Graduate Thesis Or Dissertation
 

Are there two subgroups of albacore, Thunnus alalunga, in the North Pacific? Evidence from variability in catch, seasonal migrations, and length composition for two subgroups in the coastal fishery of North America

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  • North Pacific albacore (Thunnus alalunga) is a highly migratory and commercially valuable species of tuna. All stock assessments and management decisions for North Pacific albacore are presently based on the assumption of a single, uniform stock. However, a growing body of evidence from diverse sources suggests that there are two subgroups of albacore in the coastal fishery of North America. These subgroups are believed to occur north and south of 400 North latitude. This study investigated the existence of the proposed subgroups using more than 40 years of albacore logbook and port sampler data provided by the Southwest Fisheries Science Center. I examined regional differences in the coastal albacore fishery using three metrics: 1) catch per unit effort (CPUE), 2) entry and exit date, and 3) size composition. Distinct regional differences were observed for each metric. Spatial mapping of average CPUE by month suggested that migration into the coastal fishery occurs at two distinct locations: approximately 450 North latitude and 300 North latitude. Availability within seasons, expressed as average CPUE by month, differed between the two regions. Average entry and exit dates were also different between the two regions, with the northern region starting on average one month later (July) and ending one month earlier (October) than the southern region. Moreover, albacore from the southern region averaged 2.5 cm longer than those caught in the northern region. These results support previous findings and suggest that the coastal albacore fishery is comprised of two subgroups with distinct migration routes and size characteristics. The results may have implications for the management of this fishery, particularly if these subgroups represent separate and distinct stocks. Because this study did not examine spawning distribution, further research (possibly using genetics) is needed to determine if the subgroups spawn independently in space and time.
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