ECOLOGICAL AND ECONOMIC COMPONENTS OF ALTERNATIVE FISHING METHODS TO REDUCE BY-CATCH OF MARLIN IN A TROPICAL PELAGIC ECOSYSTEM

Recently, the U.S. National Marine Fisheries Service was petitioned to list North Atlantic white marlin as an endangered species under the Endangered Species Act (ESA). The ESA requires a stock assessment of status and trends for listed species and will severely restrict U.S. commercial and recreational fisheries, but because by-catch by foreign longline fleets account for more than 95% of marlin mortality, applying the ESA to U.S. fleets would have little direct effect. We analyzed an alternative management policy by using an Ecosim model of the central north Pacific to simulate a fishing strategy that reduces marlin by-catch by removing longline hooks that fish in water less than 120 m in depth. Mortality rates might be reduced by about 50% through removal of less than 15% of hooks. After 30 yrs, blue marlin biomass had increased to 2.8 times, and striped marlin biomass to about 1.8 times, that expected under no change in fishing practices. Ecological consequences included decreased abundance of adult yellowfin tuna. This result produced an economic trade-off of lost fishery revenues (primarily yellowfin tuna) and gains due to increased catches of billfishes in recreational fisheries. A recent compilation clearly demonstrates that, as pelagic fisheries have developed over the past 50 yrs, some large predatory fishes have been reduced to a fraction of their former abundance (Myers and Worm, 2003). Associated with this increase in fishing effort is an increase in by-catch and a rising tide of opinion in favor of protecting endangered pelagic species (e.g., sea turtles, albatrosses, marine mammals; Bertness et al., 2001), overfished pelagic and migratory species (Safina, 2001), and nontarget species such as sharks and billfishes (Hinman, 1998). Attempts may therefore be launched to protect selected pelagic species from effects of fishery exploitation. Fishery practices may be altered by decree, economic sanctions, and/or market forces such as boycotts and their consequent expression in fishery targets. The ecological consequences of these actions are unknown, and we cannot assume that simply removing fishing effort will automatically “repair” the system to conditions that are optimal for all stakeholders. Applying restrictions to high-seas fisheries presents some interesting ecological, economic, and political challenges. Recently, a petition was filed with the U.S. National Marine Fisheries Service (NMFS) asking that white marlin (Tetrapturus albidus) in the Atlantic be considered for listing as an endangered species (Federal Register, 2001) because white marlin stocks appear to have declined to about 15% of estimated biomass for sustained fishing at maximum yields. Blue marlin (Makaira mazara) abundance in both the Atlantic and the Pacific reflects a similar declining trend. Both white and blue marlin are being considered for special attention by the International Commission for the Conservation of Atlantic Tunas (ICCAT). Similarly, concern is rising about the status of striped marlin (Tetrapturus audax) stocks in the Pacific (E. Peel, The Billfish Foundation, pers. comm.). Although no highly migratory pelagic fish species has been listed under the U.S. Endangered Spe