Square-mesh Codend Research Articles

Assessing T45 vs T0 and T90 meshes and covered codend effects in southeastern Australian penaeid trawls

Square-mesh codends (i.e. turned 45 degrees or ‘T45′) made from a minimum stretched mesh opening (SMO) of 27-mm mesh hung on the bar are mandated for estuarine-trawl fisheries targeting penaeids (mostly school prawns, Metapenaeus macleayi) in New South Wales, Australia. However, owing to variable mesh shrinkage over time, often slightly larger SMOs are supplied, and sometime too large to be oriented as T45 for the targeted school prawns (>13–15 cm carapace length; CL). During three experiments involving paired trawls with and without codend covers, we sought to investigate if, compared to a conventional 27-mm T45 codend, lateral openings in 30-mm SMO could be appropriately maintained by orientating diamond-shaped (‘T0′) meshes at narrow (experiment 1) or normal (experiment 2) circumferences, or turned 90 degrees (‘T90′; experiment 3). Analyses of size selection was done after first testing for species-specific cover effects, which required fish sizes to be assessed via a paired-hauls approach while sizes for school prawns were modelled using a covered-codend approach. Compared to the 27-mm T45 codend, only the T0 mesh hung at a normal circumference provided the same size selection for school prawns, with the two other alternative configurations producing wider selection. But, while incidental fish catches were low (14% of total catch), all new codends caught between ∼2 and 7 × more individuals of some small fish cross all sizes, including juveniles of an economically important species (tailor, Pomatomus saltarix). The data reiterate that among the codends tested, T45 mesh is the optimal configuration for regional penaeid trawls in terms of consistently maximining school prawn size selection and the escape of small fish, and so high-quality material in the required sizes needs to be sourced.

Effect of Codend Design and Mesh Size on the Size Selectivity and Exploitation Pattern of Three Commercial Fish in Stow Net Fishery of the Yellow Sea, China

To address the issue of minimum mesh size regulations of the stow net fishery for croaker species, we tested and compared the size selectivity and exploitation pattern for diamond- and square-mesh codends with mesh sizes 35, 45 and 55 mm for little yellow croaker (Larimichthys polyactis), silver croaker (Pennahia argentata), and flower croaker (Nibea albiflora) in the Yellow Sea, China. Our results showed that the legal codend (35 mm diamond-mesh) was inadequate to protect the juvenile croaker species because more than 75% of undersized individuals were retained, and the discard ratios were as high as approximately 60%. Irrespective of mesh shape, increasing the codend mesh sizes could significantly improve the size selectivity and exploitation pattern of croaker species. Between codends with the same mesh size, the square-mesh codends had higher size selectivity than diamond-mesh codends. Based on results, the 55 mm square-mesh codend was recommended for rational exploitation of croaker stocks. This study can provide feasibility and insight for the enforcement and reform of management strategies for sustainable fishing in Chinese stow net fisheries.

The capability of square-meshes and fixed-shape meshes to control codend size selection

Trawl codends are commonly made of diamond-mesh netting. However, diamond-mesh codends vary in mesh geometry along its length and during fishing due to catch build up. This introduces variability in the size selection process. This phenomenon compromises the rationality of regulating exploitation patterns in trawl fisheries through adjustments in codend mesh size. One technical solution often applied to achieve more well-defined size selection is turning the codend netting 45 degrees (square-mesh). However, there is a lack of evidence that square-mesh codends result in more constant size selectivity. Therefore, we aimed at quantifying the variability in size selection in square-mesh codends. We tested the size selectivity of three codends; a standard square-mesh codend, and two rigid codends where mesh geometries were fixed in diamond shape with an opening angle of 60° and square shape, respectively. The two rigid codends were used to establish baselines with limited variability in size selection. The size selectivity of these codends was compared to results - previously obtained for a standard diamond-mesh codend. Using Atlantic cod (Gadus morhua) as a case study, we demonstrated that the standard square-mesh codend had significantly larger variability in size selection compared to the fixed diamond-mesh codend. Moreover, we found no evidence that the standard square-mesh codend had lower variability in size selection than a standard diamond-mesh codend with same mesh size. These results demonstrate that the use of standard square-mesh codends is not sufficient to reduce variability in codend size selection. Additionally, we demonstrate that the sizes of fish retained is strongly dependent on mesh shape and openness. We conclude that a profound re-thinking over codend designs is required in order to achieve better control of size selection in trawl fisheries.

An overview of bottom trawl selectivity in the Mediterranean Sea

In the Mediterranean Sea, where bottom trawling for demersal species is the most important fishery in terms of landings, around 75% of the assessed fish stocks are overfished. Its status as one of the world’s most heavily exploited seas and the one subject to the highest trawling pressure has become a global concern. An extensive review of bottom trawl selectivity studies was performed to assess the sustainability of this fishery in the Mediterranean; the selectivity parameters were collected from 93 peer-reviewed publications of 10 countries, totalling 742 records and 65 species. The review highlighted that i) the catch of bottom trawls commonly employed in the Mediterranean, even complying with current regulations on codend meshes, still includes immature individuals for 64-68% of the species investigated, and individuals under the minimum conservation reference size (MCRS) for 78% of the species investigated, and that ii) the MCRS set for 59% of the species analyzed is well below their length at first maturity, and is therefore ecologically inadequate. Although square-mesh codends are slightly more selective, the models developed herein demonstrate that improving size and species selectivity would require considerably larger meshes, which may significantly reduce profitability. The urgent need to reduce the biological impacts of bottom trawling in the Mediterranean should be addressed by promoting the adoption of more ecologically sustainable fishing gears through the introduction of more selective meshes or gear modifications.

Surface selection of haddock and cod in the Norwegian demersal seine fisheries

Abstract Size selection in active fishing gears is a continuous process and undersized fish may escape during the whole fishing operation. Fish that escape during the surface hauling operation are likely to experience higher mortality due to barotrauma-related stress than fish escaping at the fishing depth during the towing process. A well-functioning selectivity device should therefore select mostly at depth for enhancing survival probabilities of escaping fish. The current gear regulation in the Norwegian demersal seine fishery is likely to cause large proportion of undersized fish to escape at the surface. In this study, we estimated surface selection of haddock and cod in demersal seine by using an automatic release system and a small meshed codend that collected fish escaping during surface hauling. The collecting bag contained 19% undersized haddock compared to 10% in the conventional square-mesh codend indicating that about 50% of undersized haddock brought to the surface were released. The proportions of undersized cod were 8% for the collecting bag and 1% for the conventional square-mesh codend. These results demonstrate that surface selection is significant for both haddock and cod. Based on this finding, we discuss methods to improve size selectivity at the fishing depth.

Comparing the size selectivity of a novel T90 mesh codend to two conventional codends in the northern shrimp (Pandalus borealis) trawl fishery

The size selectivity and usability of three codends were quantified and compared for the first time in the inshore Northern shrimp (Pandalus borealis) trawl fishery of Iceland using the covered codend method: a conventional diamond-mesh codend (T0), conventional square-mesh codend (T45), and a 90° turned mesh codend (T90) constructed of four panels and with shortened lastridge ropes. Fishers, wanting to increase the average-individual size of captured shrimp, had requested the T90 codend to be compared with conventional codends for consideration in the fishery. Results showed that, on average, the T45 and T90 codends had better size selectivity than the T0 codend in terms of releasing individuals smaller than 13 mm carapace length (Minimum References Size; MRS). The T90 codend retained significantly less Northern shrimps between 9 and 19 mm than the T0 codend and between 15 and 19 mm than the T45 codend. No significant difference of size selectivity between T45 and T0 codends was observed. All three codends presented high retention ratios of Northern shrimps above MRS (>63%) for the population encountered. However, the T0 codend was not effective at sorting out small Northern shrimps; at least 86% of Northern shrimps smaller than 13 mm were retained in the T0 codend if encountered. Catches from T45 and T90 codends had a lower proportion of shrimp below MRS. Since discarding of undersized Northern shrimps is prohibited in Iceland and fishers wanted to catch on average larger shrimp, using the novel T90 codend would enable fishers to use their quotas more efficiently.

Karadeniz’de Mezgit (Merlangius merlangus euxinus) avcılığındaki trol torba seçiciliklerinin karşılaştırılması

A large part of the total whiting (Merlangius merlangus euxinus) fishing in the Black Sea is carried out with the traditional bottom trawl nets along the coast of Turkey. Diamond mesh shape with 40 mm mesh size (40D) has been used in the codends of these trawls. In this study, the traditional trawl codend (40D) and square-mesh codend with different size (36S and 40S) were compared for whiting size selectivity in August 2014. Selectivity data were collected by using a covered codend method and analysed taking between haul variations in to account. The selectivity parameters were estimated by using CC2000 software. Results showed that commercially used 40D trawl codend is not selective enough to release immature of whiting. However, the net change in square mesh (36S and 40S) instead of a diamond-shaped (40D) mesh significantly improved the mesh selectivity for whiting. The 40S trawl codend is even higher than the first maturity size (LM50) with a length at fifty percent retention (L50) of 15.74 cm. In conclusion all square mesh codends with different mesh size tested here supplied appropriate selection considering minimum landing size (MLS) of whiting.

Predictive models for codend size selectivity for four commercially important species in the Mediterranean bottom trawl fishery in spring and summer: Effects of codend type and catch size.

Models to predict codend size selectivity for four major commercial species—European hake (Merluccius merluccius), Norway lobster (Nephrops norvegicus), deep-water rose shrimp (Parapenaeus longirostris), and Atlantic horse mackerel (Trachurus trachurus)–in Mediterranean bottom trawl fisheries were established based on data collected during fishing trials using the two legal codends: a 40 mm square-mesh codend and a 50 mm diamond-mesh codend. The models were applied to predict the extent to which size selection depend on codend type, also accounting for the potential effect of codend catch size and fishing season. The size selectivity of the two codends was evaluated and compared in identical simulated controlled conditions. Mesh type significantly affected the size selection of Norway lobster alone, with a slightly better performance of the 40 mm square-mesh codend. A high risk of retention of undersized individuals was predicted for both codends for all species except Norway lobster.

Effects of square-mesh codend on the benthic by-catch for the beam trawl fisheries of veined rapa whelk, Rapana venosa

Background. The beam trawl fisheries of veined rapa whelk, Rapana venosa (Valenciennes, 1846), cause small benthic organisms to be captured and also damage the other species living on the seafloor. To solve this problem, a proper modification that minimise the by-catch should be designed and put into practice. This study aims to compare 72 mm commercial diamond-mesh codend with 72 mm square-mesh codend to reduce the benthic by-catch in this fishery. Materials and methods. Totally 15 hauls were carried out in south-eastern Black Sea Region within 10–14 August 2017. A commercial diamond-mesh and square-mesh beam trawls were towed simultaneously behind a commercial fishing vessel to compare the by-catch compositions. Paired t-test was used to compare the catch amount of two codends based on species. GLMM (Generalised Linear Mixed Models) was used to compare the length of captured veined rapa whelk, mussel, and crab species. Results. There was a 77-percentage-point reduction in the mean number of the captured individuals as by-catch in square-mesh codend when compared with commercial diamond-mesh codend and this difference was found statistically significant (P < 0.05). A total of 17 species was captured as by-catch in both codends. Comparing to diamond-mesh codend, the catch amount of the 13 by-catch species had decreased in square-mesh codend. Conclusion. In conclusion, the square-mesh codend was found successful in decreasing the by-catch amount of many benthic species except for flatfish species and large crabs. In addition to this, the veined rapa whelk amount fell by half and this means a 25-percentage-point profit decrease for fishermen.

Improving codend selectivity in the fishery of the deep-sea red shrimp <em>Aristeus antennatus</em> in the northwestern Mediterranean Sea

The deep-sea red shrimp Aristeus antennatus is one of the most valuable demersal resources in Catalonia (northeast Spain), and Palamós is the most important harbour for this fishery in the area. Here, a management plan published in 2013 established the use of a 40-mm square-mesh codend (40s), replacing the previously used 50-mm diamond mesh codend (50d). The objective was to decrease the amount of juveniles in the catches, but the regulation did not bring the expected results. In this study, we measured and compared the selectivity of the 40s and the 50d in this fishery. In addition, we experimented with the use of a 50-mm square-mesh codend (50s). According to our results, the 40s had a lower 50% selection length (L50) than the 50d, while the 50s had a substantially higher L50 than the other two. A transition analysis showed an increase in yield per recruit after the second year from a hypothetical implementation of the 50s. Our conclusion is that the 40s does not have a higher selectivity than the 50d, which (at least partially) explains the failure to reach the management objective in Palamós. Conversely, the use of a 50s would significantly benefit the fishery, increasing gear selectivity and yield per recruit.

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

ABSTRACTThe 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.

Effect of Codend Circumference on the Size Selection of Square-Mesh Codends in Trawl Fisheries.

It is well established that increasing mesh number in the circumference of a diamond-mesh trawl codend can reduce size selection for round fish, whereas selection for flat fish species is unaffected. This effect has also been documented in Mediterranean trawl fisheries. In contrast, no information is available with regard to the effect of increasing mesh number in the circumference of square-mesh codends on the size selection of round fish and flat fish species. A field study was devised to bridge this gap and formulate proposals aimed at improving trawl fishery management. Size selection data were collected for a round fish species, red mullet (Mullus barbatus), and two flat fish species, Mediterranean scaldfish (Arnoglossus laterna) and solenette (Buglossidium luteum). Fishing trials were conducted in the Adriatic Sea (Central Mediterranean) using three square-mesh codends that differed only in mesh number around the circumference. Results demonstrated that increasing the number of meshes from 107 to 213 reduced the 50% retention length (L50) for red mullet by 2.5 cm but did not affect size selection for the two flat fish species. In some fisheries, regulatory provisions regarding the number of meshes in the circumference should therefore be carefully considered both for diamond- and square-mesh codends.

An assessment of bycatch reduction devices in Kuwait's shrimp trawl fishery

Summary Three bycatch reduction devices (BRDs), i.e. the Turtle Excluder Device (TED), Square Mesh Codend (SMC) and Fisheye (FE), were tested to evaluate their feasibility in the Kuwait shrimp-trawl fishery. All three BRDs could be fitted to a shrimp trawl and normal fishing activities could be conducted both on an industrial trawler and an artisanal dhow boat. The comparisons of the catch from a net equipped with a BRD on one side and a standard net (exact same net configuration but without a BRD device) on the other side of a double-rigged shrimp trawler (steel boat) showed different results in the three BRD types. The net fitted with a TED performed well without capturing any turtles or large animals, and did not significantly reduce shrimp catch or fish bycatch (124.4: 117.0 and 852.0: 905.8, both P > 0.05). Although the SMC reduced both shrimp catch and bycatch in general, the Wilcoxon's signed rank test showed a significant difference only among bycatch (1034.2: 1147.3, P < 0.05). Chi-square tests showed that a net fitted with SMC caught significantly larger shrimp (more large and fewer small sizes) than a standard net for all three major shrimp species (33.5: 32.5; 26.9: 25.0; 20.3: 19.3, all P < 0.01). This indicates that the SMC reduced the catch of small size shrimps as well as smaller size fish. A net fitted with a fisheye significantly reduced both the shrimp catch and bycatch (137.4: 174.6; 997.5: 1377.8, both P < 0.01); the shrimp loss was probably due to an inappropriate location of the FE in the trawl.

Is square-mesh better selective than larger mesh? A perspective on the management for Mediterranean trawl fisheries

Relatively little scientific work has been done to assess the selectivity of square-mesh codends in the highly variable multi-species conditions in the Italian trawl fisheries. Therefore this study was initiated to investigate the effect of using square-mesh and larger diamond-mesh codends on size selectivity of deepwater red shrimp (Aristaeomorpha foliacea) and red mullet (Mullus barbatus), with possible implication in future management measures. Four different codends were used in the sea trials. When size selection estimates are applied for management issues it is not sufficient only to consider the mean size selection parameters. It also needs to consider the effect of between-haul variations in the selection process. In the current study, potential consequences of the between-haul variation on the selection curves have been considered by applying a pooled curve with a double bootstrap approach. The results attained in the present study indicate that the use of 40mm square-mesh codend results in a 50% retention length (L50) similar to that of the 50mm diamond-mesh codend, but with a steeper selection range (SR). For red mullet, both 50mm diamond-mesh and 40mm square-mesh codends lead to an L50 that is higher than the minimum landing size (MLS, 11cm). Deepwater red shrimp is not subject to any MLS and may always be landed legally. However, large amounts of juveniles of deepwater red shrimp have been retained in all codends. Council Regulation (EC) No. 1967/2006 called for a discard reduction policy in waters under the jurisdiction of the European Union. As demonstrated in the current paper, to simultaneously improve the size selectivity of fish and shrimp species can be difficult due to large differences in their morphological characteristics, therefore it may question whether more sophisticated alternative of selective devices, such as grids or square-mesh panels, could be implemented in some Mediterranean fisheries.

Assessment of relative performance of a square-mesh codend design across multiple vessels in a demersal trawl fishery

Implementing gear-based solutions to reduce bycatch in trawl fisheries can be difficult owing to fishers’ concerns over the fleet-wide applicability of results from discrete, highly controlled experiments associated with the development of such innovations. In New South Wales, Australia, a codend comprising a posterior section made from 35-mm square-shaped mesh (i.e. hung on the bar) was developed and subsequently recommended for partial implementation in a multi-species demersal trawl fishery (i.e. for use when eastern king prawn –Melicertus plebejus – is the primary target species). However, industry concerns over this design have to date prevented its implementation. Here, using direct twin-tow comparisons onboard a subset of the fleet, we quantitatively assessed the performance of the square-mesh codend, relative to the range of diamond-mesh codend designs currently being used, in terms of reductions in bycatch rates and shortfalls in catch rates of retained animals. Generalised linear mixed-effects modelling across all vessels showed that when eastern king prawn was the target species, the square-mesh codend significantly reduced bycatch rates (by ∼48%), but with an accompanying significant shortfall in catch rates of eastern king prawn (∼7%) and a substantial (though not statistically significant) shortfall in catch rates of an important component of the retained byproduct (whiting, Sillago spp., ∼36%). Partitioning of data by vessel revealed that such shortfalls in retained catch were evident for only some vessels, while the square-mesh codend performed as intended for the others. When school prawn (Metapenaeus macleayi) was the target species, substantial shortfalls in catches of school prawn (∼54%) in the 35-mm square-mesh codend confirmed that a codend of a smaller square mesh might be a more appropriate design under those circumstances. Potential strategies to address intra-vessel inconsistencies in performance of the square-mesh codend and thereby facilitate implementation of square-mesh codends in the fishery are discussed.

Relative efficiency of square-mesh codends in an artisanal fishery in southern Brazil

The relative efficiency of two square-mesh codend designs with the same circumference (~2 m) but different mesh sizes and materials (32 mm polyethylene-PE and 30 mm polyamide-PA) was tested in an artisanal shrimp trawl fishery in Parana, southern Brazil. The two square-mesh codends were hauled alternately with a 26 mm diamond-shaped mesh codend (control) in a twin gear configuration. Although not significant, the mean numbers of total bycatch were reduced by 16.6 and 10.0% with the 32 and 30 mm square-mesh codends, respectively. The results indicate significant improvement in size selectivity for some species (Xiphopenaeus kroyeri and Stellifer rastrifer). The operational changes tested can be a suitable technical solution for reducing the capture of immature organisms in the shrimp trawl fishery in Parana, Brazil.

Performance of square-mesh codends in reducing discards and by-catch in the Shark Bay scallop fishery

The performance of square-mesh codends (as opposed to the conventional diamond mesh) was examined as a discard and by-catch reduction device for the Shark Bay saucer-scallop trawl fishery. Of the three different-sized square-mesh codends (50, 55, 60 mm) tested against the standard 100-mm diamond-mesh codend, the 50-mm square-mesh codend performed poorly with relatively high retention of small (&lt;85 mm shell height) scallops, whereas the 55- and 60-mm square-mesh codends retained 22–33% less smaller scallops than did the diamond-mesh codend. Overall, a mean of 5% loss in commercial-sized scallops across all three square-mesh codends and significant by-catch reductions of up to 95% were achieved compared with the diamond mesh. Catch rate of prawns by the square-mesh codend was less than 2% of that of the standard prawn net. The performance of the 60-mm square-mesh codend presents a good basis for its use in commercial trials in the Shark Bay scallop trawl fishery. A move to square-mesh codends could result in a significant reduction of small scallops and by-catch and may potentially increase the catches of commercial-sized scallops as a result in improvements in water flow and net efficiency.

Selectivity and escapement behaviour of five commercial fishery species in standard square- and diamond-mesh codends

Abstract Frandsen, R. P., Madsen, N., and Krag, L. A. 2010. Selectivity and escapement behaviour of five commercial fishery species in standard square- and diamond-mesh codends. – ICES Journal of Marine Science, 67: 1721–1731. The Danish fishery for Nephrops (Nephrops norvegicus) is often conducted in a mixed-species setting, characterized by high rates of discards of several target species, including Nephrops and cod (Gadus morhua). Experiments were conducted to investigate and compare the selective properties of a standard 70 mm square-mesh codend (standard SMC) and a standard 90 mm diamond-mesh codend (standard DMC). Selectivity estimates for five commercial species are provided for both codends. The standard SMC yielded higher estimates of length at 50% retention/mesh size (hereafter SF) for Nephrops and roundfish than did the standard DMC, but no effect of codend type on SF was found for plaice (Pleuronectes platessa). Moreover, a novel codend cover design allowed assessment of the preferred direction of escapement in the codend. Whiting (Merlangius merlangus) and Nephrops showed pronounced, but opposite, vertical preference in the direction of escapement, with whiting escaping upwards and Nephrops downwards. A significant (p &lt; 0.05) difference in the direction of escapement between the two codends was found for haddock (Melanogrammus aeglefinus) and whiting. Owing to the relatively small catches, the outcome is probably most applicable to Nephrops-directed fisheries under similar conditions, and caution should be taken not to extrapolate the results to other fisheries.

The effect of mesh configuration and codend circumference on selectivity in the Mediterranean trawl Nephrops fishery

The Pomo pit area is both the main Adriatic nursery area for European hake ( Merluccius merluccius) and Norway lobster ( Nephrops norvegicus) and an important trawling ground for the Adriatic trawl fleet. The current paper is not only restricted to the escapement of Norway lobster but also takes note of the associated white fish community (European hake, blue whiting Micromesistius poutassou and poor-cod Trisopterus minutus capelanus) which makes an important contribution to the commercial catch of trawlers. This study was undertaken to analyse the effect on trawl selectivity of inserting first a 40 mm square-mesh codend and second of increasing the circumference in the diamond-mesh codend. A traditional 40 mm diamond-mesh codend, typical of the commercial trawl fisheries, was selected as a reference codend for the comparative analysis. The square-mesh codend was always more selective than the diamond-mesh codends, while the influence of codend circumference on diamond-mesh selectivity was less evident. In addition to mesh configuration and the number of meshes around the codend, the selectivity data were modelled by estimating the individual contribution of catch size and season, but there was no evidence of a coherent effect on the selectivity of these two parameters. This study confirms that all the four investigated species make the best use of the square-mesh opening, either because of their body shape or because the form of the mesh facilitates forcing of the net. Results seem to suggest that for the main target species, Norway lobster, square-mesh codend would protect specimens under the 20 mm of MLS (carapace length). Square-mesh also improves the L50 of European hake, but it seems not to be sufficient to avoid the catch of specimens under MLS of 20 cm as fixed by the EC Reg. Nr. 1967/2006. In conclusion, enforcement of installation of square-mesh codends in Mediterranean trawl Nephrops fishing could contribute to decreasing the capture of individuals from the particular nursery area of Pomo pit.

Diamond- vs. square-mesh codend selectivity in southeastern Australian estuarine squid trawls

This study was predicated by a recent management proposal to mandate square-mesh codends in an Australian estuarine squid trawl fishery, based on their improvements to the selectivity of penaeid trawls deployed across comparable temporal and spatial scales. The assumption of no impacts of a codend cover on the performance of a squid trawl was validated before it was used to quantify the size and species selection of a conventional codend made from 41-mm diamond-shaped mesh and two new designs made from 29- and 32-mm mesh hung on the bar (i.e. square shaped). Compared to the 41-diamond codend, both square-mesh designs incrementally and significantly reduced the catches of a small legally retained teleost (whitebait Hyperlophus vittatus), but had no significant effect on discarded catches (termed ‘bycatch’). The 32-square codend also significantly increased the sizes at 50% probability of retention for the targeted squid (Luminous Bay squid Loliolus noctiluca and broad squid Photololigo etheridgei), across narrow selection ranges and, because all sizes are retained, consequently reduced their total landed catches. The results support the utility of square-mesh for consistently improving and maintaining lateral openings in codends, but the management of this fishery means that such configurations would result in economic loss, and for the 29- and 32-square codends, no concomitant reduction in bycatch. This study reiterates the need for bycatch issues to be clearly identified and defined on a fishery-specific basis, and prior to the implementation of mitigation measures.