Abstract
The Gulf of Maine, NW Atlantic Ocean, is a productive, seasonal foraging ground for Atlantic bluefin tuna (Thunnus thynnus), but commercial landings of adult size classes were up to 40% below the allocated total allowable catch between 2004 to 2008 for the rod and reel, harpoon, and purse seine categories in the Gulf of Maine. Reduction in Atlantic bluefin tuna catches in the Gulf of Maine could represent a decline in spawning stock biomass, but given wide-ranging, complex migration patterns, and high energetic requirements, an alternative hypothesis is that their dispersal patterns shifted to regions with higher prey abundance or profitability, reducing availability to U.S. fishing fleets. This study fit generalized linear models to Atlantic bluefin tuna landings data collected from fishermen’s logbooks (1979-2005) as well as the distances between bluefin tuna schools and Atlantic herring ( Clupea harengus ), a primary prey species, to test alternative hypotheses for observed shifts in Atlantic bluefin tuna availability in the Gulf of Maine. For the bluefin model, landings varied by day of year, latitude and longitude. The effect of latitude differed by day of year and the effect of longitude differed by year. The distances between Atlantic bluefin tuna schools and Atlantic herring schools were significantly smaller (p<0.05) than would be expected from a randomly distributed population. A time series of average bluefin tuna school positions was positively correlated with the average number of herring captured per tow on Georges Bank in spring and autumn surveys respectively (p<0.01, r2=0.24, p<0.01, r2=0.42). Fishermen’s logbooks contributed novel spatial and temporal information towards testing these hypotheses for the bluefin tuna fishery.
Highlights
In summer and autumn, schools of Atlantic bluefin tuna (Thunnus thynnus) (ABFT) enter the Gulf of Maine and adjacent regions to forage on lipid-rich herring and other prey before dispersing offshore and southward during winter and spring [1,2]
Our results portray the distribution of commercial size ABFT schools (>185 cm CFL) and provide evidence that a gradual longitudinal shift to the east has reduced availability to the U.S fishery and contributed to the observed decrease in U.S commercial landing in the Gulf of Maine
Fishermen’s logbooks have been used to interpret and improve the quality of information used in stock assessments [47] and provide additional context for evaluating regional shifts in CPUE- based indices of abundance
Summary
Schools of Atlantic bluefin tuna (Thunnus thynnus) (ABFT) enter the Gulf of Maine and adjacent regions to forage on lipid-rich herring and other prey before dispersing offshore and southward during winter and spring [1,2]. Atlantic bluefin tuna and their principal prey, Atlantic herring (Clupea harengus), support valuable commercial fisheries that have operated for decades in New England and the Canadian Maritimes [3,4]. From 2004-2008, commercial landings (fish >185cm curved fork length, CFL) of ABFT in the Gulf of Maine declined to approximately 40% of levels recorded in the previous two decades [5,6]. The decline in U.S commercial ABFT catches has, in part, been used to support a proposal to list this species under Appendix I of the Convention on International Trade in Endangered Species (CITES). Organizations, (e.g., U.S Fish and Wildlife Service and the Committee on the Status of Endangered Wildlife in Canada) have conducted an evaluation for this species and list them as a “species of concern” and “endangered” respectively [7,8]. The most recent ABFT stock assessment indicates spawning stock biomass has been stable between the mid-1980’s to the mid-2000’s and increased slightly over the past few years [5] indicating alternative hypotheses may explain changes in U.S commercial landings of ABFT [6]
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