An integrated approach to the foraging ecology of marine birds and mammals

Birds and mammals are important components of pelagic marine ecosystems, bu t our knowledge of their foraging ecology is limited. We distinguish six di stinct types of data that can be used in various combinations to understand their foraging behavior and ecology. We describe methods that combine conc urrent dive recorder deployment, oceanographic sampling, and hydroacoustic surveys to generate hypotheses about interactions between the physical envi ronment and the distribution, abundance, and behavior of pelagic predators and their prey. Our approach is to (1) map the distribution of whales in re lation to the distribution of their prey and the physical features of the s tudy area (bottom topography, temperature, and salinity); and (2) measure t he foraging behavior and diet of instrumented whales in the context of the fine-scale distribution and composition of their prey and the physical envi ronment. We use this approach to demonstrate a relationship between blue wh ale distribution, sea surface temperature, and concentrations of their euph ausiid prey at different spatial scales offshore of the Channel Islands, Ca lifornia. Blue whale horizontal spatial distribution was correlated with re gions of high acoustic backscatter. Blue whale dive depths closely tracked the depth distribution of krill. Net sampling and whale diet revealed that whales fed exclusively upon dense schools of Euphausia pacifica (between 10 0 and 200 m) and Thysanoessa spinifera (from the surface to 100 m). Whales concentrated foraging efforts upon those dense euphausiid schools that form downstream from an upwelling center in close proximity to regions of steep topographic relief We propose that (1) the distribution of Balaenoptera wh ales in the coastal California Current region is defined by their attractio n to areas of predictably high prey density; (2) the preferred prey of thes e whales are several species of euphausiids (E. pacifica, T. spinifera, and N. simplex) that are abundant in the California Current region; (3) blue w hales concentrate their foraging efforts on dense aggregations of euphausii ds found at discrete depths in the water column; (4) these localized areas of high euphausiid densities are predictable and sustained by enhanced leve ls of primary productivity in regions which are located downstream from coa stal upwelling centers (indicated by sea surface temperature); (5) topograp hic breaks in the continental shelf located downstream from these upwelling centers work in concert with euphausiid behavior to collect and maintain l arge concentrations of euphausiids swarms, and (6) despite seasonal and int er-annual variability, these processes are sufficiently consistent that the distribution of Balaenoptera whales can be predicted. (C) 1998 Elsevier Sc ience Ltd. All rights reserved.