Disparate movement behaviour and feeding ecology in sympatric ecotypes of Atlantic cod

Co-existence of ecotypes, genetically divergent population units, is a widespread phenomenon, potentially affecting ecosystem functioning and local food web stability. In coastal Skagerrak, Atlantic cod (Gadus morhua) occur as two such co-existing ecotypes. We applied a combination of acoustic telemetry, genotyping and stable isotope analysis to 72 individuals to investigate movement ecology and food niche of putative local “Fjord” and putative oceanic “North Sea” ecotypes – thus named based on previous molecular studies. Genotyping and individual origin assignment suggested 41 individuals were Fjord and 31 were North Sea ecotypes. Both ecotypes were found throughout the fjord. Seven percent of Fjord ecotype individuals left the study system during the study while 42 % of North Sea individuals left, potentially homing to natal spawning grounds. Home range sizes were similar for the two ecotypes but highly variable among individuals. Fjord ecotype cod had significantly higher δ13C and δ15N stable isotope values than North Sea ecotype cod, suggesting they exploited different food niches. The results suggest coexisting ecotypes may possess innate differences in feeding- and movement ecologies and may thus fill different functional roles in marine ecosystems. This highlights the importance of conserving interconnected populations to ensure stable ecosystem functioning and food web structures.

Methods
Acoustic receivers were deployed in Sandnesfjorden, Norway. Atlantic cod were caught in the fjord, a fin clip was taken for genetic origin assignment and a small muscle tissue sample was taken for stable isotope analysis. The fish were tagged with acoustic tags and their behaviour in the fjord was tracked.

生态型（ecotype）——即遗传分化的种群单元——的共存是一种广泛存在的现象，其或可影响生态系统功能与本地食物网的稳定性。在斯卡格拉克海岸海域，大西洋鳕鱼（Gadus morhua）存在两种共存的生态型。我们结合声学遥测、基因分型与稳定同位素分析技术，对72尾个体展开研究，以探究推定的本地“峡湾型”与海洋性“北海型”生态型的运动生态学特征与食物生态位——这两种生态型的命名基于此前的分子生物学研究。基因分型与个体起源归属分析结果显示，41尾个体属于峡湾型，31尾属于北海型。两种生态型均分布于整个峡湾区域。研究期间，7%的峡湾型鳕鱼个体离开了研究区域，而北海型个体的离开比例达42%，这些个体可能会洄游至其出生的产卵场。两种生态型的家域范围大小相近，但个体间差异极大。峡湾型鳕鱼的δ¹³C与δ¹⁵N稳定同位素值显著高于北海型鳕鱼，这表明二者利用了不同的食物生态位。研究结果表明，共存的生态型可能在摄食与运动生态学方面存在先天差异，因此在海洋生态系统中承担着不同的功能角色。这一发现凸显了保护连通种群对于维持稳定的生态系统功能与食物网结构的重要性。

方法
声学接收器部署于挪威桑讷峡湾（Sandnesfjorden）。研究人员在峡湾内捕获大西洋鳕鱼，采集鳍组织样本用于遗传起源归属分析，同时采集小块肌肉组织样本用于稳定同位素分析。对这些鱼类植入声学标记标签，并追踪其在峡湾内的行为活动。