Young-Of-Year Atlantic Cod and Saithe Differ in Individuality and Structure of Movement Traits.

Individual differences in behavior impact ecological interactions and potentially influence life-history strategies. However, comparative studies examining how behavioral variation is structured in closely related, ecologically similar species remain limited. We investigated the expression of movement traits and the structure of behavioral variation in young-of-year (YOY, 0-group) Atlantic cod (Gadus morhua) and saithe (Pollachius virens) using repeated laboratory trials over 3 months in solitary open-field and mirror contexts, followed by additional paired conspecific trials using the same experimental contexts. Several movement traits were quantified, and activity and acceleration were analyzed using Bayesian mixed-effects models to estimate species differences, context-dependent plasticity, and among-individual variation across time and social contexts. Species differed modestly in behavioral expression; saithe were quicker to explore and exhibited higher acceleration than Atlantic cod. Cod activity increased, and acceleration declined over time, whereas saithe exhibited little change. Context produced contrasting effects for both species: mirror exposure reduced activity (no change in acceleration), paired trials increased activity in both species, while acceleration showed no clear change in cod but decreased in saithe. Beyond species mean responses, species differed in the organization of behavioral variation. Cod exhibited stronger and more structured among-individual differentiation, including strong intercept-slope relationships, but weaker coupling among movement traits. Saithe showed tighter integration among movement traits, higher residual variability, and more variable responses across contexts. These results demonstrate that closely related, ecologically similar gadoids differ not only in behavioral expression but in how behavioral variation is partitioned among and within individuals, and this is apparent even during the first months of life. The noted differences align with ecological contrasts between demersal, facultatively social Atlantic cod and pelagic, more strongly shoaling saithe.