On the movements, aggregations and the foraging habitat of bluefin tuna (Thunnus thynnus and Thunnus orientalis)

To exploit the ocean's patchy resources, large open ocean fish species have evolved highly migratory foraging strategies. In this thesis, a synoptic study of Atlantic bluefin tuna (Thunnus thynnus) and Pacific bluefin tuna (Thunnus orientalis) and their environment was conducted to identify feeding behavior and foraging related horizontal and vertical movements, and to elucidate foraging habitat. First, electronic archival tags (n=561) were used to examine seasonal movements (1996-2005), aggregations and diving behaviors of Atlantic bluefin tuna (ABFT) to better understand their migration ecology and oceanic habitat utilization. Throughout the North Atlantic mean diving depth was significantly correlated with thermocline depth and dive behavior changed in relation to the stratification of the water column. Distribution behavior was characterized by seasonal aggregations and rapid movement phases. Throughout the North Atlantic, high residence times (167 +/- 33 days) were identified in four spatially confined regions on a seasonal scale. In these regions, mean diving depths were significantly shallower and dive frequency and internal temperature variance were significantly higher than during transit movements between the regions, suggesting foraging behavior. Residence time in high-use areas was correlated to primary productivity in northern latitudes and these areas represent critical foraging habitats with seasonally abundant prey.; To be able to study feeding in wild bluefin tunas, the heat increment of feeding (HIF) in response to known quantities and caloric value of food ingested by captive individuals was evaluated. Feeding experiments were conducted using stomach and peritoneal placed archival tags in captive Pacific bluefin tuna (PBFT, n=31). Peritoneal temperature measurements indicate a significant correlation between food energy content and the duration from the start of post-grandial heat increment to the maximum thermal excess (TXmax ) in all ambient temperatures measured. An algorithm developed from these laboratory experiments was applied to data collected in the wild (n=1,562 days). Pacific bluefin tuna off the Californian coast foraged regularly (2.1+/-0.4 feeding events/day) with an estimated daily intake of 1059+/-482 kcal (4.8+/-1.8% body mass). Feeding events occurred most often in morning hours and feeding temperature and depths were significantly colder and deeper than overall PBFT thermal and depth preference. Track-based area restricted search indices and daily diving frequency were significant and consistent predictors of the number of feeding events per day.; Building on this new understanding of tuna feeding ecology I examined the relationship between foraging ABFT (n=7; 2,756 days) in North Atlantic and chlorophyll concentration and net primary productivity. Individuals conducted area restricted search primarily in the productive North Atlantic Drift Province above 40°N, where they resided from spring to late fall and were associated with chlorophyll blooms. Using ocean current data and cross-correlation techniques, I determined significant time lags between intensity of tuna searching behavior and primary productivity indices in backtracked water masses. Off of the continental shelves in the western North Atlantic, there was a mean lag of three weeks between peak chlorophyll blooms and presence of individual bluefin tuna. In the eastern Atlantic, time lags were significantly shorter, ranging from 1-2 weeks. Overall, primary productivity levels were significantly higher than at zero lag and area restricted search intensity was significantly correlated with levels of backtracked chlorophyll concentration.