Challenges in the Assessment and Management of Highly Migratory Bycatch Species: A Case Study of the Atlantic Marlins
Patrick D. Lynch, John E. Graves, and Robert J. Latour
In marine ecosystems, highly migratory species (HMS) are characterized as having vast geographical distributions, with extensive individual migrations often spanning entire oceans. Dispersal on this scale can promote ocean-wide population connectivity, resulting in many HMS exhibiting genetic homogeneity. From a biological perspective, these species often comprise a single unit stock within an ocean basin. Since single stocks can be distributed throughout multinational and international waters (as with the tunas [Family Scombridae]), sustainable management of these harvested stocks requires cooperation between all fishing nations. An international governing organization is imperative to ensure cooperation, and in the Atlantic Ocean, the member nations of the International Commission for the Conservation of Atlantic Tunas (ICCAT) are responsible for management of highly migratory fishes. The main objective of ICCAT is to maintain stocks at levels that produce maximum sustainable yield (MSY) (ICCAT 2007a), a goal that is likely shared among most fishing nations. However, numerous HMS exhibit spatial and temporal overlap, which creates management challenges since large quantities of nontarget HMS are often caught incidentally. The various HMS captured may not have the same intrinsic population growth rates or carrying capacities; therefore, their populations may not exhibit the same responses to a given level of fishing effort. Since fishers often seek productive stocks, nontarget species may be depleted at a rate faster than target species, thus sustainable management of all stocks may require a reduction of effort well below that which maximizes yield of the target species. For many fishing nations, this can result in substantial declines in commercial revenues. Since the overall importance of nontarget species inevitably varies between stakeholders, the international cooperation that is essential for management of HMS may break down when incidental catch is considered.