Improving release efficiency of cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) in the Barents Sea demersal trawl fishery by stimulating escape behaviour

Abstract

Abstract:We tested the ability of stimulators to improve the release efficiency of cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) through the meshes of a square mesh section installed in a trawl. The section was tested in three different configurations: without any stimulation device, with a mechanical stimulation device, and with LED light stimulation devices. We analysed and compared the behaviour of cod and haddock in all three configurations based on release results and underwater recordings. Parallel to the fishing trials, we carried out fall-through tests to determine the upper physical size limits for cod and haddock to be able to escape through the square meshes in the section. This enabled us to infer whether lack of release efficiency was due to fish behaviour or release potential of the square meshes in the section. The results showed that the escape behaviour of haddock can be triggered by mechanical stimulation. In contrast, cod did not react significantly to the presence of mechanical stimulators. LED light stimulation had some effect on the behaviour of haddock, but not on cod.Résumé : Nous avons vérifié la capacité de stimulateurs d'améliorer l'efficacité de la libération de morues (Gadus morhua) et d'aiglefins (Melanogrammus aeglefinus) par les mailles d'une section de mailles carrées installée dans un chalut. La section a été mise à l'essai selon trois configurations différentes, soit sans aucun instrument de stimulation, avec un instrument de stimulation mécanique et avec des instruments de stimulation à lumière DEL. Nous avons analysé et comparé le comportement des morues et des aiglefins dans les trois configurations à la lumière des résultats de libération et d'enregistrements sous l'eau. Parallèlement aux essais de pêche, nous avons mené des essais de passage par gravité afin de déterminer les limites supérieures des tailles auxquelles les morues et aiglefins peuvent s'évader par les mailles carrées dans la section. Cela nous a permis d'établir si l'absence d'une libération efficace était due au comportement des poissons ou au potentiel de libération des mailles carrées dans la section. Les résultats montrent que le comportement d'évasion des aiglefins peut être déclenché par une stimulation mécanique. Les morues, au contraire, n'ont pas présenté de réaction significative à la présence de stimulateurs mécaniques. La stimulation à la lumière DEL a eu un certain effet sur le comportement des aiglefins, mais pas sur celui des morues. [Traduit par la Rédaction]

Combination of a sorting grid and a square mesh panel to optimize size selection in the North-East Arctic cod (Gadus morhua) and redfish (Sebastes spp.) trawl fisheries

“…This downward swimming behavior of cod is well documented in earlier studies (e.g. Engås and Godø, 1989;Wardle, 1993;Grimaldo et al, 2017) and was observed for 80.3 % (95% CI: 70.4-88.7 %) of the 71 cod observed entering the section. Compared with cod, redfish entered the section relatively evenly distributed, a behavior also documented in the literature (e.g.…”

“…This high contact value is in line with results for the double steel grid system presented by Larsen et al (2016), which showed that redfish were effective at contacting the upper grid of the section tested. This indicates, that compared with cod, which have been reported multiple times to seek outlets in a mainly downwards direction (Engås and Godø, 1989;Wardle, 1993;Grimaldo et al, 2017), redfish…”

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

“…When white LED lights were installed on the SMP or in the panel right below the SMP, the release efficiency through the SMP was not significantly improved for either hake or blue whiting. Grimaldo et al (2017) tested whether green LED lights could improve the release efficiency of cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) through a square mesh section. No significant differences were found for cod, whereas the results for haddock only indicated an effect for the smaller individuals.…”

“…Stimulators are designed to trigger fish escape behaviour, but the results obtained so far have shown varying degrees of success (e.g. Glass and Wardle 1995, Herrmann et al 2014, Grimaldo et al 2017). Mechanical stimulators have been shown to reduce the retention rate of some juvenile fish species (e.g.…”

Can a square-mesh panel inserted in front of the cod end improve size and species selectivity in Mediterranean trawl fisheries?

“…The poor contact probability is likely due to the excessive distance of the panel from the codline ), a part of the net where fish have no reason to change direction and swim up through the panel meshes. This could be changed by placing stimulation devices in the vicinity of the square-mesh panel to alter swimming patterns, forcing fish to interact with the panel meshes (Brewer et al 1998;Eayrs 2007;Glass et al 1995;Herrmann et al 2015;Krag et al 2016;Grimaldo et al 2017). In this study a square-mesh panel inserted in front of the trawl failed to improve size and species selection; as a result, the present findings are to all effects negative.…”