FISHERIES OCEANOGRAPHY Fish. Oceanogr. 10 (Suppl. 1), 1–13, 2001 © 2001 Blackwell Science Ltd. 1 Blackwell Science Ltd Ecosystem controls of juvenile pink salmon (Onchorynchus gorbuscha) and Pacific herring (Clupea pallasi) populations in Prince William Sound, Alaska ROBERT T. COONEY, 1, * J. R. ALLEN, 2 M. A. BISHOP, 3 D. L. ESLINGER, 4 T. KLINE, 5 B. L. NORCROSS, 1 C. P. MCROY, 1 J. MILTON, 6 J. OLSEN, 6 V. PATRICK, 7 A. J. PAUL, 1 D. SALMON, 8 D. SCHEEL, 9 G. L. THOMAS, 5 S. L. VAUGHAN 5 AND T. M. WILLETTE 10 1 Institute of Marine Science, University of Alaska Fairbanks, Fairbanks, AK 99775–7220, USA 2 Alaska Digital Graphics, Anchorage, AK 99521, USA 3 Copper River Delta Institute, Cordova, AK 99574, USA 4 NOAA Coastal Services Center, Charleston, SC 29405–2413, USA 5 Prince William Sound Science Center, Cordova, AK 99574, USA 6 Prince William Sound Aquaculture Corporation, Cordova, AK 99574, USA 7 University of Maryland, Advanced Visualization Laboratory, College Park, MD 20742, USA 8 Presently unafilliated, Cordova, AK 99574, USA 9 Department of Biology, Alaska Pacific University, Anchorage, AK 99520, USA 10 Alaska Department of Fish and Game, Soldotna, AK 99699, USA ABSTRACT Five years of field, laboratory, and numerical modelling studies demonstrated ecosystem-level mechanisms influ- encing the mortality of juvenile pink salmon and Pacific herring. Both species are prey for other fishes, seabirds, and marine mammals in Prince William Sound. We identified critical time-space linkages between the juve- nile stages of pink salmon and herring rearing in shallow- water nursery areas and seasonally varying ocean state, the availability of appropriate zooplankton forage, and the kinds and numbers of predators. These relationships defined unique habitat dependencies for juveniles whose survivals were strongly linked to growth rates, energy reserves, and seasonal trophic sheltering from predators. We found that juvenile herring were subject to substan- tial starvation losses during a winter period of plankton diminishment, and that predation on juvenile pink salmon was closely linked to the availability of altern- ative prey for fish and bird predators. Our collaborative study further revealed that juvenile pink salmon and age- 0 herring exploit very different portions of the annual production cycle. Juvenile pink salmon targeted the cool-water, early spring plankton bloom dominated by diatoms and large calanoid copepods, whereas young-of- the-year juvenile herring were dependent on warmer conditions occurring later in the postbloom summer and fall when zooplankton was composed of smaller cala- noids and a diversity of other taxa. The synopsis of our studies presented in this volume speaks to contemporary issues facing investigators of fish ecosystems, including juvenile fishes, and offers new insight into problems of bottom-up and top-down control. In aggregate, our results point to the importance of seeking mechanistic rather than correlative understandings of complex natural systems. Key words: Salmon, herring, Alaska, Prince William Sound INTRODUCTION At a time when marine ecosystems are under increasing threat from over-fishing and growing levels of pollution, and questions about climate change and its effects are being debated, there is an accelerating need to better understand the fundamental mechanisms whereby envir- onmental perturbations are translated into biological change on time scales of seasons, years, and decades. This is not a new inquiry. The roots of modern fisheries oceanography extend back to the great cod, herring and plaice fisheries of the late 1800s and early 1900s in the North and Baltic Seas (Hjort, 1914; Hardy, 1959). A rich traditional knowledge derived from fishermen pointed to linkages predictive of fishing success even then: ‘Strongly weedy water acts negatively upon the shoal- ing of the herring. Therefore in May and June, slimy bright green or brown-green plankton indicator discs as well as green mists in samples collected by Hensen nets, with a rather sharp smell, characterize waters unfavourable for the fishing of herring’ (Manteufel, 1941). These early qualitative observations, and later those describing relationships between ocean currents, spawn- ing grounds and nursery areas for commercial species led *Correspondence. windsong@montana.com