Data from: Shifts in North Sea forage fish productivity and potential fisheries yield

Forage fish populations support large scale fisheries and are key components of marine ecosystems across the world, linking secondary production to higher trophic levels. While climate-induced changes in the North Sea zooplankton community are described and documented in literature, the associated bottom-up effects and consequences for fisheries remain largely unidentified.

We investigated the temporal development in forage fish productivity and the associated influence on fisheries yield of herring, sprat, Norway pout and sandeel in the North Sea. Using principal component analysis, we analysed 40 years of recruitment success and growth proxies to reveal changes in productivity and patterns of synchroneity across stocks (i.e. functional complementarity). The relationship between forage fish production and Calanus finmarchicus (an indicator of climate change) was also analysed. We used a population model to demonstrate how observed shifts in productivity affected total forage fish biomass and fisheries yield.

The productivity of North Sea forage fish changed around 1993 from a higher average productivity to lower average productivity. During the higher productivity period, stocks displayed a covariance structure indicative of functional complementarity. Calanus finmarchicus was positively correlated to forage fish recruitment, however, for growth, the direction of the response differed between species and time periods. Maximum sustainable yield (MSY) and the associated fishing mortality (Fmsy) decreased by 33%–68% and 26%–64%, respectively, between the higher and lower productivity periods.

Synthesis and applications. The results demonstrate that fisheries reference points for short-lived planktivorous species are highly dynamic and respond rapidly to changes in system productivity. Furthermore, from an ecosystem-based fisheries management perspective, a link between functional complementarity and productivity, indicates that ecosystem resilience may decline with productivity. Based on this, we advise that system productivity, perhaps monitored as forage fish growth, becomes an integral part of management reference points; in both single species and ecosystem contexts. However, to retain social license of biological advice when fish catch opportunities are reduced, it is crucial that shifts in productivity are thoroughly documented and made apparent to managers and stakeholders.

饵料鱼（forage fish）种群支撑着全球大规模渔业，同时也是全球海洋生态系统的关键组成部分，可将次级生产与更高营养级生物联结起来。尽管学界已对气候驱动的北海浮游动物群落变化开展了描述与记录，但与之相关的上行效应及其对渔业的影响，目前仍未得到充分探明。

本研究针对北海鲱鱼、西鲱、挪威稚鳕及沙鳗的饵料鱼生产力时间动态，及其对渔业产量的关联影响展开了调查。研究采用主成分分析（principal component analysis）方法，对40年的种群补充量与生长替代指标进行分析，以揭示各渔业种群的生产力变化与同步性模式（即功能互补性）。同时，本研究还分析了饵料鱼生产与飞马哲水蚤（Calanus finmarchicus，气候变化指示物种）之间的关联关系，并通过种群模型阐释了观测到的生产力变化如何影响饵料鱼总生物量与渔业产量。

北海饵料鱼的生产力在1993年左右发生了显著转变，从较高的平均生产力水平转向较低的平均生产力水平。在高生产力时期，各渔业种群的协方差结构呈现出功能互补性特征。飞马哲水蚤与饵料鱼种群补充量呈正相关关系，但针对生长指标的响应方向则因物种与时期而异。在高、低生产力两个时期之间，最大可持续产量（MSY）及对应的最优捕捞死亡系数（Fmsy）分别下降了33%~68%与26%~64%。

综合与应用。研究结果表明，短命浮游食性鱼类的渔业参考点具有高度动态性，可快速响应系统生产力的变化。此外，从基于生态系统的渔业管理视角来看，功能互补性与生产力之间的关联暗示，生态系统韧性可能随生产力水平下降而降低。基于此，我们建议将系统生产力（可通过饵料鱼生长状况进行监测）纳入单一物种与生态系统层面的渔业管理参考点体系。然而，为在捕捞配额缩减时维持生物建议的社会认可度，必须对生产力变化进行充分记录，并向管理者与利益相关者清晰阐明这些变化。