Abstract Small mesh codends applied in the brown shrimp (Crangon crangon L.) fishery in the North Sea cause large bycatch of fish, invertebrates and undersized shrimp. We investigated the most obvious and enforceable mitigation measure: the increase of mesh size beyond 20 mm, which was the medium mesh size of the fleet before 2016. The predicted selectivity from different codend designs was applied in a yield-per-recruit model able to reproduce the seasonal pattern of landings and biomass by sex, size, and age. The model enables assessment and quantifying population responses to possible management actions, also considering other influential variables such as natural mortality, which vastly decreased during the 1990s to the contemporary low level due to the decrease of cod in the Southern North Sea. Model results show that the selectivity of 20 mm codends leads to the poorest fishery and population indexes. Any increase in mesh size within the range considered and in the present low-predator situation would lead to a larger shrimp stock and higher catches. Increasing mesh size reduces catches of undersized and small commercial-sized shrimps entering the fishery mainly in summer time, thus leading to surplus catches in autumn and spring. Model results reveal that an increase of mesh size diminishes growth overfishing and allows the formation of a larger stock that may better cope a fluctuating natural mortality and the steadily upwards creeping fishing intensity.

中文翻译：

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小步大跃进：北海褐虾渔业改变编码网目尺寸的生物经济效应

摘要 在北海褐虾（Crangon crangon L.）渔业中使用的小网眼编码器导致大量鱼类、无脊椎动物和小虾的误捕。我们调查了最明显和可执行的缓解措施：将网格尺寸增加到 20 毫米以上，这是 2016 年之前机队的中等网格尺寸。来自不同编码器设计的预测选择性应用于每次招募的产量模型，能够按性别、大小和年龄重现上岸量和生物量的季节性模式。该模型能够评估和量化种群对可能的管理行动的反应，同时考虑其他有影响的变量，例如自然死亡率，由于北海南部鳕鱼的减少，自然死亡率在 1990 年代大幅下降到当代的低水平。模型结果表明，20 毫米编码的选择性导致最差的渔业和人口指数。在所考虑的范围内以及在目前的捕食者数量较少的情况下，任何网目尺寸的增加都会导致虾种群增加和捕获量增加。增加网目尺寸会减少主要在夏季进入渔业的体型过小和小型商业规格虾的捕获量，从而导致秋季和春季的捕捞量过剩。模型结果表明，网眼尺寸的增加会减少过度捕捞的增长，并允许形成更大的种群，从而更好地应对波动的自然死亡率和稳步上升的捕捞强度。在所考虑的范围内以及在目前的捕食者数量较少的情况下，任何网目尺寸的增加都会导致虾种群增加和捕获量增加。增加网目尺寸会减少主要在夏季进入渔业的体型过小和小型商品虾的捕获量，从而导致秋季和春季的捕获量过剩。模型结果表明，网眼尺寸的增加会减少过度捕捞的增长，并允许形成更大的种群，从而更好地应对波动的自然死亡率和稳步上升的捕捞强度。在所考虑的范围内以及在目前的捕食者数量较少的情况下，任何网目尺寸的增加都会导致虾种群增加和捕获量增加。增加网目尺寸会减少主要在夏季进入渔业的体型过小和小型商品虾的捕获量，从而导致秋季和春季的捕获量过剩。模型结果表明，网眼尺寸的增加会减少过度捕捞的增长，并允许形成更大的种群，从而更好地应对波动的自然死亡率和稳步上升的捕捞强度。