Abstract
This study investigated the size selectivity and catch efficiency in the North-east Atlantic demersal trawl fishery targeting cod (Gadus morhua) and haddock (Melanogrammus aeglefinus). The use of a sorting grid followed by a size selective codend is compulsory in this fishery. Experimental fishing was conducted with a commercially rigged trawl, flexigrid section, and diamond-mesh codend. The flexigrid section is a dual sorting system and all fish that escaped from the two grids in the flexigrid system and the codend were retained in separate covers. This allowed us to estimate the combined size selectivity of the system, as well as the separate contributions from the first grid, second grid, and codend. The catch efficiencies for cod and haddock below and above the minimum reference length (MRL) were quantified by estimating the values for a set of exploitation pattern indicators. The results showed that most fish escaped from the flexigrid section. Only a few fish below the MRL entered the codend and nearly all of them subsequently escaped through the codend meshes. The probability of retaining undersized cod and haddock was low with the combined system. Furthermore, the results showed that the probability of escape was high through the second grid in the system for fish above the MRL, and this finding was supported by the exploitation pattern indicators. In particular, the estimated exploitation pattern indicators showed that using the current MRL and the fish size distributions encountered during the trials, no haddock and only 2.3 % of cod below the MRL were retained by the gear. However, 77.4 % of the haddock and 16.0 % of the cod above the MRL were found to escape, thereby indicating poor catch efficiency, especially for haddock. These results demonstrate the importance of supplementing size selectivity research with estimates of the actual catch efficiency using the gear employed, which were quantified using exploitation pattern indicators in this study.
Highlights
Several types of size and species selective processes take place during trawling
Considering the issues mentioned above, the present study investigated the performance of a gear setup comprising a flexigrid and legal diamond mesh codend in order to address the following research questions: i) What are the individual contributions of the flexigrid and legal diamond mesh codend to the overall size selectivity of the gear setup used in the North-east Atlantic gadoid fishery? ii) What is the catch efficiency of fish above and below the minimum reference length (MRL) for the compulsory size selective sorting device used in this fishery? iii) Is there any difference in the size selective performance of a new flexigrid section with the intended grid angles of 25° and a well-used section with lower grid angles?
For the first time, the present study determined the individual contributions and combined size selectivity of the grids and codend in the compulsory sorting system comprising a flexigrid and diamond mesh codend used in the North-east Atlantic cod and haddock fishery
Summary
Several types of size and species selective processes take place during trawling Some of these are unintended, e.g., escape under the fishing line (Ingólfsson and Jørgensen, 2006; Brinkhof et al, 2017a,b), whereas others aim to manipulate the species (Engås and Godø, 1989; Krag et al, 2010) and size (Sistiaga et al, 2010, 2016) composition in the codend catch, e.g., various sorting grids and codend mesh sizes. Unlike Sort-V, which comprises a single steel grid and offers a single escape opportunity for fish, the flexigrid comprises two flexible grids and it enables a dual sequential size selection process (Fig. 1a).
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