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Fishery Bulletin 98(1) 



Punt and Butterworth (1995) estimated west-to- 

 east transfer at about 7% and east-to-west transfer 

 at about 1.5-3%, assuming a diffusion model. They 

 also state that the higher end of the range (3%) sug- 

 gests a far larger size for the western population 

 than do models that assume no migration. Cooke 

 and Lankester (1996) test both diffusion and over- 

 lap models and concluded that the overlap model fits 

 the data better. Under that model, they estimated 

 exchange rates at 7.3% east-to-west and 9.8% west- 

 to-east, but with no statistical difference between the 

 two. Powers and Cramer ( 1996) examined the impli- 

 cations of a range of migration rates and degrees of 

 spawning site fidelity. Although they made no con- 

 clusions about which scenario is most likely, they 

 pointed out the extreme sensitivity of the results to 

 the assumptions. 



Eastern and western Atlantic bluefin tuna popu- 

 lations have markedly different life history parame- 

 ters (Turner, 1994). The western population spawns 

 from mid-April to mid-June (Richards, 1976). West- 

 ern bluefin tuna sometimes mature as early as age 

 6 and are considered fully mature by age 8, at a 

 weight of 135 kg ( Baglin, 1982; NRC, 1992 ). The east- 

 ern population spawns from June through August 

 (Dicenta and Piccinette, 1980) and matures at an 

 earlier age and smaller size than the western pop- 

 ulation. Eastern bluefin tuna mature as early as 

 age 3, at a weight of 15 kg ( Rodriguez -Roda, 1967; 

 Baglin, 1982), and are fully mature by age 5 (Rodri- 

 guez-Roda, 1967; Baglin, 1982; ICCAT-). 



The contrast in size and age at maturity of western 

 and eastern Atlantic bluefin tuna allows an inferen- 

 tial test of spawning site fidelity. Because the Gulf of 

 Mexico is the only known spawning ground for west- 

 ern Atlantic bluefin tuna, and the vast majority of 

 fish collected in the Gulf are large adults that are 

 present only during and just prior to the spawning 

 season (January-June), we assumed that all bluefin 

 tuna in the Gulf during this time period are there to 

 spawn. 



If eastern Atlantic bluefin tuna that migrate to the 

 west mature according to the eastern Atlantic matu- 

 ration schedule, then the size distribution of bluefin 

 tuna in the Gulf of Mexico should reveal the pres- 

 ence of eastern migrants within the western spawn- 

 ing population. Finding small fish (<135 kg) on the 

 Gulf of Mexico spawning grounds would support the 

 diffusion hypothesis and suggest that trans-Atlan- 

 tic migrants from the east mix with western fish 

 during spawning. In contrast, the absence of small 

 fish on the Gulf of Mexico spawning grounds would 

 imply that eastern migrants do not spawn in the 

 west, supporting the overlap model and indicating 

 strong spawning site fidelity. We know that small 



bluefin tuna from the east Atlantic swim west at 

 least occasionally. All tagged eastern Atlantic blue- 

 fin tuna recaptured in the west have been small fish 

 (n=19, all captured outside the Gulf), although very 

 few large fish, and relatively few bluefin tuna over- 

 all, have been tagged in the east, compared with tag- 

 ging in the west (NRC, 1994). 



Methods 



We analyzed the size distribution of bluefin tuna 

 caught in the Gulf of Mexico prior to and during the 

 spawning season (the only time of year when bluefin 

 tuna are present in the Gulf) for fish between the 

 known size of first breeding in the Mediterranean 

 and the known size of first breeding for west Atlan- 

 tic bluefin tuna. Any individuals smaller than the 

 known size of first spawning in the west would pre- 

 sumably be of eastern Atlantic origin. 



A weight-frequency distribution (WED) of bluefin 

 tuna on the Gulf spawning grounds was constructed by 

 using data reported to National Marine Fisheries Ser- 

 vice (NMFS) by the commercial fishing industry oper- 

 ating in the Gulf This data set included the weight 

 and date of capture for every bluefin tuna legally 

 caught and landed in the Gulf We used data from 1980 

 through 1992, because beginning in 1993 only bluefin 

 tuna over 178 centimeters ( 70 inches) fork length were 

 legally permitted to be retained and sold.^ 



To estimate the proportion of smaller eastern 

 spawning fish expected at a given east-to-west annual 

 transfer rate (i.e. fish remain with the western popu- 

 lation), we created a simple model of the number of 

 sexually mature eastern migrants that arrive in the 

 west each year: 



where S^^. = the number of age-7 or younger spawn- 

 ing fish of eastern origin arriving in 

 yeary; 

 P„ = the percentage of sexually mature 



adults in eastern age class a; 

 T^ = the east-to-west transfer rate; and 

 A^^^ ^ - the number in eastern age class a in 

 yeary. 



East-to-west transfer was modeled as an instan- 

 taneous process that occurs prior to the spawning 

 season. The parameter P was taken from the lit- 



■• National Marine Fisheries Service. 1995. Supplemental draft 

 environment impact statement for a regulatory amendment for 

 the western Atlantic bluefin tuna. U.S. Dep. Commer. NMFS. 

 NOAA, Silver Spring, MD, 131 p. 



