Apex Predators in the Pelagic Zone

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Exploitation of apex predators in pelagic food webs should result in a set of ecological changes at several scales. We will determine the dynamics of compensatory responses by apex predators (large tunas, billfishes and sharks) and some of their primary prey (squids, small tunas and juvenile mahimahi) in the pelagic food webs of the Central North Pacific (CNP). We will test for fishery-induced changes in growth rates, predator-prey interactions (diets), life history characteristics such as size or age at maturity, and species composition of the fish community. Analyses of food web dynamics will be used to evaluate the ecological consequences of changes in exploitation. We will use retrospective analyses of long-term data sets provided by collaboration with the Honolulu Laboratory, National Marine Fisheries Service. Responses to `pulse` manipulations will derive from analyses of recent evidence for major, rapid changes in exploitation effects, prey availability, and environmental effects (oceanic regime shifts). Responses to `press` manipulations of apex predator populations will derive from the 35-50 year records of basic biological data, life history studies and catch data maintained by NMFS. Contemporary data on growth rates, diets, life history characteristics, and species composition will be compared with those collected since 1948. This effort will employ three classes of analytical tools. Time-series methods and Bayesian models will be used to discern signal from noise and to identify the interactions of causative variables. Bioenergetics models will provide quantitative estimates of the changes in food web linkages for individual predators, their prey and the cumulative change for all members of the apex trophic guild. A new simulation model (Ecosim II) creates the food web context of an entire ecosystem and will be used to explore responses to changing mortality rates, different life history strategies and the role of a habitat refuge created around the Northwest Hawaiian Islands. The recent rise of conservationist concern for large predatory fishes is based on their unknown role in sustaining the ecological integrity of pelagic ecosystems. It is imperative that we better understand the effects of these migratory predators and their interactions with habitats encompassed by current and future refuges for pelagic fishes. That understanding must be set in the context of environmental changes that dampen or amplify food web effects.