Hawaii FEP - Western Pacific Fishery Council

the mesopelagic community migration indicates a clear link between the nearshore and oceanicecosystems in the Hawaiian Islands, which in turn affects the presence and abundance of thepelagic predator species.Studies near the Hawaiian Islands indicate that concentrations of spawning tuna near the islandsmay be due to increased forage species in these areas associated with elevated primaryproductivity (Itano 2000). Spawning in yellowfin tuna has been correlated to sea surfacetemperatures (SSTs), mainly above 24 - 26°C and may also be correlated with frontal areas suchas the edge of Western Pacific Warm Pool (WPWP). The WPWP is the largest oceanic body ofwarm water with surface temperatures consistently above 28°C (Yan et al. 1992 in Itano 2000).The edge zones of this warm area are convergence zones which bring up nutrient rich waters andcreate high productivity areas resulting in high densities of tuna forage (i.e., baitfish such asanchovy) and thus large numbers of tuna. Offshore areas of high pelagic catch rates andspawning frequencies were found around several productive seamounts which also exhibit highproductivity due to interactions of submarine topography, current gyres and being located in thelee of the main Hawaiian Islands (Itano 2000). Trophic linkages such as those evident in tunaswhereby ocean anchovy are a primary forage species [of tuna] which themselves feed primarilyon copepods provide a critical link between zooplankton and larger pelagic fishes (Ozawa andTsukahara 1973 in Itano 2000). Understanding these linkages is an essential component ofsuccessful ecosystem-based fishery management.Phytoplanktons contribute to more than 95 percent of primary production in the marineenvironment (Valiela 2003) and represent several different types of microscopic organisms thatrequire sunlight for photosynthesis. Phytoplankton primarily live in the upper 100 meters of theeuphotic zone of the water column and provide primary production in the marine ecosystem asfood for zooplankton, which in turn, feeds small organisms such as crustaceans and so forth onup the food chain. For example, large pelagic species are commonly most concentrated nearislands and seamounts that create divergences and convergences, which concentrate foragespecies, and also near upwelling zones along ocean current boundaries and along gradients intemperature, oxygen, and salinity. Swordfish and numerous other pelagic species tend toconcentrate along food-rich temperature fronts between cold upwelled plankton-rich water andwarmer oceanic water masses (NMFS 2001).These frontal zones have been identified as likely migratory pathways across the Pacific forloggerhead turtles (Polovina et al. 2000). Loggerhead turtles are opportunistic omnivores thatfeed on floating prey such as the pelagic cnidarian Vellela vellela (“by the wind sailor”) and thepelagic gastropod Janthia spp., both of which are likely to be concentrated by the weakdownwelling associated with frontal zones (Polovina et al. 2000).Migration patterns of pelagic fish stocks in the Pacific Ocean are not easily understood orcategorized, despite extensive tag-and-release projects for many of the species. This isparticularly evident for the more tropical tuna species (e.g., yellowfin, skipjack, bigeye) thatappear to roam extensively within a broad expanse of the Pacific centered on the equator.Although tagging and genetic studies have shown that some interchange does occur, it appearsthat short life spans and rapid growth rates restrict large-scale interchange and genetic mixing ofeastern, central, and far-western Pacific stocks of yellowfin and skipjack tuna. The movement of68