Gear modifications for bycatch reduction in the Bay of Biscay demersal trawl fishery

Bottom trawl fisheries have great social and economic importance for coastal communities in the Basque Country. However, the activity of the demersal trawl fishery in the Bay of Biscay, which includes the coastline of the Basque country, can be compromised due to its multispecies nature, high proportion of unwanted species in the catch, and the increasingly strict legislation implemented aiming to ensure sustainable fisheries. In this scenario, developing gear modifications to reduce unwanted bycatch is increasingly important. This thesis presents results from six selectivity research papers that address unwanted catch issues in the Bay of Biscay bottom trawl fishery. Hake (Merluccius merluccius), horse mackerel (Trachurus trachurus), blue whiting (Micromesistius poutassou), and mackerel (Scomber scombrus) are some of the most relevant species for the bottom trawl fishery in the Bay of Biscay and constitute the main choke species in the fishery. Papers I and II present selectivity results for different square mesh panel (SMP) designs aimed to improve fish-SMP contact probability and release efficiency for these species. Among the designs tested, the results demonstrate that modifying SMP size and position can increase fish contact probability with the SMP. Paper III investigates the size selection process through SMP and codend meshes for blue whiting based on fish morphology and behavior. The results demonstrate that SMP size selection can be explained by different fish contact angles with SMP meshes, which allows making accurate predictions for fish size selectivity. Paper IV explores the effect of alternative SMP and codend mesh combinations on the size selectivity of hake and blue whiting and on the fishery exploitation pattern for a variety of fish population scenarios. The results demonstrate that changes both in SMP and, especially, codend designs can have a significant effect on the size selectivity and exploitation patterns of hake and blue whiting. This paper also outlines new ways for investigating and illustrating the effect of multiple gear changes on the size selectivity and exploitation pattern indicators by means of diagrams named treatment trees. These may aid in the identification of promising gear designs and help the industry in the pursuit of specific catch goals. In Paper V a trawl configuration for species separation is tested. This new configuration intends to guide those species that hold themselves close to the lower panel of the trawl through a horizontal grid into a lower codend, while the rest of the species are directed to an upper codend. The findings in Paper V demonstrate that, under the conditions in which this fishery operates, the trawl configuration tested is not able to efficiently separate species based on their behavior. Finally, in Paper VI the effect of shortening codend lastridge ropes on codend size selectivity compared to a standard codend is tested, and fish escape chances estimated based on fish morphology. The results show that a codend with shortened lastridge ropes can improve the size selectivity of horse mackerel and blue whiting, while the selectivity of hake was not affected. The results indicate species-dependent variability in the ability to utilize open meshes located at different places. In general, the work presented in this thesis provides technological advances and knowledge that contributes with guidance on how to reduce unwanted bycatch and generate alternative exploitation patterns in the Bay of Biscay demersal trawl fishery

Escape of hake (Merluccius merluccius), horse mackerel (Trachurus trachurus) and blue whiting (Micromesistius poutassou) in codends with shortened lastridge ropes

Diamond meshes in trawl codends have limited openness, which reduces escape opportunities for roundfish. Shortening the lastridge ropes (LR) attached to codend selvedges can increase the availability of open meshes resulting in higher chances of escape. However, this availability does not imply optimal mesh openness, nor does it guarantee use. We estimate the escape probability of hake, horse mackerel and blue whiting through a 20% shortened LR codend and a standard codend, and quantify the contribution of different mesh opening angles (OAs) to their size selectivity. The results confirm that high OAs increase escape opportunities for all species. However, shortened LR only improved size selectivity significantly for horse mackerel and blue whiting. This difference between species may be related to behavioural differences. The mesh openness achieved with 20% shortened LR was below that necessary to obtain optimal escape opportunities for these species. The study highlights the relevance of considering fish morphology and behaviour to optimally exploit size selectivity when designing shortened LR codends.publishedVersio

Release efficiency and selectivity of four different square mesh panel configurations in the Basque mixed bottom trawl fishery

Reduction of discards from the Basque mixed bottom trawl fishery is a challenge. To improve the selective properties of the gear used by the fleet and supplement codend size selection, a square mesh panel (SMP) installed in the upper panel of the trawl was introduced in 2006. However, recent studies have shown that the release efficiency of this SMP is low due to lack of contact between the fish and the SMP. In this study, we tested the release efficiency of the SMP for four different gear configurations. We tested the effect of adding LED lights at two different positions and altering panel size and panel position in the trawl. The analyses were focused on two species: hake (Merluccius merluccius) and blue whiting (Micromesistius poutassou). The results showed that the position of LED lights did not significantly affect the SMP’s release efficiency for any species. However, increasing panel size had a significant positive effect on the release efficiency of blue whiting, and placing the SMP in the lower panel improved the release efficiency of hake. These results highlight the challenge of simultaneously improving the selective properties of gear for species with different behaviour, especially in mixed demersal fisheries.acceptedVersio

Stimulating release of undersized fish through a square mesh panel in the Basque otter trawl fishery

Discards of regulated species in the Basque mixed trawl fishery are a challenge. In 2006, a square mesh panel (SMP) was introduced in the fishery to increase the release efficiency of undersized fish. However, studies have shown that the selectivity in this fishery is based on codend selectivity and the release through the SMP is inefficient due to low contact between fish and the SMP. In order to improve contact, we tested four different gear configurations that use different stimulators to lead fish to the panel: without stimulation, with stimulation based on ropes, with stimulation based on ropes and floats, and with stimulation based on LED lights. The experiment was carried out on three of the potential choke species for the fishery: hake (Merluccius merluccius), horse mackerel (Trachurus trachurus), and blue whiting (Micromesistius poutassou). The results showed that stimulators did not significantly improve the release efficiency of hake and horse mackerel through the panel. For blue whiting, stimulation with floats had a significant positive effect on release efficiency, whereas LED light-based stimulation had the opposite effect. In general, the contribution of the SMP to the overall release efficiency of the selective system (SMP + codend) was low. Underwater recordings confirmed that the stimulators generally were not able to lead fish towards the SMP.Stimulating release of undersized fish through a square mesh panel in the Basque otter trawl fisherysubmittedVersio

Anthropic Impact Effect on the Distribution of Stramonita haemastoma (Linnaeus 1766) (Gastropoda: Muricidae) In the North Coast of Tenerife

Las zonas costeras han sido históricamente un lugar de asentamiento humano y de aprovechamiento de recursos por su elevada productividad. En las islas Canarias concretamente, el marisqueo de la «púrpura» o «cañadilla» (Stramonita haemastoma) ha sido uno de los principales motivos de explotación del intermareal isleño. El objetivo del presente estudio fue analizar el efecto del impacto antrópico sobre la distribución, abundancia y tamaño de este molusco. Para ello, se muestrearon dos zonas del norte de Tenerife, la Punta del Hidalgo y Finca El Apio, lugares muy homogéneos en sus características físicas, pero con una diferencia importante, su cercanía al núcleo urbano. Los resultados muestran diferencias en la abundancia y la talla de Stramonita haemastoma entre las dos localidades, encontrando mayor número de individuos, así como individuos de mayor tamaño, en la localidad menos impactada por el ser humano. Del mismo modo, se muestra un cambio en la distribución natural de esta especie. Este estudio podría ser una herramienta para la gestión adecuada y sostenible de este recurso, a través del conocimiento de su estado actual.Historically coastal areas have been a human settlement place and exploitation area for its resources and its high productivity. In the Canary Islands, one of the main reasons to exploit the islands intertidal zone has been the shellfishing of “purpura o cañadilla” (Stramonita haemastoma). The main objetive of this study was to analyze the human impact’s effect on the distribution, abundance and size of this mollusk. To do so, we sampled two locations at the North of Tenerife, with similar characteristics, la Punta del Hidalgo and Finca El Apio. The results show differences on the abundance and size of Stramonita haemastoma among localities, being more abundant and larger at the less human-impacted locality. In addition, there are changes in the natural distribution of this species. This study could be useful tool for sustainable management actions of this resource throughout the knowledge of its actual status

Is there a limit to the potential effects of shortening lastridge ropes on the size selectivity of diamond mesh codends?

Shortening codend lastridge ropes can be an effective fishing gear modification to improve the size selection properties in diamond mesh codends. Lastridge ropes attached to codend selvedges withstand the longitudinal forces created by the catch building up and therefore, prevent the codend meshes from closing. However, the extent to which the lastridge ropes should be shortened to maximize the effect of this measure is unclear. Besides opening codend meshes, shortening lastridge ropes can also lead to net folding, which can potentially have negative consequences for size selectivity. In the present study, we tested the size selective properties of a 129 mm diamond mesh codend in three different configurations: 0 %, 15 % and 30 % shortened lastridge ropes. Selectivity data were collected for cod (Gadus morhua), haddock (Melanogrammus aeglefinus) and redfish (Sebastes spp.) in the Barents Sea gadoid bottom trawl fishery. Shortening the lastridge ropes by 15 % had a significant effect on the release efficiency of haddock between 35 and 50 cm, whereas to obtain a similar result for cod, the lastridge ropes had to be shortened by 30 %. However, the use of shortened lastridge ropes significantly increased the retention of fish below 35 cm for both species, especially when the lastridge ropes were shortened from 15 % to 30 %. The effect on redfish size selectivity was in general limited. Exploitation pattern indicators showed that there was no added benefit from shortening them further from 15 % to 30 % for any of the three species. This study concludes that, while shortening lastridge ropes can contribute to improved size selection of diamond mesh codends, reducing them beyond 15 % is not recommended because it can substantially increase the retention of undersized fish, probably due to net folding.publishedVersio

Optimizing size selectivity and catch patterns for hake (Merluccius merluccius) and blue whiting (Micromesistius poutassou) by combining square mesh panel and codend designs

Gear modifications in fisheries are usually implemented to obtain catch patterns that meet management objectives. In the Basque bottom trawl fishery, gear regulations include the use of a square mesh panel (SMP) placed at the top panel of the extension piece of the trawl to supplement diamond mesh codend selectivity. However, the catch patterns obtained with this combination have raised concern among scientists and authorities. This study combines new data on different SMP and codend designs with existing data from the literature to produce new results that are applied to predict the size selectivity and catch patterns of different gear combinations for a variety of fishing scenarios. A systematic approach based on the concept of treatment trees was outlined and applied to depict the effect of individual and combined gear design changes on size selectivity and catch patterns for hake (Merluccius merluccius) and blue whiting (Micromesistius poutassou). This approach led to identification of the gear combination with the most appropriate exploitation pattern for these two species and improved the readability and interpretation of selectivity results. The results demonstrated that changes both in SMP and, especially, codend designs have a significant effect on hake and blue whiting size selectivity and catch patterns. Therefore, we believe that further research should prioritize codend size selectivity, and additional selection devices may be added once codend designs with good selective properties are achieved

Optimizing size selectivity and catch patterns for hake (Merluccius merluccius) and blue whiting (Micromesistius poutassou) by combining square mesh panel and codend designs

Gear modifications in fisheries are usually implemented to obtain catch patterns that meet management objectives. In the Basque bottom trawl fishery, gear regulations include the use of a square mesh panel (SMP) placed at the top panel of the extension piece of the trawl to supplement diamond mesh codend selectivity. However, the catch patterns obtained with this combination have raised concern among scientists and authorities. This study combines new data on different SMP and codend designs with existing data from the literature to produce new results that are applied to predict the size selectivity and catch patterns of different gear combinations for a variety of fishing scenarios. A systematic approach based on the concept of treatment trees was outlined and applied to depict the effect of individual and combined gear design changes on size selectivity and catch patterns for hake (Merluccius merluccius) and blue whiting (Micromesistius poutassou). This approach led to identification of the gear combination with the most appropriate exploitation pattern for these two species and improved the readability and interpretation of selectivity results. The results demonstrated that changes both in SMP and, especially, codend designs have a significant effect on hake and blue whiting size selectivity and catch patterns. Therefore, we believe that further research should prioritize codend size selectivity, and additional selection devices may be added once codend designs with good selective properties are achieved

Optimizing size selectivity and catch patterns for hake (Merluccius merluccius) and blue whiting (Micromesistius poutassou) by combining square mesh panel and codend designs

Gear modifications in fisheries are usually implemented to obtain catch patterns that meet management objectives. In the Basque bottom trawl fishery, gear regulations include the use of a square mesh panel (SMP) placed at the top panel of the extension piece of the trawl to supplement diamond mesh codend selectivity. However, the catch patterns obtained with this combination have raised concern among scientists and authorities. This study combines new data on different SMP and codend designs with existing data from the literature to produce new results that are applied to predict the size selectivity and catch patterns of different gear combinations for a variety of fishing scenarios. A systematic approach based on the concept of treatment trees was outlined and applied to depict the effect of individual and combined gear design changes on size selectivity and catch patterns for hake (Merluccius merluccius) and blue whiting (Micromesistius poutassou). This approach led to identification of the gear combination with the most appropriate exploitation pattern for these two species and improved the readability and interpretation of selectivity results. The results demonstrated that changes both in SMP and, especially, codend designs have a significant effect on hake and blue whiting size selectivity and catch patterns. Therefore, we believe that further research should prioritize codend size selectivity, and additional selection devices may be added once codend designs with good selective properties are achieved.publishedVersio

Prediction of square mesh panel and codend size selectivity of blue whiting based on fish morphology

Square mesh panels (SMPs) are selective devices used extensively to supplement codend size selectivity in trawl fisheries. Therefore, predictions of the effect of mesh size in both SMPs and codends on size selectivity are valuable. Here, we established a framework to predict the size selection of blue whiting through different SMPs and diamond mesh codends based on the morphological characteristics of this species. We hypothesized that size selection for an SMP is determined by different fish contact angles, whereas different mesh opening angles determine size selection for the codend. Based on these hypotheses, we first developed a model that enabled us to predict which sizes of blue whiting are able to pass through meshes of different sizes and shapes. We then tested whether the selectivity for blue whiting of the SMP, the codend, and the combination of both could be explained by the models. Finally, we predicted the size selectivity of multiple combinations of SMPs and diamond mesh codends. The method presented here can potentially be applied to make predictions for species other than blue whiting