Origin, ecotype and ontogeny shape fine-scale vertical segregation in juvenile Atlantic cod

Abstract Ecotypic divergence in Atlantic cod ( Gadus morhua ) is well established for adult fish, but the extent to which juveniles differ is still poorly understood. We used acoustic telemetry to track 94 juvenile cod (1–3 years old) genetically assigned to inshore and offshore ecotypes, originating from shallow Dýrafjörður and the deeper Ísafjarðardjúp fjord system in northwest Iceland. Over a two-year study period, we quantified residency events, in-fjord residency, and vertical distribution to test the effects of ecotype, capture origin, and body size. Inshore cod from Dýrafjörður displayed longer residencies, less seasonal vertical migration, and reduced ontogenetic deepening compared with offshore cod, which were more likely to shift to deeper waters based on body size and season. Capture origin strongly shaped behaviour: fish from the larger Ísafjarðardjúp fjord system resided at greater depths and were more likely to leave the fjord than conspecifics from Dýrafjörður. While translocation influenced the likelihood of exiting the fjord, depth-use differed by capture origin even when restricting the comparison to individuals that became resident. Furthermore, translocated fish exhibited ecotype-specific depth-use patterns parallel to local fish. Together, these results provide the first evidence of fine-scale vertical and residency segregation between age 1–3 juvenile Atlantic cod ecotypes but also show pronounced site-associated differences and within-ecotype heterogeneity. Examples include the very dense residency kernels of Dýrafjörður inshore juveniles and the diverse nursery strategies observed within all groups. This heterogeneity can have important implications for how environmental change in coastal nursery areas may affect cod populations and should be considered for coastal conservation.