Unraveling the Influence of Behavioural Ecotypes on Fish Gut Microbiome: Focus on the Atlantic cod (Gadus morhua) in Icelandic Waters.

This study investigated the gut microbiome of Atlantic cod in northern Icelandic waters to assess how resident (coastal) and migratory (frontal) behavioural ecotypes influence gut microbial community composition, with a focus on the effects of ecological adaptation and the potential drivers of microbial shifts within this species. A total of 81 intestinal samples from coastal and frontal Atlantic cod collected in northern Iceland in December 2021 and January 2024 were analysed using 16S rRNA gene metabarcoding. Environmental datasets from the Copernicus Marine Service were used to assess the relationship between annual shifts in microbiome composition and changes in environmental conditions. Our results highlight that the Icelandic cod gut microbiome is a highly dynamic system primarily shaped by environmental shifts, such as warming trends and anthropogenic stressors. Because different behavioural ecotypes are inherently exposed to distinct environmental configurations, these distinct macro-scale exposures may indirectly translate into ecotype-specific microbial signatures that shape the differentiation between coastal and offshore habitats; the effects were particularly evident in the coastal populations, underscoring the greater vulnerability of nearshore habitats to environmental shifts and anthropogenic stressors. These findings suggest the role of the microbiome in ecological plasticity of Atlantic cod; concurrently, they reveal distinct ecotype-specific sensitivities to environmental shifts, particularly those of anthropogenic origin, providing a valuable framework that may support future marine conservation strategies.