Nemerson et al.: Spawning site fidelity in Thunnus thynnus 



123 



ference in size and age at 

 maturity between east and 

 west Atlantic populations does 

 not appear related to dif- 

 ferences in growth rate be- 

 cause recent growth models 

 indicate little difference be- 

 tween populations (Cort, 1991; 

 Turner et al., 1991; Table 1). 



Similarly, if differences in 

 age or size at maturity are 

 affected by environmental 

 conditions, for example tem- 

 perature, we would expect this 

 effect to be manifested pri- 

 marily by changes in growth 

 rate. Again, the similarity in 

 growth rate between east and 

 west Atlantic bluefin tuna 

 suggests that environmental 

 conditions are unlikely to 

 explain the difference in size 

 and age at maturity. 



Alternatively, if age at ma- 

 turity is a heritable trait, then a long period of size- 

 selective fishing mortality could shift genotype fre- 

 quencies in the population because few late-maturing 

 fish are likely to survive to reproduce (Trippel, 1995), 

 resulting in a younger age or smaller size at matu- 

 rity, or both (Policansky, 1993; Trippel, 1995). Experi- 

 ments with guppies indicate that increased mortality 

 (as through fishing) selects for earlier maturity at 

 smaller size (Reznick, 1993). Bluefin tuna in the east 

 Atlantic has a longer history of exploitation and a 

 much larger population than west Atlantic bluefin 

 tuna. Assuming that the very large difference in 

 population sizes results in a comparable difference 

 in stock density, then an eastern bluefin tuna with 

 a genetic propensity to mature at or before age 5 

 in the east Atlantic should, upon migrating to the 

 west Atlantic, find itself in a relatively resource-rich, 

 lower-density environment, which should certainly 

 not delay maturation or inhibit spawning. Thus, 

 although genetic effects are difficult to establish, 

 such a large difference in size and age at maturity 

 as between east and west Atlantic bluefin tuna is 

 unlikely to be a result of density-dependent or envi- 

 ronmental differences. Further, it seems unlikely 

 that a sexually mature five- or six-year-old east 

 Atlantic bluefin tuna would revert to immaturity 

 upon migrating to the west Atlantic, and then remain 

 immature for two or three years until finally spawn- 

 ing at age eight. 



Another possible explanation for these results is 

 that the size-at-maturity data on which this analy- 



sis depends are incorrect. In fact. Clay (1990) crit- 

 icized both the Baglin (1982) and Rodriguez-Roda 

 (1967) studies, citing small sample sizes and inad- 

 equate temporal and spatial coverage. As Clay (1990) 

 pointed out, the Rodriguez-Roda (1967) study dealt 

 with fish that were on their way to the spawning 

 grounds and thus may have over-estimated the per- 

 centage of small, mature fish in the total population. 

 Although this is a valid criticism, it is worth noting 

 that the collection of Rodriguez-Roda also contained 

 immature fish, impl3dng that not all fish in his sample 

 were on their way to the spawning grounds. 



Further, although our method clearly relies on the 

 assumption that bluefin tuna of eastern origin first 

 spawn at a smaller size than western fish, only a 

 small proportion of age classes 4 through 7 need be 

 mature for our results to prevail. We investigated 

 the sensitivity of our model to a ten-fold reduction in 

 the maturity parameter P (i.e. to 0.05 on age-4 and 

 0.1 on ages 5 through 7) and found that small blue- 

 fin tuna would still be significantly rarer (chi-square 

 test, P<0.005) in the Gulf than predicted under the 

 diffusion model with a 2% east-to-west transfer rate, 

 given our assumptions. 



Finally, for the purpose of this study, we have 

 made the most conservative assumption, i.e. that 

 any bluefin tuna (except larvae) found in the Gulf of 

 Mexico is spawning. Thus, it is possible that some of 

 the smaller bluefin tuna in the Gulf of Mexico land- 

 ing records were not actually spawning fish, or were 

 of west Atlantic origin, or both. 



