Spawning site fidelity in Atlantic bluefin tuna, Thunnus thynnus: The use of size-frequency analysis to test for the presence of migrant east Atlantic bluefin tuna on Gulf of Mexico spawning grounds

The Gulf of Mexico is the only known spawning area for bluefin tuna (Thunnus thynnus thynnus) in the western Atlantic. Although it is known from tag recaptures that eastern Atlantic bluefin tuna travel to the western Atlantic, whether or not these fish spawn in the western Atlantic is of critical importance in interpreting the significance of this movement. East Atlantic bluefin tuna mature at a younger age (4-5 yr) and smaller size (45 kg) than western bluefin tuna (8 yr and 135 kg), and tag recaptures indicate that some young fish make the trans-Atlantic swim. Thus the presence of small (<135 kg) bluefin tuna in the Gulf of Mexico during spawning season would constitute evidence that bluefin tuna of east Atlantic origin spawn in the west. We used size-frequency analysis to test the hypothesis that Atlantic bluefin tuna of eastern and western origins mingle on the Gulf of Mexico spawning grounds. We created a simple model to estimate the proportion of small eastern spawning fish that should be found in the Gulf of Mexico catch, assuming a 2% east-to-west transfer rate and complete mixing of eastern and western fish. Using conservative assumptions, the model predicts that between 5% and 10% of the bluefin tuna catch in the Gulf should consist of fish that are less than 135 kilograms in weight, and thus are presumably eastern migrants. We analyzed Gulf of Mexico catch records from 1980 to 1992 for the presence of bluefin tuna less than 135 kg. These small fish represented from 0% to 0.9% of the catch annually, and only 0.3% for the entire period. We conclude that eastern migrant tuna do not mix completely, if at all, with western bluefin tuna on the Gulf of Mexico spawning grounds.