Historical Effects of Fishing on Age Structure and Stock Mixing in Northwest Atlantic Bluefin Tuna

Title of Document: HISTORICAL EFFECTS OF FISHING ON AGE STRUCTURE AND STOCK MIXING IN NORTHWEST ATLANTIC BLUEFIN TUNA Matthew Ryan Siskey, Master of Science, 2015 Directed By: Professor David H. Secor Marine Estuarine Environmental Sciences Bluefin tuna support important fisheries in the Northwest Atlantic Ocean, which have declined in yield from intense, size-selective exploitation. Age structure, size-at-age, and stock composition were investigated as principal responses to exploitation, utilizing otolith microstructural and chemical analysis. To evaluate otoliths as ageing structures, annulus formation was compared to temperature-associated oscillations in otolith strontium:calcium. Evaluation of otolith stable isotope measures used in stock composition analyses indicated significant differences in δO measurements between laboratories, but not δC values. Comparisons of age structure, size-at-age, and stock composition over three periods (1974-1978, 1996-2002, 2009-2014) coinciding with the cycle of exploitation intensity suggest size-selective fishing caused (1) age truncation, where median age declined (14 to 6 years); (2) minor changes in size-at-age; and (3) fluctuating stock composition, with peak mixing in the 1990s (48% eastern stock contribution). Size-specific reductions in fishing mortality could contribute to recovery through more frequent production of strong year-classes. HISTORICAL EFFECTS OF FISHING ON AGE STRUCTURE AND STOCK MIXING IN NORTHWEST ATLANTIC BLUEFIN TUNA