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）种群支撑着规模化渔业，同时是全球海洋生态系统的关键组成部分，将次级生产传递至更高营养级。现有文献已对气候变化驱动的北海浮游动物（zooplankton）群落变化进行了系统描述与记录，但由此产生的上行效应及其对渔业的影响仍未得到充分探明。 本研究针对北海鲱鱼、西鲱、挪威棱鳕与沙鳗的饵料鱼生产力时序变化，及其对渔业产量的影响展开调查。研究采用主成分分析（principal component analysis）方法，分析了40年的种群补充成功率与生长替代指标数据，以揭示种群生产力的变化规律与各鱼群间的同步性模式（即功能互补性）。此外，本研究还分析了饵料鱼生物量与作为气候变化指示物的飞马哲水蚤（Calanus finmarchicus）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总饵料鱼生物量与渔业产量。 北海饵料鱼的生产力在1993年前后发生了结构性转变，从较高的平均生产力水平转为较低的平均水平。在高生产力阶段，各鱼群呈现出体现功能互补性的协方差结构。飞马哲水蚤与饵料鱼的种群补充成功率呈正相关关系，但对于生长指标而言，不同物种与不同时期的响应方向存在差异。在高低生产力两个阶段之间，最大可持续产量（maximum sustainable yield, MSY）及其对应的捕捞死亡系数（fishing mortality, Fmsy）分别下降了33%~68%与26%~64%。 综合与应用：本研究结果表明，短命浮游食性鱼类的渔业参考点具有高度动态性，可对系统生产力变化做出快速响应。此外，从基于生态系统的渔业管理（ecosystem-based fisheries management）视角出发，功能互补性与生产力之间的关联表明，生态系统恢复力（ecosystem resilience）可能随生产力水平下降而降低。据此我们建议，将系统生产力（可通过监测饵料鱼生长情况实现）纳入单一物种与生态系统双重维度的渔业管理参考点体系，作为其核心组成部分。然而，当鱼类捕捞配额缩减时，为维持生物管理建议的社会认可度，全面记录生产力的变化情况并向管理者与利益相关者（stakeholders）清晰传达这些变化，显得尤为关键。