The strength of interspecific competition and predator–prey interactions depends on the area of co-occurrence of the interacting species. Therefore, it is necessary to quantify the changes in the spatial overlap of trophically connected species to understand the outcomes of species interactions. In the Baltic Sea, the interplay between cod and flounder has previously been neglected. In this study, we use four decades of data on cod and flounder distributions covering the southern and central Baltic Sea to: (1) model and map the changes in the distributions of the two species using generalized additive models; (2) quantify the temporal changes in the potential competitive and predator–prey interactions between them using spatial overlap indices; (3) relate these changes in overlap to the known dynamics of the different cod and flounder populations in the Baltic Sea. Competition overlap has continuously increased for cod, from the beginning of the time-series. This is a possible cause of the observed decline in feeding levels and body condition of small and intermediate sized cod. Flounder overlap with large cod instead has decreased substantially, suggesting a predation release of flounder, potentially triggering its increase in abundance and distribution range observed in the last decades.

中文翻译：

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波罗的海鳕鱼和比目鱼之间空间重叠的长期变化

种间竞争和捕食者-猎物相互作用的强度取决于相互作用物种的共生区域。因此，有必要量化营养连接物种空间重叠的变化，以了解物种相互作用的结果。在波罗的海，鳕鱼和比目鱼之间的相互作用以前被忽视了。在本研究中，我们使用涵盖波罗的海南部和中部的鳕鱼和比目鱼分布的四个十年数据来： (1) 使用广义可加模型对这两个物种的分布变化进行建模和绘制；(2) 使用空间重叠指数量化它们之间潜在竞争和捕食者-猎物相互作用的时间变化；(3) 将这些重叠变化与波罗的海不同鳕鱼和比目鱼种群的已知动态联系起来。从时间序列开始，鳕鱼的竞争重叠不断增加。这是观察到的中小型鳕鱼的摄食水平和身体状况下降的可能原因。比目鱼与大鳕鱼的重叠反而大幅减少，表明比目鱼的捕食释放，可能引发过去几十年观察到的比目鱼丰度和分布范围的增加。