend has relatively higher L₅₀ values than the commercial codend. However, except for blue whiting, even this codend is rather unselective when 50% maturity lengths (L_M50) are considered.     

Comparison of the size selectivity of diamond (PA) and square (PE) mesh codends for deepwater crustacean species in the Antalya Bay, eastern Mediterranean

The aim of this study was to compare selectivity results of currently used 44 mm nominal polyamide (PA) diamond mesh- and alternatively suggested 40 mm nominal polyethylene (PE) square mesh- codends in the deepwater crustacean trawl fishery in the Antalya Bay, eastern Mediterranean. Selectivity experiments were carried out during targeted trawling of four commonly harvested crustacean species: giant red shrimp Aristaeomorpha foliacea , 'blue and red' shrimp Aristeus antennatus , rose shrimp Parapenaeus longirostris , and pandalid shrimp Plesionika martia . A conventional bottom trawl of 600 meshes around the fishing circle was operated onboard a commercial stern trawler between 6 and 18 June 2007. Depth of the fishing area varied between 441 and 630 m. Data were collected using the covered codend technique, and analyzed using a logistic equation with maximum likelihood for individual and pooled hauls. The commercially used trawl codend was unable to release immature crustaceans. Selectivity parameters of the three species of crustaceans were distinctly lower when collected with the polyamide diamond mesh than with the polyethylene square mesh, except in the case of giant red shrimp for which values were similar. However, the present and previous results show that in square mesh codends, mesh sizes must be more than 40 mm in order to keep catches clear of specimens below minimum landing sizes or 50% sexual maturity sizes of crustaceans in the Mediterranean. This study suggests that regulating mesh size by requiring square mesh openings during deep water crustacean trawling of the eastern Mediterranean is essential for the release of immature individuals.     

Selectivity of diamond (PA) and square (PE) mesh codends for commercially important fish species in the Antalya Bay,eastern Mediterranean

Size selectivity of the 40 mm nominal polyethylene (PE) square mesh codend and the 44 mm nominal polyamide (PA) diamond mesh codend were determined under commercial conditions in the demersal trawl fishery. Data were collected using the covered codend technique and analyzed by logistic equation with a maximum likelihood method. Changing the mesh from a 44 mm PA diamond to a 40 mm PE square increased the mean retention lengths (L₅₀s) of red mullet (Mullus barbatus), picarel (Spicara maena) and bogue (Boops boops) and decreased the selection ranges (SR) for common pandora (Pagellus erythrinus). For large‐eye dentex (Dentex macrophthalmus) and axillary sea bream (Pagellus acarne) mean L₅₀ and SR values were 9.6 and 11.8, and 2.4 and 2.4 cm, respectively, only in the DM44PA codend. For golden banded goatfish (Upeneus moluccensis) values for SM40PE were only 15.0 and 2.7 cm, respectively. Results showed that the 40 mm PE square mesh codend provided higher selectivity for most of the Mediterranean fishes. The results also showed that regulating mesh size and requiring square mesh openings during trawling is essential for the release of immature individuals. This practice will result in a reduction in overfishing and permit recovery of overfished stocks.     

Sorting grid trials to improve size selectivity of red mullet (Mullus barbatus) and annular sea bream (Diplodus annularis) in Turkish bottom trawl fishery

Sorting grids with two different bar spacings (12 mm and 14 mm) were tested to improve size selectivity of the commercially important fish species, red mullet (Mullus barbatus) and annular sea bream (Diplodus annularis), in Turkish bottom trawl fishery. Fishing trials were carried out with R/V ‘Egesüf’ between April and May 2003 in Izmir Bay, Aegean Sea, using a traditional bottom trawl. Selectivity data were collected by the top cover method and analyzed by means of a logistic equation with the maximum likelihood method. Selectivity parameters for individual hauls were obtained with the software program cc 2000 . Mean selectivity was also estimated and compared using the EC Model software that takes between‐haul variations into account. The codend catch size as an additional explanatory variable was used in the comparison. The Kolmogorov–Smirnov (K–S) test was also applied to detect differences between length‐frequency distributions in the upper and lower bags of the 12 and 14 mm bar spacings for red mullet and annular sea bream. The mean L₅₀ values of red mullet were 8.7 and 10.0 cm with the 12 and 14 mm grids, respectively. The L₅₀ value of 14 mm was comparable with the L₅₀ value of the codend mesh size; however, the 12 mm value was rather low. The mean L₅₀ values of 12 and 14 mm bar spacings were 8.8 and 10.4 cm for annular sea bream, respectively; the value of 14 mm bar spacing was very close to 50% size at sexual maturity of the species (10.5 cm). The K–S test indicated length distributions of red mullet and annular sea bream in the 12 and 14 mm upper and lower bags as significantly different (P < 0.05). These results show that improving the size selectivity in a multi‐species fishery using a single selective device is rather difficult. However, higher size selectivity can be obtained when considering the minimum landing size or the 50% sexual maturity size for a given species.     

Efficiency of flexible sorting grids to improve size selectivity of the bottom trawl in the Balearic Islands (western Mediterranean), with comparison to a change in mesh cod-end geometry

In recent years several studies have been developed to improve the selectivity of the Mediterranean bottom trawl fisheries, which exert high fishing pressure on young individuals. The focus has been mainly on increasing the mesh size in the cod‐end or changing its mesh geometry; few studies have assessed the efficiency of the sorting grid systems. Analysis of 21 trawl hauls carried out at depths of 117–697 m off the Balearic Islands (western Mediterranean) in October–November 2005, studied size selectivity of the trawl using flexible sorting grids with 15 (SG15) and 20 (SG20) mm bar spacing. A divided polyamide bottom trawl designed for commercial purposes demonstrated high efficiency for the simultaneous use of both sorting grid sizes at different depth intervals (DS: deep shelf; US: upper slope; MS: middle slope). The results were also compared with those from the authors’ previous studies in the area where 40 mm diamond (DM) and square (SM) mesh cod‐ends were used. A saturation effect was detected on the DS for both sorting grids, due to the large amount of biomass captured at this depth interval. Size selectivity of 11 species was modelled, which showed an increase in length at first capture (L₅₀) from SG15 to SG20. Values of L₅₀ estimated for the main target species were: 10.9 cm total length (TL) with SG15, and 18.9 cm TL with SG20 for Merluccius merluccius; 21.1 mm carapace length (CL) with SG15, and 23.8 mm CL with SG20 for Nephrops norvegicus; and 15.9 mm CL with SG15, and 19.6 mm CL with SG20 for Aristeus antennatus. Comparison of size selectivity parameters between the sorting grids and cod‐ends of different geometry showed clear differences. For most species, the highest value of L₅₀ was obtained with SM (e.g. 26.6 mm CL for N. norvegicus and 22.1 mm CL for A. antennatus). Exceptions were the round‐fish M. merluccius, the flatfish Lepidorhombus boscii, and the crustacean Parapenaeus longirostris, with values of SG20 (18.9 cm TL, 12.1 cm TL, and 25.7 mm CL, respectively) higher than with SM (15.2 cm TL, 10.2 cm TL, and 20.2 mm CL, respectively). Sorting grid selection ranges were broader than those estimated for DM and SM. Experience shows that the exchange of the DM for the SM and/or the introduction of SG20 can improve selectivity in the Mediterranean trawl, SM being more efficient than SG20 in the Balearic Islands.     

The impact of fisheries on vulnerable habitats: the case of trawling on circa-littoral grounds in the Strait of Sicily (central Mediterranean Sea)

The Council Regulation (EC) no. 1967/2006 introduced the adoption of the 40 mm square-mesh or alternatively a 50 mm diamond-mesh codend for trawlers of EU Member States. In this context, the main aim of the current study was to compare the catch composition of Lampedusa’s shallow-water trawl fishery, in terms of retained and discarded fraction, using the 24 mm square-mesh codend (illegal and supposed unselective) and the 50 mm current diamond-mesh codend (legal and supposed selective). During 2012, 21 hauls were performed, under commercial conditions, with the 24 mm square mesh codend and 27 with the 50 mm diamond mesh codend in an area of the Strait of Sicily at depths from 40 to 80 m. The introduction of a 50 mm diamond-mesh, although not significantly, reduced the CPUEs of retained and discarded fraction compared with the 24 mm mesh. Moreover, for most target species and their size classes (especially immature individuals), the legal mesh recorded a lower mean number of specimens than the banned mesh size. All these advantages aside, the discard of the shallow-water trawl fishery, in Lampedusa, represented a very important part of the total catch (more than three-quarters) by both mesh sizes and a large amount of this was represented by species and habitat of conservation concern. Our results suggest that spatial restrictions, such as no-take zones, could be a more effective management tool to protect this sensitive ecosystem in the area than the technical measures (e.g. reduction of mesh size) imposed today by the EU.     

Gillnet selectivity for Chilean hake (Merluccius gayi gayi Guichenot, 1848) in the bay of Valparaíso

To determine the selectivity of gillnets in the Chilean artisanal fishery for the Chilean hake Merluccius gayi gayi, an experimental net was used with three mesh sizes (5.2, 6.8 and 7.6 cm). A total of 2279 specimens of Chilean hake were caught (24–56 cm total length, TL), mainly via gilling and snagging. The catch proportion below the size of sexual maturity SSM_50% (37 cm TL) for each mesh size tested was estimated. Models were fitted separately according to the sex to compensate for differences in size compositions, with the males fitted to a binormal model and females fitted to the normal location model. Analysing both sexes combined, the model with the lowest deviance was lognormal, with estimated modal lengths of 30.9, 40.2 and 43.9 cm TL for meshes of 5.2, 6.8 and 7.6 cm, respectively. The estimated selectivity factor (SF) indicates that the modal size matches the SSM_50% with a mesh size of 6.2 cm, while the length of retention of 50% (l₅₀) coincides with the SSM_50% with a mesh size of 7.6 cm.     

Selectivity of 50 mm T0 and T90 codends for commercial shrimp species in the Turkish deepwater trawl fishery,Eastern Mediterranean

The study compared the selectivity of 50 mm mesh size conventional (T0) and 90° turned mesh (T90) demersal trawl codends for four commercial shrimp species (Aristaeomorpha foliacea, Aristeus antennatus, Parapenaeus longirostris and Plesionika martia) in the Bay of Antalya in the eastern Mediterranean. Data were collected from 27 valid hauls using the covered codend method. Selectivity parameters were obtained using the logistic equation with the maximum likelihood method and by taking into account the between‐haul variations. Results show that changing the codends from T0 to T90 increases the percentage of escapees for all four targeted shrimp species. According to species, a change in the mesh configuration significantly increases L₅₀ by 7–14%. However, for the two commercially important red shrimp species (Aristaeomorpha foliacea and Aristeus antennatus), even the L₅₀ values obtained in the T90 codend (23.3 mm) remain rather low for the release of sufficient numbers of immature specimens. First maturity sizes (FMS) for these two species are 38 and 30 mm, respectively; therefore, for their sustainable exploitation, additional management measures must be investigated including larger mesh sizes.     

Exploitation and mortality rates of European hake (Merluccius merluccius) in the Aegean Sea (Izmir Bay,Turkey)

Experimental trawl surveys in Izmir Bay (Aegean Sea) were taken seasonally between 2007 and 2009. A total of 1353 specimens were sampled ranging from 5.9 cm (1.4 g) to 44.4 cm (670 g). The main size group of Merluccius merluccius was between 14 and 25 cm total length (TL). In total, the mean catch per unit effort (CPUE) for hake by number and weight was 101.4 ± 18.4 ind h^?1 and 10.7 ± 1.5 kg h^?1, respectively. Highest mean CPUE by number and biomass was determined as 257.1 ± 15.1 ind h^?1 and 15.1 ± 4.9 kg h^?1 in summer. Mortality (M, F, Z) ratios of hake were 0.58 year^?1, 1.66 year^?1 and 2.24 year^?1, respectively. Because of the many smaller specimens caught in this study, this seems to indicate that there may be heavy fishing pressures on the hake population; the high exploitation rate (E = 0.74) appears to confirm this conclusion.     

Implications for catch composition and revenue in changing from diamond to square mesh codends in the northeastern Mediterranean

The objective of this study was to estimate the potential short‐term economic loss as a result of replacing the commercial diamond codend with a square mesh codend, and to compare fish lengths captured in 40 mm square (S40) vs 44 mm commercial hand‐woven diamond (CD44) mesh codends for red mullet (Mullus barbatus), common pandora (Pagellus erythrinus), bogue (Boops boops), European hake (Merluccius merluccius), axillary seabream (Pagellus acarne), and brushtooth lizardfish (Saurida undosquamis). A total of 20 hauls (10 hauls for S40 and 10 hauls for CD44) were conducted onboard a commercial trawler between 27 February and 11 April 2012 in Mersin Bay in the northeastern Mediterranean. Results showed that the 40 mm square mesh codend caught significantly fewer juveniles of all aforementioned species. However, when changing from the commercial diamond codend to the 40 mm square mesh codend the potential economic loss of revenue was found to be 40% in the study period, which covered the final 6 weeks of the fishing season.     

Length-weight relationships of 10 fish species from the coastal waters of the East China Sea

The present study provides the length-weight relationships (LWRs) for 10 marine fish species collected from the coastal waters of the East China Sea by bottom trawl surveys. The open width of the sampling net was 40 m and the mesh size of the codend was 20 mm. Specimens were collected from November 2015 to November 2019 at seasonal intervals. The measurement accuracy of weight and length were 0.1 grams and 0.1 centimeters, respectively. This study provides new information of LWRs for nine species which have not yet been reported in FishBase. Also, this study updates the information of maximum length for two species.     

Length–weight relationships of 11 deep‐sea fishes from the western Bay of Bengal and Andaman waters,India

Length–weight relationships (LWRs) are presented for 11 deep‐sea fishes caught in the western Bay of Bengal and Andaman waters during August 2010 using a 38 m high speed demersal trawl II (HSDT II, crustacean version, codend mesh size 40 mm) and a 45.6 m Expo model demersal trawl (codend mesh size 30 mm). The b values ranged from 2.34 to 3.3 and the coefficient of variation (r²) ranged from .82 to .98. LWR estimates of eight deep‐sea fishes are provided for the first time. The estimated LWR values were compared with the Bayesian LWR estimates available in FishBase, based on models developed to improve the accuracy and predictability of species‐specific growth parameters of data‐poor species.     

Trawl codend design (44 mm diamond PE mesh) and the effect on selectivity for Pagellus erythrinus and Pagellus acarne, two species with different morphometrics

The effects of different body shapes on size selectivity were analysed. Relationships between total length and fork length, height, width and girth were estimated and the selectivity parameters of a 44 mm PE diamond mesh codend determined for common pandora (Pagellus erythrinus) and axillary sea bream (Pagellus acarne). Two data sets were collected during demersal trawl trials carried out in Izmir Bay (Aegean Sea) between May 1996 and February 1997. Regression analyses were used to determine the relationships between total length and other measured dimensions of the species. However, selectivity parameters were estimated from pooled data by using the logistic equation with the maximum likelihood method. Fork length, height, width and girth were found to increase linearly with total length for both species (R² > 0.90, except TL–W for axillary sea bream, 0.76). Ratios of average body thickness (W/H) were 0.45 (±0.002) for common pandora and 0.52 (±0.002) for axillary sea bream. L₅₀ and SR values were found as 12.4 (±0.44) and 2.2 (±0.51) from the eight valid hauls for common pandora and as 13.6 (±0.13) and 1.9 (±0.26) from the three valid hauls for axillary sea bream, respectively. The difference between the size selectivity of the two species in the same family can be explained by the body shape and fish behaviour distinctions.     

Alternative codends to improve the size selectivity for nylon shrimp (Heterocarpus reedi) and yellow squat lobster (Cervimunida johni) off the Chilean coast

The size selectivity of alternative codends for nylon shrimp and yellow squat lobster was studied off the Chilean coast using the covered codend technique. Various mesh sizes (47 and 63 mm mesh opening) and shapes (diamond and square) in the codends were used. For both species, substituting the DM47 codend with the DM63 codend significantly increased 50% retention length (l₅₀). The change of diamond (DM47) to square mesh (SM47) had a positive effect in size selectivity for nylon shrimp. For yellow squat lobster, no significant effect was observed when changing the diamond (DM63) to square mesh (SM63). Selectivity data were modeled by considering explanatory variables such as ‘codend catch weight’ and ‘vessel’. Increases in the catch significantly reduced the l₅₀ for both species whereas the vessel factor was not significant. Finally, using the length at first maturity as a reference value, the authors concluded that the DM47 codend failed to protect the immature portion of either stock, thus the DM63 codend is recommended.     

Length–weight relationships of 13 fish species from the Lijiang River,China

The length–weight relationships (LWRs) of 13 species belong to three orders and four families from the Lijiang River, China. Samples were obtained between 2013 April and 2015 January using gillnets (25 × 12 m, mesh size 0.5 cm), trawl nets (50 × 12 m, mesh size 0.5 cm) and fishhooks (0.5 × 0.5 × 10 m, mesh size 0.5 cm mesh). Exponent b varied from 2.639 to 3.549 and r² values varied from .962 to .995.