*Corresponding author. Fax: 001 408 459 4882; e-mail: dcroll@cats.ucsc.edu Deep-Sea Research II 45 (1998) 1353 — 1371 An integrated approch to the foraging ecology of marine birds and mammals Donald A. Croll*, Bernie R. Tershy, Roger P. Hewitt, David A. Demer, Paul C. Fiedler, Susan E. Smith, Wesley Armstrong, Jacqueline M. Popp, Thomas Kiekhefer, Vanesa R. Lopez, Jorge Urban, Diane Gendron Institute of Marine Sciences, University of California, Santa Cruz, CA 95064 USA  Southwest Fisheries Science Center, National Marine Fisheries Service, 8604 La Jolla Shores Dr., La Jolla, CA 92037 USA Pacific Cetacean Group, Monterey CA, USA Universidad Autonoma de Baja California Sur, A.P. 19-B, La Paz, B.C.S., Mexico,USA Centro Interdisciplinario de Ciencias Marinas-IPN. A.P. 592, La Paz, B.C.S., Mexico, USA Received 3 June 1997; received in revised form 15 February 1998; accepted 1 March 1998 Abstract Birds and mammals are important components of pelagic marine ecosystems, but our knowledge of their foraging ecology is limited. We distinguish six distinct types of data that can be used in various combinations to understand their foraging behavior and ecology. We describe methods that combine concurrent dive recorder deployment, oceanographic sampling, and hydroacoustic surveys to generate hypotheses about interactions between the physical environment and the distribution, abundance, and behavior of pelagic predators and their prey. Our approach is to (1) map the distribution of whales in relation to the distribution of their prey and the physical features of the study area (bottom topography, temperature, and salinity); and (2) measure the foraging behavior and diet of instrumented whales in the context of the fine-scale distribution and composition of their prey and the physical environment. We use this approach to demonstrate a relationship between blue whale distribution, sea surface temper- ature, and concentrations of their euphausiid prey at different spatial scales offshore of the Channel Islands, California. Blue whale horizontal spatial distribution was correlated with regions of high acoustic backscatter. Blue whale dive depths closely tracked the depth distribu- tion of krill. Net sampling and whale diet revealed that whales fed exclusively upon dense schools of Euphausia pacifica (between 100 and 200 m) and ¹ hysanoessa spinifera (from the surface to 100 m). Whales concentrated foraging efforts upon those dense euphausiid schools 0967-0645/98/$ — see front matter  1998 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 4 5 ( 9 8 ) 0 0 0 3 1 - 9