We present a study on the diet composition of Western Baltic cod (WBC) Gadus morhua based on 3150 stomachs sampled year-round between 2016 and 2017 using angling, gillnetting, and bottom trawling, which enhanced the spatio-temporal coverage of cod habitats. Cod diet composition in shallow areas (< 20 m depth) was dominated by benthic invertebrate species, mainly the common shore crab Carcinus maneas. Compared to historic diet data from the 1960s and 1980s (limited to depth >20m), the contribution of herring Clupea harengus decreased and round goby Neogobius melanostomus occurred as a new prey species. Statistical modelling revealed significant relationships between diet composition, catch depth, fish length and season. Generalized additive modelling identified a negative relationship between catch depth and stomach content weight, suggesting reduced food intake in winter when cod use deeper areas for spawning and during peak summer when cod tend to avoid high water temperatures. Our results highlight the importance of shallow coastal areas as major feeding habitats of adult cod in the Western Baltic Sea, which were previously unknown because samples were restricted to deeper trawlable areas. Our results strongly suggest that historic stomach analyses overestimated the role of forage fish and underestimated the role of invertebrate prey. Eventually, our study demonstrates the importance of a comprehensive habitat coverage for unbiased stomach sampling programs to provide a more reliable estimation of top predator diet, a key information for food web analyses and multi-species models.

中文翻译：

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被遗忘的觅食地：西波罗的海成年鳕鱼的季节性和特定深度食物摄入模式

我们基于 2016 年至 2017 年全年使用钓鱼、刺网和底拖网捕捞的 3150 只胃，对西波罗的海鳕鱼 (WBC) Gadus morhua 的饮食成分进行了研究，这增强了鳕鱼栖息地的时空覆盖率。浅海（< 20 m 深）的鳕鱼饲料成分以底栖无脊椎动物物种为主，主要是常见的岸蟹 Carcinus maneas。与 1960 年代和 1980 年代的历史饮食数据（限于深度 > 20m）相比，鲱鱼 Clupea harengus 的贡献减少，圆形虾虎鱼 Neogobius melanostomus 作为新的猎物物种出现。统计模型揭示了饮食成分、捕获深度、鱼长和季节之间的显着关系。广义加法模型确定了捕获深度和胃内容物重量之间的负相关关系，建议在冬季减少食物摄入量，因为鳕鱼在更深的区域产卵，而在夏季高峰期，鳕鱼往往会避开高水温。我们的研究结果强调了浅海沿岸地区作为西波罗的海成年鳕鱼主要觅食栖息地的重要性，这在以前是未知的，因为样本仅限于更深的可拖网区域。我们的结果强烈表明，历史胃部分析高估了饲料鱼的作用，低估了无脊椎动物猎物的作用。最终，我们的研究证明了无偏胃采样计划的全面栖息地覆盖的重要性，以提供对顶级捕食者饮食的更可靠估计，这是食物网分析和多物种模型的关键信息。