Diet and condition of Atlantic bluefin tuna (Thunnus thynnus) in the Gulf of Maine, 2004–2008

Abstract

Atlantic herring (Clupea harengus) are the primary forage for Atlantic bluefin tuna (Thunnus thynnus; ABFT) in the Gulf of Maine. Recent studies have documented significant declines in ABFT somatic condition and shifts in their size and spatial distribution in the Gulf of Maine, which may be linked to trophic changes. We collected stomachs (n = 122) as well as liver (n = 110) and white muscle (n = 382) samples for carbon and nitrogen stable isotope analysis and lipid analysis to determine if diet composition had changed relative to the late 1980’s to early 2000’s for large, commercially harvested ABFT (≥185 cm curved fork length (CFL)). Samples of smaller ABFT (<185 cm CFL) were also collected (stomachs: n = 21; liver: n = 17; white muscle: n = 19) to compare diet between size classes. Large ABFT diet was similar among current and historic studies, Atlantic herring being the main prey (39.5 to 52.8 % weight). Small ABFT fed at a lower trophic position (TP = 3.9) than larger individuals (TP = 4.9) due to higher consumption of sand lance (Ammodytes spp.) and euphausiids (65.6 vs. 4.5 % weight). Mean and maximum lipid stores of large ABFT increased from spring through fall, but lean fish were observed in all seasons. In the fall, lean ABFT were lighter with lower nitrogen and higher carbon isotope values than co-occurring ABFT with higher lipid stores. These patterns are consistent with shelf vs. offshore isotope baseline differences in the western North Atlantic and variable arrival and residency patterns for ABFT in the Gulf of Maine.