Atlantic Blue Marlin and Yellowfin Tuna: Comparative Population Vulnerability to Fishing Mortality
C. Phillip Goodyear
Abstract.—Atlantic blue marlin are primarily harvested as bycatch in fisheries targeting tunas and swordfish. These target species are managed for maximum sustainable yield (MSY) based largely on guidance from surplus production models that lack age structure. Simulation models were constructed around the life history characteristics of Atlantic blue marlin and yellowfin tuna, one of the target species. Each simulated population was exposed to fishing mortality and the resulting time streams of catches and abundances were used as surplus production model inputs using the computer program ASPIC. The slopes of the stock-recruitment curves of the simulated populations were adjusted until the ASPIC estimates of the intrinsic growth rates for the simulations were equivalent to the estimates derived for these populations in the last ICCAT stock assessments. The equilibrium curves of production on fishing mortality for the age-structured populations were then compared to the logistic production model fitted by ASPIC. For blue marlin, the underlying production curve shifted to the left, and F MSY was lower than the value estimated by ASPIC. For yellowfin tuna, the underlying production curve shifted to the right and F MSY occurred at a higher fishing mortality rate than that estimated by ASPIC. These results suggest that the Atlantic blue marlin stock is more vulnerable to fishing mortality than indicated by the production model fitted in the last assessment. Also, the fishing mortality rate that produces MSY for yellowfin is near the extinction level for blue marlin. This characteristic is likely shared by other highly productive stocks that support the fisheries in which blue marlin are killed as bycatch. These results indicate that fishing target species at MSY may result in continued serious depletions of Atlantic blue marlin unless the catchability can be reduced relative to the catchability of the target species.