118 



Abstract.-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 east- 

 em 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 indi- 

 cate 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 blue- 

 fin 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 east- 

 ern spawning fish that should be found 

 in the Gulf of Mexico catch, assuming a 

 2% east-to-west transfer rate and com- 

 plete 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 0'7c to 0.9% 

 of the catch annually, and only 0.3% 

 for the entire period. We conclude that 

 eastern migrant tuna do not mix com- 

 pletely, if at all, with western bluefin 

 tuna on the Gulf of Mexico spawning 

 grounds. 



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 



David Nemerson 



National Audubon Society 



550 South Bay Avenue 



Ishp, New York 11751 



Present address Marine Field Station 



Institute of Manne and Coastal Sciences 



Rutgers University 



132 Great Bay Boulevard 



Tuckerton, New Jersey 08087-2004 

 E-mail address nemersoniatmcsrutgers edu 



Steven Berkeley 



Oregon State University 

 Hatfield Manne Science Center 

 Oregon State University 

 Newport, Oregon 



Carl Saflna 



National Audubon Society 

 550 South Bay Avenue 

 Islip, New York 11751 



Manuscript accepted 1 December 1998. 

 Fish Bull. 98:118-126 12000). 



Atlantic bluefin tuna ( Th un n us thyn - 

 nus thynnus) is a highly migratory 

 pelagic species that ranges through- 

 out the Atlantic between 60°N lati- 

 tude and the equator, although it 

 has not been encountered south 

 of 20°N since the 1960s. Two blue- 

 fin tuna breeding sites are known 

 in the North Atlantic: the Gulf 

 of Mexico and the Mediterranean 

 Sea. No other regular spawning site 

 has been identified in the North 

 Atlantic (Richards, 1976;McGowan 

 and Richards, 1989; NRC, 1994). 

 Intensive fisheries exist for bluefin 

 tuna along the North American and 

 European coasts, and to a lesser 

 degree in the high seas of the North 

 Atlantic. Although fish tagged on 

 both sides of the ocean have been 

 recovered on the side opposite from 

 their release, it is not known if blue- 

 fin tuna return to their natal spawn- 



ing ground to reproduce (Turner 

 and Powers, 1995; Cooke and Lank- 

 ester, 1996). This question is of 

 utmost importance in evaluating 

 the significance of trans-Atlantic 

 movement and the scale at which 

 management must operate to be 

 effective. 



The behavior of trans-Atlantic 

 migrating bluefin tuna is unknown, 

 but the possibilities are bounded 

 by two extremes. At one extreme, 

 an emigrant may join the popula- 

 tion on the side of the ocean to 

 which it migrates, becoming indis- 

 tinguishable from the population 

 it joins with respect to the proba- 

 bility, timing, and locale of future 

 life history events, such as matura- 

 tion, spawning, and migration. At 

 the ether extreme, a migrant may 

 always return to its natal side prior 

 to the next spawning season. 



