Vertical behavior and diet of albacore tuna (Thunnus alalunga) vary with latitude in the South Pacific Ocean

Abstract

Albacore tuna (Thunnus alalunga) are an important upper tropic-level oceanic predator with a circum-global distribution. Little is known of the movements and diet of albacore tuna in the South Pacific Ocean and how variability in both might influence the vulnerability of albacore tuna to fisheries across their range. We coupled data derived from satellite-tagged albacore tuna with stomach samples collected from individuals at the same locations to characterize the vertical behavior, thermal and dietary habits of albacore tuna at tropical (New Caledonia and Tonga) and temperate (New Zealand) latitudes. A total of 18 pop-up satellite archival tags deployed on albacore tuna remained attached for 0–50 days. Position estimates, calculated from 11 tags, described short-term movements of predominantly less than 500km, although one fish moved more than 1000km over a period of 50 days. Vertical behavior and diet differed substantially between tropical and temperate latitudes. At tropical latitudes, albacore tuna showed a distinct diel pattern in vertical habitat use, occupying shallower, warmer waters above the mixed layer depth (MLD) at night, and deeper, cooler waters below the MLD during the day. In contrast, there was little evidence of a diel pattern of vertical behavior in albacore tuna at temperate latitudes, with fish limited to shallow waters above the MLD almost all of the time. Spatial patterns of species composition in stomach contents were consistent with vertical movement patterns, with significantly more deepwater prey species consumed in tropical waters than in temperate waters. Albacore in tropical waters also consumed significantly greater diversities of prey than in temperate waters, predominately preying on fish species, whereas those in temperate waters predominately preyed on crustacea. Our results indicate that the vertical distribution of albacore is constrained either by thermal preferences with diet reflecting these preferences, by the vertical distribution of prey species, which may be affected by the thermal structure of the ocean, or most likely by a mixture of both. Spatial differences in the vertical distribution of albacore tuna suggest that vulnerability of albacore to oceanic fisheries varies with latitude. Changes in the thermal structure of oceanic waters in temperate areas associated with climate change might influence the vertical distributions and, therefore, vulnerability of albacore to oceanic fisheries into the future.