An examination of population structure, movement patterns and environmental preferences in northern bluefin tuna

Electronic tagging technologies are providing new methodologies for tracking open ocean fish. In this thesis, acoustic, archival and pop-up satellite tags were used to examine the movement patterns and behavior of Atlantic bluefin (Thunnus thynnus) and Pacific bluefin (Thunnus orientalis ) tunas. Data from all tag types showed similar patterns in bluefin tuna behavior. 177 Atlantic bluefin tuna were tagged with pop-up satellite tags off the coast of North Carolina between 1999 and 2004. Tuna resided in the waters of North Carolina and the South Atlantic Bight throughout the winter months, predominately over the shallow continental shelf. It is hypothesized that a large aggregation of spawning prey fish is what attracts bluefin tuna to this area. In the spring, fish moved offshore, predominately into the western Atlantic Ocean along the northern edge of the Gulf Stream. In the summer months, tuna were found predominately in the waters offshore of New England, in the Gulf of Maine. Movement patterns of 143 Pacific bluefin tuna in the California Current were also examined using archival tags. Tagged fish showed latitudinal movement patterns that were correlated with peaks in coastal upwelling induced primary productivity. Bluefin tuna were found farthest south in the spring when they were located off southern Baja California, Mexico and farthest north in the fall when fish were found predominately off central and northern California, USA. Overall geographical area occupied by tagged bluefin varied with primary productivity, with fish being more tightly clustered when in areas of high productivity. Distribution of bluefin in both the Atlantic and Pacific Oceans varied by age class of fish as well as by tracking year. In addition to electronic tracking population subdivision was examined in Atlantic bluefin tuna through sequencing of the control region of the mitochondrial genome. A total of 168 samples from the spawning grounds in the Gulf of Mexico, Bahamas and Mediterranean Sea were analyzed. Bluefin tuna populations from the Gulf of Mexico and the Mediterranean Sea were found to be genetically distinct based on phi st, sequence nearest neighbor and AMOVA analyses, suggesting these two major spawning areas support independent stocks. It is hypothesized that the fidelity to individual spawning grounds identified in electronic tracking records maintains the genetic differentiation among stocks.