Pre-spawning habitat use of Atlantic bluefin tuna (Thunnus thynnus) inferred from stable isotope analysis

Atlantic bluefin tuna (ABFT; Thunnus thynnus) spawn primarily in the Gulf of Mexico and Mediterranean Sea, but migrate to foraging habitats throughout the North Atlantic where they are the target of commercial and recreational fisheries. Their natal origin has been characterized through otolith oxygen isotope analysis to link fish on both spawning grounds and foraging habitats to their spawning ground origins, but connectivity on a shorter, seasonal timescale is still not completely understood. Nitrogen isoscapes in the North Atlantic include a distinct separation of productive, nearshore and more oligotrophic open ocean foraging habitats. We used linear discriminant analysis of bulk nitrogen isotope data to estimate the percent of ABFT that occupied shelf or open ocean foraging habitats prior to capture on eastern and western Atlantic spawning grounds. ABFT in the Gulf of Mexico were mainly classified as previous shelf foragers (91%), while ABFT associated with eastern Atlantic spawning grounds primarily had an open ocean/Mediterranean Sea classification (96% Morocco, 79% Strait of Gibraltar, 91% Balearic Sea, 100% Adriatic Sea). Amino acid nitrogen isotope data of ABFT from the Gulf of Mexico confirmed that observed bulk nitrogen isotope differences were due to baseline rather than trophic variability and source amino acid values generally aligned most closely with literature values from shelf and slope waters rather than open ocean habitats. These data provide insight into the foraging habitats that support eastern and western Atlantic spawning assemblages.