Divergent thermal modulation of juvenile cod and saithe in subarctic nearshore nurseries

Shallow coastal habitats function as key nurseries for many marine fishes, providing food, shelter, and favorable temperatures for growth. However, species differ markedly in how they occupy and persist in these environments and in their sensitivity to coastal temperatures. We examined seasonal and interannual patterns in the presence, size, and condition of young-of-the-year Atlantic cod Gadus morhua and saithe Pollachius virens in northwest Iceland using beach-seine surveys conducted across multiple fjords and years (2015-2021). Generalized additive mixed models were used to relate juvenile catch numbers, length, and Fulton’s K to anomalies in coastal temperature, total juvenile count, and seasonal progression. The results showed species-specific variation in phenology and temperature modulation of coastal nursery use. Cod size was determined by temperature, and their facultative residency was shaped by an interaction of season and weekly temperature deviation later in the season, a pattern that aligns well with the high intraspecific diversity and behavioral flexibility of Atlantic cod. Saithe, by contrast, were dominant in those habitats during summer, followed by declining and intermittent use later in the season. Saithe showed weak temperature dependence, whether at annual, regional, or seasonal scales, perhaps as their high mobility reduces coupling to the coastal thermal environment. The results reinforce the ecological importance of fucoid-adjacent nurseries for both cod and saithe in Icelandic coastal ecosystems and suggest that coastal warming may influence the 2 species differently, with potential implications for their coexistence, growth trajectories, and recruitment processes.