The New PFRP Tag: Using Stable Isotope Techniques to Better Understand the Trophic Ecology and Migration Patterns of Tropical Tunas

(continued on page 2) Trade winds generally blow to the west across the equatorial Pacific Ocean, piling up warm surface water in the west. In addition, there is a divergence of surface water at the equator due to Ekman transport away from this region. These forces set up a strong upwelling zone that extends westward from the coast of South America. This upwelling zone eventually encounters a large pool of warmer water in the west warm pool-cold tongue system (WPCT) (Figure 1). The eastern cold-tongue system is characterized by high levels of primary production, whereas the western warm pool, a region that supports highly productive tuna populations, has lower levels of primary production. Tuna forage accumulates in the convergence zone between the warm pool and equatorial upwelling boundary. Lehodey et al. (1997) demonstrated that movements of tuna populations are linked to large zonal shifts of the WPCT boundary that are driven by inter-annual variability in the trade wind strength (i.e., ENSO events). Within this dynamic WPCT system, the objectives of our PFRP study are to 1) define the trophic structure of the pelagic ecosystem in different regions and 2) establish isotope-derived, biogeography of pelagic predators. By doing so, we will establish the major ecosystem linkages in the equatorial Pacific and gain insight into how energy and organic matter flow through the upper trophic levels. Finally, the information from our first two goals will form the basis for identifying natural isotope “tags” useful to 3) characterize regional residency and migration patterns of tropical tunas and other apex predators.