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

• Main Author: Krag, Ludvig Ahm
• Format: Book
• Language: English
• Published: Technical University of Denmark 2009
• Subjects: /dk/atira/pure/sustainabledevelopmentgoals/life_below_water; SDG 14 - Life Below Water; Kattegat; Gadus morhua
• Online Access: https://orbit.dtu.dk/en/publications/64a840a3-7ba9-4e3d-bd19-4e85121d467a; https://backend.orbit.dtu.dk/ws/files/3781639/PhD%20thesis%20Ludvig%20Ahm%20Krag.pdf

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 ...