Mesh Codend Research Articles

A general catch comparison method for multi-gear trials: application to a quad-rig trawling fishery for Nephrops

Expeditious uptake of quad-rig trawling in the economically important Irish fishery for Nephrops norvegicus outpaced technical understanding of this newly introduced gear. The main driver for its introduction is increased catch rates of Nephrops. Higher Nephrops discard rates associated with quad-rig trawling are likely to be problematic under the landing obligation unless size selectivity can be improved. Catch comparison methods are suitable for assessing the performance of fishing gear modifications to reduce discards. Utilizing a quad-rig potentially increases the number of gears that can be included in a catch comparison study to four but current modeling methods are limited to two gears. Our study provides a new general multinomial mixed effects modeling framework that can be applied to two or more gears, elucidates how case-specific and choice-specific covariates may influence catch at length, and facilitates discussion on appropriate gear based management measures. Application of the method to catches from four different cod-end mesh sizes revealed significant effects of carapace length, catch weight and net position, on the numbers of Nephrops retained in each cod-end. Results suggest that management measures which specifically address different catch profiles associated with different numbers of trawling rigs are required to optimize Nephrops size selectivity.

Mortality of non-target flatfishes escaping from demersal trawl codends

Mortality of non-target flatfishes that had escaped from various sizes (40, 44 and 50 mm) of codend meshes was determined by capturing and holding the escapees in cages for 7 days. Although three flatfish species were studied, sufficient data for detailed analyses were obtained only for Buglossidium luteum (Risso, 1810) (solenette). Trawl hauls (48 in total) with 15 min towing durations were conducted in the summer and winter seasons of 2011 and 2012. A tailored bottom trawl with 800 meshes around the mouth was used with covers to capture fish. Covers were used as an observation cage, and were fixed to the sea bottom at depths of 3.0–8.2 m. The monitoring period was 7 days. It was estimated that average escape mortality of B. luteum for all test cages (96.5%) in summer was significantly higher (p < .01) than that in winter (56.5%). Mesh size and shape was not a significant factor (p > .01), with the exception of the 2011 winter; however, the mortality rate was inversely related to fish length in most cases (p < .01). High mortalities (~100%) were observed for Arnoglossus laterna (Walbaum, 1792) (Mediterranean scaldfish) and Citharus linguatula (Linnaeus, 1758) (spotted flounder) in all hauls in both summer and winter.

Impact of codend mesh sizes on selectivity and retention of Acadian redfish Sebastes fasciatus in the Gulf of Maine trawl fishery

A trouser trawl was used to determine the size selectivity of three sizes of mesh opening (114, 140 and 165mm double 5mm twine diamond) on a commercial fishing vessel fishing off Provincetown, Massachusetts, USA. Fifty-six tows were completed in March and April 2013, catching over 42,000kg of Acadian redfish (Sebastes fasciatus) and about 6000kg of other species. Robust models for the mean L50s and selection ranges, and confidence intervals, were developed for all three tested codends, incorporating both within and between haul variability. L50 and selection ranges were determined for the nominal 114mm (L50:22.3cm; SR:3.3cm), 139mm (L50:29.2cm; SR:4.4cm), and 165mm (L50:33.6cm; SR:5.0cm) codends. All measures of model validity were positive. These models are fully adequate to provide guidance to managers and fishermen on size retention of redfish and appropriate codend mesh size. Additionally, simulation of fishing of the three tested codends on the observed population indicated that substantial escape of redfish through codend meshes occurs (51–96%), suggesting that investigation of escape of redfish is warranted to support a sustainable fishery. The observed population also indicates that inadequate numbers of larger redfish may be available to support a higher-priced market.

Restoring the ecosystem creates wealth. The case of the Northern coast of Tunisia’s deep-water rose shrimp trawl fishery

The demersal trawl fishery of the north Tunisian coast primarily targets the deep-water rose shrimp, Parapenaeus longirostris, and secondarily a variety of demersal fish species. These fishes include hake (Merluccius merluccius), common pandora (Pagellus erythrinus), red mullet (Mullus barbatus), surmullet (Mullus surmuletus), Atlantic horse mackerel (Trachurus trachurus), bogue (Boops boops), picarel (Spicara smaris) and spotted flounder (Citharus linguatula). A bioeconomic model was used to test management measures through scenarios that ran over eleven years to estimate the viability of the fishery according to biological and economic results. The most beneficial scenario was the combination of several management measures including a temporal closure of two months, the replacement of the 40mm diamond mesh codend with 40mm square mesh, the removal of both the biological recovery tax and of fuel subsidies, and an 83% reduction in fishing capacity. This results in an annual private profit higher by 9.3M TND (Tunisian Dinar) (6.9M USD) that of 2008, and an economic rent that was higher by 13.3M TND (9.9M USD) than by the end of the projected period. Shrimp and fish biomasses have doubled compared with 2008, and trawling damages would be reduced to the equivalent of a five- fold reduction in fishing effort. This study shows that the objectives of improving demersal ecosystems and improving individual and collective wealth can be achieved through the synergistic effects of various regulatory measures.

Population dynamics of Philocheras trispinosus (Crangonidae) with abundance of other caridean shrimps (Crustacea: Decapoda) caught by beam trawl in the Southern Black Sea, Turkey

Objective: To determine growth and reproduction biology of Philocheras trispinosus (P. trispinosus) with abundance of other caridean shrimps caught by beam trawl in the Southern Black Sea. Methods: Samplings were carried out monthly with a beam trawl of 2 m length and 15 mm cod-end mesh size between December 2012 and November 2013. All individuals were sampled between 1 and 40+ m water depth from 146 hauls. Results: A total of 550 P. trispinosus, 618 Palaemon adspersus, 12 Palaemon serratus, 11 Philocheras fasciatus and 10 Crangon crangon were sampled during the study period. The crangonid shrimps started appearing during the winter and spring and the number of individuals increased to reach their highest value in mid-winter and mid-spring. The seasonal von Bertalanffy growth parameters for P. trispinosus were estimated as L∞ = 29.3 mm total length, K = 0.860/year, t0 = –0.900 year, C = 0.180, and ts = 0.010. The start of the slow growth period was at the beginning of July (WP = 0.510). Ovigerous females appeared in the sampling area between January and April. Size at sexual maturity was estimated as 25.69 mm total length. Conclusions: The results supported the distribution and abundance of caridean shrimp species from the study area and the population dynamics of the most abundant shrimp species, P. trispinosus. The current study can be answered as baseline data prior to management strategies to ensure sustainable conservation of the shrimp species.

An Analysis of Innovation Decision Process among the Mechanized Fishing Boat Operators

The study was conducted among the large mechanised wooden trawl boat operators involved in marine fishing in three selected major fishing harbours of the country viz ., Visakhapatnam in Andhra Pradesh, Thoothukudi in Tamil Nadu and Veraval in Gujarat. The socio-economic and psychological characteristics of fishermen were assessed. The overall innovation decision efficiency index (IDEI) score was 55.25%. The findings on centre-wise and technology-wise IDEI scores are also discussed. The respondents from the three fishing centres differed significantly (p<0.05) in their innovation decision behaviour pertaining to adoption of the technologies viz ., FRP sheathing for fishing vessels, marine anticorrosive painting, marine antifouling painting, turtle excluder device (TED), square mesh codend and use of adequate ice onboard. The regression coefficients of four variables viz., investment on fishing craft and gears, innovativeness, training undergone and extent of linkage with research and extension systems significantly and positively influenced the innovation decision process (p<0.05). The R2 value was found to be 0.838, indicating that the 17 characteristics taken together accounted to 83.80% of variations in the innovation decision efficiency level. The constraints in the innovation adoption decision process pertaining to the identified technologies were also documented.

Bell-shaped size selection in a bottom trawl: A case study for Nephrops directed fishery with reduced catches of cod.

Monotonous size selection curves have traditionally been sufficient to describe the size selection in the aft end of a bottom trawl. Such modelling is a good approximation when the size selective system consists of a single selective device. However, in some fisheries the demands for species and size selectivity have motivated the development of selective systems in trawl fisheries that utilize more than one selective device simultaneously. An example can be found in the Swedish demersal trawl fishery targeting Norway lobster (Nephrops norvegicus), which simultaneously aims at avoiding catches of Atlantic cod (Gadhus morhua). In this fishery, the selective system consists of a Nordmøre type sorting grid followed by a size selective square mesh codend. The size selection curve for this system has a characteristic bell-shaped curvature, which cannot be sufficiently described by a monotonous selection curve. An approach that can handle a bell shaped curvature is to use a more flexible empirical size selection model. However, such models primarily use a curve fitting procedure, and do not allow the possibility to investigate the contribution of the individual parts of the selection system. Therefore, we choose to use a structural-based model that directly models the contributions of the individual selectivity devices to the overall performance of the system. We demonstrate that this approach can appropriately describe the experimental size selection data for both Nephrops and cod in a system composed of a sorting grid followed by a size selective codend. Furthermore, this approach provides a direct quantification of the selective processes of the individual parts of the system to the overall size selection in the fishing gear. In addition, we demonstrate how this approach can provide fisheries managers with a new tool when trying to develop more sustainable fisheries through improving fishing gear size and species selectivity.

여수와 진도 연안 낭장망에 어획된 어류 혼획률 월 변화 및 미성어 비율

We examined monthly changes in the rate of fishes by-caught by gape net with wings and their immature ratio in the coast of Yeosu and Jindo Island, Korea. A minus correlation between bycatch rate, the ratio of fishes except for anchovy, Engraulis japonicus, to all fishes collected by gape net with wings, and individuals of the collected anchovy was significantly observed, implying that as the anchovy catch decreased and the bycatch rate increased. Immature ratios by the dominant bycatch species during the study is the following; Leiognathus nuchalis was 72.7~99.0%, Sphyraena pinguis was 84.0%, Sardinella zunasi was 90.0%, others (Leptocephalus, Trichiurus lepturus, Ammodytes personatus, Sphyraena pinguis, Trachurus japonicas, Mugil cephalus and Erisphex pottii) were 100.0%. In order to decrease the high bycatch rates of immature fishes in spring and autumn, our study suggested increasing of codend mesh size and developing suitable bycatch reduction devices in a gape net with wings.

Selectivity of commercial and experimental codends for the demersal trawl fishery of the deep-water rose shrimp, Parapenaeus longirostris (Lucas, 1846), in the Aegean Sea

In this study, the selectivity of commercial and alternative (experimental) codends for the deep-water rose shrimp, Parapenaeus longirostris, in the Aegean Sea demersal trawl fishery was investigated. Tested were the 44-mm diamond mesh codend as used in commercial fisheries (D44), in addition to a 50 mm diamond (D50), a 40 mm square (S40), and a 40 mm 90° turned mesh (T40), respectively. Sea trials were carried out in the eastern Aegean Sea, with a local commercial trawler using a modified trawl net with 620 meshes circular mouth opening. A total of 35 valid daytime hauls (8 with D44, 9 with D50, 8 with S40 and 10 with T40) were carried out. Selectivity data were collected using the covered codend method and analysed by means of a logistic equation with the maximum-likelihood method. Mean selection curves were estimated and compared using the between-haul variations model. The mean 50% retention carapace length values (LC50) were estimated as 21.2 ± 0.1, 23.2 ± 0.3, 20.8 ± 0.2 and 20.5 ± 0.4 mm CL for D44, D50, S40, and T40, respectively. Selection range (SR) values were determined as 4.9 ± 0.1 mm in D44, 8.7 ± 0.3 mm in D50, 4.0 ± 0.3 mm in S40, and 10.1 ± 0.3 mm in T40. While haul duration and total catch among the parameters did not affect the LC50 values for the investigated codends, the value of the parameter describing the weight of P. longirostris caught in that codend showed an effect for the D50 and T40 codends. Consequently, all investigated codends yielded sufficient LC50 results when considering the minimum landing size (MLS) value imposed by the EU (20 mm CL). In conclusion, the legally allowed S40, which gave an LC50 value close to that of D44, and which would be the preferable codend in terms of its releasing characteristics for individuals under the MLS value (89%), can be used as a good alternative for the D44 (preferred by the fishermen) in the deep-water rose shrimp fishery.

Questioning the effectiveness of technical measures implemented by the Basque bottom otter trawl fleet: Implications under the EU landing obligation

The selective properties of a bottom trawl fitted with a 70mm diamond mesh codend and a 100mm top square mesh panel (SMP) for hake (Merluccius merluccius), pouting (Trisopterus luscus and Trisopterus minutus) and red mullet (Mullus surmuletus) were investigated over the period 2011–2013. The experiments were carried out over three separate cruises aboard two commercial Basque bottom otter trawlers in the Bay of Biscay area. “Fall-through” experiments were also undertaken to estimate the potential size selection of 100mm square mesh for the same species. Results from the “Fall-through” experiments and the at-sea selectivity cruises demonstrated that a 100mm SMP has the potential to enable undersized and immature individuals to escape through the meshes. However, the selectivity cruises demonstrated that in practice, the SMP was largely ineffective at releasing undersized individuals as only a small fraction of the fish entering the trawl attempted to escape through the SMP during their drift towards the codend. The fraction attempting to escape was quantified by the “SMP contact probability” and was less than 4% for hake and red mullet and less than 15% for pouting. Furthermore, for each species, the release potential for the diamond mesh codend was found to be significantly lower than the length-at-maturity and the legal minimum conservation reference size. On average, the proportions of the total catch of undersized individuals of each species retained by the gear, were 52%, 17% and 45% for hake, pouting and red mullet respectively. Based on our findings, we conclude that the gear currently deployed by the Basque bottom otter trawl fleet operating in the Bay of Biscay is largely ineffective at releasing undersized hake, pouting and red mullet. The introduction of the obligation to land all catches, under the 2013 reform of the EU Common Fisheries policy will create new challenges for the Basque bottom otter trawl fleet and thereby an incentive to improve selectivity to avoid unwanted catches of undersized individuals.

Could a T90 mesh codend improve selectivity in the Belgian beam trawl fishery?

In an effort to improve the selectivity of beam trawls in the Belgian flatfish fishery, codend size selectivity of three flatfish and two roundfish species were compared testing 80mm diamond-shaped mesh (T0) versus 80mm mesh turned 90° (T90) using the covered codend method. Eight models were considered to describe the size selective performance of the T0 and T90 codends for each species; four traditional models, along with four models that considered contact probability (the fraction of fish that will not make proper contact with the codend meshes for size-dependent escape). Fifteen hauls were completed in the North Sea, fishing both codends simultaneously. The results demonstrated that the T90 codend significantly increased size selectivity for the two roundfish species investigated, whiting (Merlangius merlangus) and pouting (Trisopterus luscus), and significantly decreased size selectivity of the three flatfish species, sole (Solea solea), plaice (Pleuronectes platessa), and dab (Limanda limanda). Selection range (SR) values were similar for both roundfish species with each codend, but were significantly reduced for all flatfish species by the T90 codend. Models that considered contact probability provided the best model for four of the species with T90 (pouting, sole, plaice, and dab), and one species with T0 (sole). Lack of adequate contact with the T90 meshes could be a result of the combined effects of increased flow through the T90 codend due to T90 meshes remaining more open than T0 meshes during towing, and the relative high towing speeds of beam trawls. This study concludes that although a T90 codend reduces the size selectivity of flatfish, the lower SR of the T90 codend could be paired with a larger T90 codend mesh size for improved flatfish selectivity, and that a T90 mesh codend increases the size selectivity for commonly encountered roundfish in the Belgian beam trawl fishery.

Behavior of different trawl codend concepts

The overall objective of this study was to evaluate the behavior of different codend designs to provide basic information that is relevant for implementing technical management measures, improving selectivity and catch quality, and reducing energy consumption. Six different codend designs were evaluated: a traditional diamond mesh codend; a T90 codend (meshes turned 90°); a Bacoma codend with diamond meshes in the lower panel and square meshes in the upper panel; a Bacoma codend with the square meshes orientated in the T0 direction; a two panel square mesh codend; and a four panel square mesh codend. The codends were tested in a flume tank with flow of 1.8knots/h. A simulated catch of 500kg was placed in the tested codend. A motion tracking system was used with four track markers placed on each of three successive cross-sections and a single marker at the end of the codend. This made it possible to assess and compare the movements of the codend and the netting in three dimensions. The drag of the codends also was measured and compared.

Understanding the size selectivity of red mullet (Mullus barbatus) in Mediterranean trawl codends: A study based on fish morphology

Red mullet (Mullus barbatus) is one of the most important commercial species for the multi-species Mediterranean demersal trawl fisheries. Several experimental studies have been carried out to assess the codend size selectivity for red mullet in the last two decades. However, while the number of experiment-based selectivity studies on red mullet are considerable, very little theoretical work exists which tries to understand and explain the size selectivity for this species in trawl codends. Therefore the main objective of this study was to establish a theoretical framework to explain and predict the size selection of red mullet in bottom trawl codends of different mesh size and configuration. We identified the important morphological characteristics of red mullet which determine the size selection in towed fishing gears using the FISHSELECT methodology and further used this methodology to predict size selection for a wide range of mesh configurations through simulations. For example we predicted the 50% retention length for red mullet to be in the range 9.2–15.8cm for the legal 50mm mesh size diamond mesh codend and in the range 10.6–14.2cm for the legal 40mm mesh size square mesh codend. The simulated size selection was found to agree well with previously published results for codend size selection of red mullet obtained from sea trials. Thus based on fish morphology we were able to understand and explain previous results leading to a better understanding of the codend size selection for this species. Further the morphologically based results enable predictions of size selection of red mullet in codend designs not previously tested experimentally.

Survival of trawl-caught Norway lobster (Nephrops norvegicus L.) after capture and release—Potential effect of codend mesh type on survival

Survival estimates were obtained for the Norway lobster, Nephrops norvegicus, retained and escaping from a standard 70mm mesh size diamond and a modified 55mm square mesh codend, on board a commercial vessel in fishing grounds off the Portuguese south coast. Ten hauls were carried out, five with each experimental codend, plus three creel sets for the capture of a control group in an adjacent non-trawled area. A total of 571 lobsters were sampled upon arrival on vessel deck, either captured in the codend or retained in the codend cover. In addition, a total of 25 individuals were caught with creels. They were assessed for physical damage and vitality and subsequently placed in cages which were deployed in the same adjacent area for 48h. Average survival rates were 0.18 and 0.17 for retained individuals and 0.17 and 0.30, for individuals escaping through diamond and square meshes, respectively, and 0.84 for creeled individuals. A discussion is carried out stressing the difficulty in disentangling the influence of the different factors contributing to condition and mortality of individual lobsters.

Selective characteristics of a shark-excluding grid device in a Mediterranean trawl

Galeus melastomus (blackmouth catshark) is often caught as bycatch in demersal trawls in the Mediterranean. In order to reduce bycatches of shark we tested an excluder grid with 90mm bar spacing during experimental fishing in the Tyrrhenian Sea (Western Mediterranean). Data collected made it possible to simultaneously evaluate catch losses of two commercial species: Nephrops norvegicus (Norway lobster) and Phycis blennoides (greater forkbeard). The escape outlet ahead of the grid and the codend were both mounted with a cover in order to collect escaped fish ahead of the grid and through the codend meshes. We used a structural model to estimate the contribution of the individual selective processes consisting of the excluder grid and the size selective codend. The 90mm excluder grid did not prove to be efficient in excluding G. melastomus, while it excuded more of P. blennoides. Catches of N. norvegicus were also affected by the presence of the grid, but not as much as the catches of other two species. The results obtained for the experimental grid+codend setup were then compared with the estimated selectivity for the “codend alone” setup. Furthermore, by way of explorative simulation with other grid bar spacing, we concluded that reducing the grid bar spacing to 70mm would provide better compromise between the reduction of G. melastomus as bycatch and the catch rate of P. blennoides and N. norvegicus.

Selectivity and retention of pollock Pollachius virens in a Gulf of Maine trawl fishery

Measuring the size selectivity of pollock, or saithe, Pollachius virens, has been challenging due to the patchy, elusive nature of the species. By using data collected opportunistically during a redfish Sebastes fasciatus selectivity study, selectivity of three sizes of mesh opening (114, 140 and 165mm double 5mm twine diamond) for pollock was determined using a commercial fishing vessel fishing off Provincetown, Massachusetts, USA. Fifty-six tows were completed in March and April 2013; 21 included sufficient catches of pollock to estimate size selectivity. Robust, simple models for the mean L50s (50% selection length) and SR (selection ranges), and confidence intervals, were developed for all three tested codends, incorporating both within and between haul variability. Selection parameters and selection ranges were determined for codends with nominal mesh sizes of 114mm (L50: 34.8cm; SR: 2.4cm), 139mm (L50: 45.6cm; SR: 3.1cm), and 165mm (L50: 52.4cm; SR: 3.6cm). All measures of model validity were positive, indicating robust and reliable findings that can be used to provide guidance to fishery managers, stock assessment scientists, and fishermen on size-dependent retention of pollock by codend mesh size.

Predicting size selection of cod (Gadus morhua) in square mesh codends for demersal seining: A simulation-based approach

Demersal seining is an important fishing method to harvest cod (Gadus morhua) in Norwegian fisheries. Knowledge about size selectivity of cod in this type of fishing gear is therefore of importance for managing the exploitation of cod resources. However, limited data exist on the size selection of cod in the square mesh codends mostly applied in this fishery. By using knowledge of fish morphology and the computer-based simulation method FISHSELECT, we investigated the potential for size selection of cod in square mesh codends for demersal seining. We were able to explain and understand existing experimental selectivity results and predict the effect of design changes in the codend. The results showed that the currently applied codend designs are adequate to ensure low catches of cod below the minimum size for this fishery, but they also indicated that a considerable part of the size selection may occur through slack meshes. Thus, it is likely that part of the codend mesh selection may occur when the gear is at the surface.

Bottom trawl selectivity parameters of four fish species from Karataş coasts of Turkey in Northeastern Mediterranean Waters

The bottom trawl selectivity parameters (22mm cod-end mesh size) were determined for Saurida undosquamis , Mullus barbatus , Upeneus pori , and Pagellus erythrinus along Karatas coasts of Turkey in northeastern Mediterranean waters during fishing season for the year 2002-2003. The lengths at first capture (L 50 ) values for S. undosquamis , M. barbatus , U. pori and P. erythrinus were 12.57, 9.27, 10.20 and 7.20cm, respectively. When lengths at first maturity of these species were taken into consideration, it was concluded that bottom trawl fisheries carried out in breeding areas creates a high fishing pressure (overfishing) on all species and enlarging the mesh size of the net is not enough to prevent the high fishing pressure on these species.

Analysis of body size selectivity of by-catch using the cover net method for a shrimp beam trawl

Selectivity and by-catch are the main features used to define fish catch for fisheries management or to determine fishing gear efficiency. A fish girth distribution analysis was carried out to determine the retention rate of juveniles discarded and to establish common selectivity for a multispecies catch. Total body length, body weight, and girth of juvenile fish retained in the cod-end and cover net separately were collected using an 18-mm mesh cover net during 12 fishing trials with a 41-mm cod-end beam trawl. The by-catch weight ratio in the cod-end was twice that of the by-catch ratio in number. The 50% selectivity of body length and 50% girth of redfin velvet fish were not significantly different between the cod-end and cover net, whereas those of other fish species were significantly different between the cod-end and cover net. The difference in 50% selectivity girth of other fish between the cod-end and cover net was similar between cod-end mesh size and cover net mesh size. Furthermore, the difference in 50% body length selectivity of other fish between the cod-end and cover net was almost double the difference in girth. Girth selectivity in a multispecies catch using towed fishing gear was effectively used to determine fish and net mesh size.

Size selection in codends made of thin-twined Dyneema netting compared to standard codends: A case study with cod, plaice and flounder

In an experimental trawl fishery, diamond mesh codends made out of 2.5mm flexible Dyneema twine were tested. The principle aim was to investigate the effect of the number of meshes in the codend circumference, the number of twines (single or double), and netting orientation (T0 or T90) on the size selection of cod (Gadus morhua), plaice (Pleuronectes platessa) and flounder (Platichthys flesus). In addition, the obtained size selectivity for the codends made of Dyneema netting were compared to results obtained for T90 codends made of standard 5mm single twine PE netting, and to previous results for other PE codends used in the same fishery. It was observed that the selective performance of the Dyneema netting codends was very high compared to other T0 and T90 codends of the same mesh size and number of meshes in the codends circumference. This demonstrates the high selective potential of the thin and flexible Dyneema netting compared to nettings which are traditionally used in the construction of trawl codends. Furthermore, reducing the number of meshes in the codend circumference for the Dyneema codends significantly increased the size selection of cod but did not affect the size selection of plaice and flounder. This difference between the species is likely linked to their different morphologies. No differences in the size selection of the T0 and T90 designs made of Dyneema twine tested were observed for any of the three species. For cod, the effect of using single or double twine in the Dyneema codends was also tested, however, no significant differences were found.