Seasonal shifts and individual variation in juvenile Atlantic cod diel migration at nearshore nurseries

Diel migration (DM) is a common behavior among aquatic species, supporting optimal foraging, predator avoidance, and energy balance. While often perceived as uniform, DM patterns vary significantly across populations and individuals due to both intrinsic and extrinsic factors. We investigated diel horizontal migration in acoustically tagged juvenile Atlantic cod Gadus morhua in nearshore nurseries in Iceland and Norway, examining spatial, seasonal, and individual variation. By applying a novel method to estimate horizontal DM from acoustic telemetry data for individuals too small to carry depth-logging transmitters, we detected clear DM patterns when a distinct day-night cycle was present. In Iceland, fish consistently migrated nearshore at night and farther from land during the day. In contrast, Norwegian G. morhua displayed both typical and reverse DM patterns. During the Icelandic summer, when daylight was continuous, DM was not observed, and fish remained farther from shore. Migration timing at both sites aligned closely with seasonal changes in daylength, particularly with shifts in light levels at dawn and dusk, indicating that seasonal variation in DM is largely governed by photoperiod. While seasonal DM patterns were broadly consistent across sites and between individuals, variation in the tendency to migrate and migration direction between individuals suggests that intrinsic factors specifically influence finer-scale variation in DM. Understanding these dynamics is critical for predicting behavioral responses to shifting light regimes and habitat availability under climate-driven changes, including poleward range expansions.