From migrants to mossbacks: Tracer- and tag-inferred habitat shifts in the California yellowtail Seriola dorsalis

The California yellowtail Seriola dorsalis (YT) is an economically and ecologically valuable predator in both coastal and pelagic regions of the California Current Ecosystem. Delineating size-structured migration patterns can help assess population connectivity and predict effects of regional fishing pressure. We used chemical tracers (stable isotope analysis and mercury analysis) and conventional tagging to evaluate the dynamics of a potential ontogenetic shift in habitat from pelagic waters to coastal regions. Stable isotope analysis revealed a shift in habitat use at intermediate sizes (fork length, FL = 76 to 87.5 cm). Smaller YT were isotopically similar to pelagic yellowfin tuna Thunnus albacares, while larger YT were isotopically similar to the coastal white seabass Atractoscion nobilis. Tag recaptures from a small number of fish (48 deployments, 15 recaptures) corroborated an ontogenetic shift from offshore to coastal habitats, suggesting local, residential populations of larger YT in nearshore areas. Mercury concentrations increased directly after the observed habitat shift (FL = 88.3 cm), which is likely a result of both bioaccumulation with age and a shift to higher Hg prey inshore. Residential behavior of mature YT > 80 cm (∼4 to 12+ yr old) suggests that regional size distributions could be influenced by local fishing pressure and inshore movement dynamics, as recruitment of migrants from southern waters will likely be comprised of smaller, younger fish.