HISTORICAL EFFECTS OF FISHING ON AGE STRUCTURE AND STOCK MIXING IN NORTHWEST ATLANTIC BLUEFIN TUNA

• Main Author: Siskey, Matthew Ryan
• Other Authors: Secor, David H, Digital Repository at the University of Maryland, University of Maryland (College Park, Md.), Marine-Estuarine-Environmental Sciences
• Format: Thesis
• Language: English
• Published: 2015
• Subjects: Aquatic sciences; age structure; Atlantic bluefin tuna; fisheries; size-selective fishing; stock mixing; Northwest Atlantic
• Online Access: http://hdl.handle.net/1903/17126; https://doi.org/10.13016/M2X35Z

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 δ18O measurements between laboratories, but not δ13C 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.