Evidence of trans-Atlantic movement and natal homing of bluefin tuna from stable isotopes in otoliths

Abstract

Stable carbon and oxygen isotopes in the otoliths of Atlantic bluefin tuna Thunnus thyn- nus were used to investigate trans-Atlantic movement and natal homing behaviors. Otolith δ 13 C and δ 18 O of yearling bluefin tuna from 6 different year classes (1999 to 2004) were characterized to vali- date the approach and assess inter-annual variation in these markers for individuals from eastern (Mediterranean Sea/eastern Atlantic Ocean) and western (USA Atlantic Ocean) nurseries. Otolith δ 18 O of yearlings from the eastern nursery were enriched relative to their western counterparts, while otolith δ 13 C was relatively similar between nurseries. Temporal variability in otolith δ 13 C and δ 18 O of bluefin tuna was observed; nevertheless, classification success to eastern and western nurseries was relatively high (87%) when all year classes were pooled. Otolith δ 13 C and δ 18 O of yearlings were then used as a baseline sample ('eastern' and 'western' nursery signature) to predict the origin of adoles- cents (20 to 70 kg) from 1 foraging area (USA Atlantic Ocean) and adults (>140 kg) from 1 spawning ground (Mediterranean Sea) based on otolith core signatures (corresponding to yearling period). Mixed-stock analysis indicated that approximately 60% of the adolescent bluefin tuna collected from foraging areas in the USA Atlantic Ocean originated from the eastern nursery, suggesting that sub- stantial trans-Atlantic movement of adolescents from east to west occurred. In addition, natal homing was well developed, with 94% of the adult bluefin tuna collected in the Mediterranean Sea derived from the eastern nursery. Findings from this study suggest that USA fisheries depend upon migrants of Mediterranean origin and that mixing across the 45° W management boundary is substantially higher than assumed previously.