Habitat Use of Juvenile Atlantic Bluefin Tuna

Between 50% and 90% of the U.S. recreational fishery for Atlantic Bluefin Tuna (hereafter tuna) consists of juveniles and a large portion (25%-83%) of these are subsequently released. However, unlike adult tuna, little is known about movement, habitat use, or mortality of juveniles. We believe that juvenile tuna do not utilize the water column in the same manner as adults. To investigate habitat use of juvenile tuna, we deployed 20 pop-up tags from New Jersey and Massachusetts between June and September 2012. Because these fish are visual predators, we hypothesized that time of day and moon phase would affect their use of the water column and therefore, would be reflected in the mean depth and temperature occupied. Additionally, we hypothesized that date and location may influence habitat use. Using general linear mixed models with repeated measures, we determined that date and time of day accounted for a significant portion of the total variation in occupied habitats. As the season progressed, juvenile tuna occupied deeper, cooler waters. Diurnal habitat use varied such that juvenile tuna were more likely to use deeper, cooler waters during dawn compared with any other time. We suggest these habitat changes are related to foraging behavior.