122 



Fishery Bulletin 98(1) 



ADAPT VPA run supplied by NMFS (Table 2). Table 

 2 shows that, if the no-memory assumption is cor- 

 rect and the trans-Atlantic transfer rate is at least 

 2%, we would expect about 5% to 10% offish spawn- 

 ing in the west to be smaller than 135 kg. (Note that 

 for this comparison to be valid, either all mature 

 fish in the west and all small migrant spawning fish 

 must go to the Gulf of Mexico to spawn or the same 

 proportion of each group must go there each year. ) 

 In fact, only 0% to 0.3% of fish on the Gulf spawn- 

 ing grounds between 1980 and 1992 weighed less 

 than 135 kg (Fig. 3), significantly less than predicted 

 (X2=353, P<0.0001). Of 4688 fish for which NMFS 

 has recorded weights, 15 were less than 135 kg, and 

 10 of those were between 120 and 135 kg (Fig. 2). 



Note that this analysis considered only newly 

 arrived migrants each year, and ignored the possible 

 accumulation of sexually mature migrants from pre- 

 vious years that had not yet reached 135 kg. The con- 

 tinued presence of prior migrants would have raised 

 the expected number of small spawning fish. Even 

 without the cumulative effect of small migrants, the 

 actual proportion of small spawning fish in the Gulf 

 catch was about 5—10% of that predicted by the 

 model with a 2% transfer rate. Higher transfer rates 

 would imply that even greater numbers of small 

 spawning fish should appear in the Gulf 



It is clear from our results that small bluefin tuna 

 are not present among spawning fish in the Gulf of 

 Mexico in the numbers that would be expected for 



even the lowest of hypothesized trans-Atlantic trans- 

 fer rates. Our interpretation is that young adult blue- 

 fin tuna of eastern origin seldom or never spawn in 

 the Gulf of Mexico and presumably do not contribute 

 significantly to the spawning biomass of the western 

 population. There are at least three possible alterna- 

 tives: 1) eastern migrants may either delay spawn- 

 ing in the west until they reach 135 kg or remain in 

 the east until they reach 135 kg, making them indis- 

 tinguishable from western spawning fish; 2) migrant 

 eastern tuna may be spawning in the west but not 

 in the Gulf of Mexico; or 3) small migrants may be 

 spawning in the Gulf but are avoiding detection or 

 are being under-reported. 



Size and age at maturity 



If size and age at maturity are environmentally 

 determined, then eastern migrants might follow the 

 west Atlantic maturity schedule and thus be unde- 

 tected with our methods. For example, changes in 

 size and age at maturity may be a response to differ- 

 ences in interspecific or intraspecific population den- 

 sity. A lower population density reduces competition 

 for food and increases per capita food intake, resulting 

 in faster growth. When experiencing such low inter- 

 or intraspecific population densities and enhanced 

 growth, fish may mature at about the same size but 

 would attain this size at a younger age (Trippel, 

 1995). However, for bluefin tuna, the reported dif- 



