Improving the size- and species selectivity of cod (Gadus morhua) in demersal mixed-species trawl fisheries

Research output: Book/ReportPh.D. thesisResearch

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For the last decade, the Kattegat-Skagerrak and the North Sea cod stocks have been at a critically low level. Several management initiatives were introduced to protect and aid the recovery of these cod stocks. Most fisheries in these areas are conducted in a multispecies setting, where several different species, including cod, are caught together. Demersal trawling is the predominant fishing method in Denmark, as measured by both catch value and volume. Demersal trawls also account for the highest discard rates of juvenile fish, including cod. The focus of this work was on improving the selectivity of demersal trawling with regard to cod. This Ph.D. thesis consists of a review and four supporting papers (1-4). Two main techniques are used to aid fish in escaping from fishing gear. The first technique involves a process of mechanical sorting based on fish size and the second is based on the use of species-specific behaviour patterns. Paper 1 describes an experiment in which square mesh panels (SMPs) were used to study what effects the SMP have, the position of the SMPs in the extension and codend, of the mesh size of the SMP, and of the general increase in the codend mesh size. The goal of this study was to improve the size selection for cod in the fisheries directed towards Nephrops. Paper 2 describes a new methodology, tools, and software that have been developed and used to measure the morphological parameters that determine the ability of cod to penetrate different mesh types, sizes, and openings. The results show that the morphology-based simulations of size selectivity of cod can be used to explain a large part of both the within-haul and the between-haul variations previously reported from sea trials. The method can further predict the selection parameters (L50 and SR) for cod for different mesh types and sizes. Paper 3 describes separation of cod from haddock based on differences in behaviour between the two species. The design of the gear used in this study is based on the tendency of cod to stay close to the seabed and of haddock to rise as they enter the trawl; the gear, therefore, was designed to keep the fishing line raised above the seabed to avoid catching cod. The majority of the economically important species in the Danish demersal fisheries, including cod, all enter the trawl low and close to the seabed, thus behavioural-based separation of cod and the other species is not possible at the mouth of the trawl. Paper 4 describes an experiment in which behaviour was studied in the extension piece of a demersal trawl equipped with a vertical separator frame. A second vertical separator frame with raising bars was used to determine if the observed vertical distribution could be stimulated with the aim of separating species. The morphology and behaviour of cod and other commercially important species in relation to fishing gear and the numerous different fisheries and gear designs in which cod are caught make it very difficult to develop a universal design that can substantially reduce the catch of cod without simultaneously reduce the catch of the target species. To optimise the trade-off between discard and loss of marketable catch, solutions have to be specific to particular fisheries or populations of fish. Papers 1 and 3 provide specific examples of how size- and species selection, respectively, has been improved for cod. Paper 4 demonstrates that the vertical behaviour of cod and other species can change along the longitudinal axis of a trawl and that these vertical preferences can be stimulated by simple means. The new methodology described in Paper 2 offers new possibilities for studying and predicting size selection in a systematic way based on the morphology of fish and the shape of the mesh with which the fish is presented. New gear designs can thereby be optimised theoretically before being subjected to expensive and time-consuming sea trials. Papers 1, 2, and 4 have been published in scientific journals and Paper 3 has been submitted to Fisheries Research. This review will take a broader perspective and will examine the capturing process, which is the basis for the selection process. Moreover, it discusses the existing methods and knowledge in the fields of size selection and species selection relevant to the demersal trawl fisheries in the Kattegat-Skagerrak and the North Sea where cod is caught.
Original languageEnglish
Place of PublicationCharlottenlund, Denmark
PublisherTechnical University of Denmark
Number of pages47
Publication statusPublished - 2009

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