Increases in the relative abundance of mid-trophic level fishes concurrent with declines in apex predators in the subtropical North Pacific, 1996–2006

The North Pacific subtropical gyre is a large oceanic gyre bounded on the south by the North Equatorial Current, on the west by the Kuroshio Current, on the north by the Kuroshio Extension Current and the North Pacific Current, and on the east by the California Current (Pickard and Emery, 1990). Although low in primary productivity, the warm, vertically stratified oligotrophic waters of the subtropical gyre contain a highly diverse food web populated by tunas, sharks, and billfishes at the top trophic levels (Seki and Polovina, 2001; Kitchell et al., 2002). Since the 1950s, the tunas, billfishes, and other large predators in this ecosystem have been targeted by large-scale fisheries. Several studies have indicated possible ecosystem impacts from fishing (Ward and Myers, 2005a; Kitchell et al., 1999; 2002). A comparison of catch, size, and species composition between a research longline survey in the 1950s and observer data from commercial longliners in the 1990s indicated a substantial decline in the abundance of large predators, in the mean size of these predators, and gave some evidence of an increased abundance of formerly rare species (Ward and Myers, 2005a). Models of the North Pacific subtropical gyre were generated with Ecopath with Ecosim, vers. 5 (EwE) modeling software (available from http://www.ecopath. org/index.php) to investigate whether the ecosystem contained any keystone species (Kitchell et al., 1999; 2002). The results indicated that there was not any single species group that functioned as a keystone, but that a broad reduction of apex predators due to fishing might result in an increase in prey (Kitchell et al., 1999; 2002). In effect, the fishing fleet is the keystone predator (Kitchell et al., 1999). However another modeling effort with an EwE model that incorporated some sizeclass structure revealed that although fishing decreased predator abundance, there was limited evidence (based on the decline in predators) of trophic cascades or other ecosystem impacts (Cox et al., 2002). The longline logbook and observer data from the Hawaii-based fishery are valuable data sources for investigating the spatial and temporal dynamics of the exploited subtropical ecosystem. The fishery operates over a large portion of the central North Pacif ic, from the equator to 40°N latitude and from 140°W longitude to the International Date Line. Federally mandated logbooks completed by fishermen provide catch and effort Increases in the relative abundance of mid-trophic level fishes concurrent with declines in apex predators in the subtropical North Pacific, 1996–2006