Nemerson et a\ Spawning site fidelity in Thunnus thynnus 



121 



erature, and is for ages 

 0-3, 0.5 for age 4, and 1 for 

 ages 5 and beyond (Baglin, 

 1982; ICCAT"). We assumed 

 that any migrant of age 8 or 

 greater would be the same 

 size as western spawning fish 

 and would not be distinguish- 

 able from western spawning 

 fish of the same size (Cort, 

 1991; Turner et al., 1991; 

 Table 1 ). We used a transfer 

 rate T of 29^ per year, east-to- 

 west. This rate is at the low 

 range of published estimates. 

 In this initial test, we did not 

 consider fish less than age 4 

 that could have migrated to 

 the west as immature fish in 

 prior years and then reached 

 age 4 and maturity in the 

 current year. Thus, our esti- 

 mate of the expected number 

 of spawning fish of eastern 

 Atlantic origin in the western 



Atlantic should represent a minimum estimate and 

 provide a conservative test for the presence of east- 

 ern migrants. N^a.\ was taken from yearly age-spe- 

 cific population estimates supplied by NMFS from a 

 run of the ADAPT virtual population analysis (VPA) 

 program with 27c east-to-west and a 1% west-to-east 

 transfer rates, assuming no memory. Note that the 

 population estimates from this VPA run resulted in 

 poor fits to the indices of abundance used to tune the 

 VPA. '5 We used these population estimates because 

 they provided a conservative test of our assump- 

 tions. 



Results and discussion 



Bluefin tuna smaller than the accepted size at first 

 spawning of western fish are very rare in the Gulf. 

 Catches of fish less than 135 kg ranged from 0% to 

 0.9% of annual catch from 1980 to 1992 and aver- 

 aged 0.3*^ over the entire period (Table 2). A com- 

 plete weight frequency distribution is presented in 

 Figure 2. 



These percentages are not consistent with the 

 low end of published migration rate estimates under 

 the diffusion model. That is, if 27( of each age class 



Weight (kg) 



Figure 2 



Weight-frequency distribution of bluefin tuna caught in the Gulf of Mexico between 1980 

 and 1992. The numbers over the bars indicate the total number of fish caught in the 

 weight interval indicated. 



^ Porch, C. 1995. National Marine Fisheries Service. South- 

 east Fisheries Science Center, 75 Virginia Beach Drive. Miami, 

 FL 33149. Personal commun. 



migrated from east to west and joined the western pop- 

 ulation, we would expect to see many more small fish, 

 i.e. fish of eastern origin, spawning in the west. ( Recall 

 that the diffusion, or no-memory model, implies that 

 the migrant does not "remember" its natal spawning 

 gi'ound but does "remember" its maturation sched- 

 ule.) The model predicts that between 8483 and 

 14,655 sexually mature migrants smaller than 135 

 kg would have arrived each year in 1980-92. We 

 compared these numbers with the numbers of sex- 

 ually mature fish estimated for the west from the 



