BLOCK, BA; Stanford University: Hot Tuna in Cold Seas: The Ecological Physiology and Conservation of Giant Bluefin

Tuna have historically been a challenge to study due to their size, speed and range over the vast oceanic habitat. Electronic tagging and remote sensing technologies are revolutionizing the study of tunas in the open ocean. Critical information on their distribution, physiology and ecology has resulted from the expanding use of electronic tags. Surgically implanted archival tags reveal that Atlantic bluefin make extensive feeding migrations along the North American continental shelf and offshore North Atlantic waters. Bluefin migrate into subpolar waters during feeding aggregations that extend over 7 months of the year. These same fish make shorter breeding migrations into warm temperate and tropical waters. These two ecological phases place distinct demands on the bluefin tuna�s physiology. Archival tagging data indicate that bluefin tuna encounter the coldest temperatures during feeding migrations (3�-12�C) while maintaining endothermic muscle and visceral temperatures of 23�-30�C. During breeding migrations, ambient and body temperatures range as warm as 33�C creating a high metabolic demand for oxygen. The hearts of bluefin operate at ambient temperature due to close proximity to the gills and the absence of a heat exchanger in the coronary circulation. This creates a physiological paradox as the tuna heart must meet the oxygen demands of warm, aerobic tissues in both cold and warm waters. We have investigated the cardiovascular physiology of the bluefin tuna. The data indicate that the bluefin tuna�s exceptional cardiac performance may be a key adaptation supporting the wide thermal niche of bluefin tuna in the wild. This thermal niche expansion puts the giant bluefin tuna at risk due to the inability of international fishers to agree on how to effectively manage the species.