An individual-based model of North Pacific albacore tuna seasonal migratory behaviour and climate sensitivity.

Juvenile North Pacific albacore tuna (Thunnus alalunga) undertake annual long distance migrations between offshore waters and the California Current Large Marine Ecosystem (CCLME), yet the drivers of the timing of these movements remain unclear. Highly migratory marine predators like albacore often use environmental cues to track seasonal resources and optimize foraging. Mixed layer depth (MLD), defined as the well-mixed surface layer of the ocean, has previously been associated with important albacore physiological and behavioral patterns. Using electronic tagging data and an individual-based model (IBM) we show MLD has a pivotal role in influencing albacore migration timing and depth preferences. Albacore actively expand their vertical habitat in correspondence with wintertime MLD deepening and appear to utilize a 30m MLD threshold to initiate preemptive movements to reach seasonally and spatially explicit foraging resources. Model simulations using MLD-based rules and an ocean sea surface temperature (SST) constraint successfully capture the seasonality of movements and distribution of albacore. Climate projections suggest that by 2070–2099, SST warming will shift albacore distributions poleward and MLD shoaling will prolong their coastal residence, potentially increasing albacore concentrations in the Northern CCLME. These findings highlight the relevance of subsurface ocean conditions to the movement of highly migratory species and demonstrate the utility of IBMs in the study of complex migratory behaviors.