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.

中文翻译：

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双分选网格和菱形网编码底拖网在东北大西洋鲆渔业中的尺寸选择性和捕捞效率

摘要 本研究调查了东北大西洋底层拖网渔业以鳕鱼 (Gadus morhua) 和黑线鳕 (Melanogrammus aeglefinus) 为目标的大小选择性和捕捞效率。在该渔业中，必须使用分拣网格，然后是尺寸选择性编码。实验捕鱼是用商业装备的拖网、flexigrid 部分和菱形网编码器进行的。flexigrid 部分是一个双分拣系统，所有从 flexigrid 系统中的两个网格和代码中逃脱的鱼都被保留在单独的盖子中。这使我们能够估计系统的组合大小选择性，以及来自第一个网格、第二个网格和编码器的单独贡献。低于和高于最小参考长度 (MRL) 的鳕鱼和黑线鳕的捕捞效率通过估计一组开发模式指标的值进行量化。结果表明，大多数鱼从柔性网格部分逃逸。只有低于 MRL 的几条鱼进入了密码器，几乎所有的鱼随后都通过了密码器网格逃脱。组合系统保留小鳕鱼和黑线鳕的可能性很低。此外，结果表明，高于 MRL 的鱼类通过系统中的第二个网格逃逸的可能性很高，这一发现得到了开发模式指标的支持。特别是，估计的开发模式指标表明，使用当前 MRL 和试验期间遇到的鱼大小分布，没有黑线鳕，只有 2。渔具保留了低于 MRL 的 3% 鳕鱼。然而，发现超过 MRL 的 77.4% 的黑线鳕和 16.0% 的鳕鱼逃逸，从而表明捕捞效率低下，尤其是黑线鳕。这些结果证明了使用所用网具的实际捕捞效率估计来补充大小选择性研究的重要性，在本研究中使用开发模式指标对其进行量化。