Physical forcing and the dynamics of the pelagic ecosystem in the eastern tropical Pacific: simulations with ENSO-scale and global-warming climate drivers

We used a model of the pelagic ecosystem in the eastern tropical Pacific Ocean to explore how climate variation at El Niño – Southern Oscillation (ENSO) scales might affect animals at middle and upper trophic levels. We developed two physical-forcing scenarios: (1) physical effects on phytoplankton biomass and (2) simultaneous physical effects on phytoplankton biomass and predator recruitment. We simulated the effects of climate-anomaly pulses, climate cycles, and global warming. Pulses caused oscillations to propagate through the ecosystem; cycles affected the shapes of these oscillations; and warming caused trends. We concluded that biomass trajectories of single populations at middle and upper trophic levels cannot be used to detect bottom-up effects, that direct physical effects on predator recruitment can be the dominant source of interannual variability in pelagic ecosystems, that such direct effects may dampen top-down control by fisheries, and that predictions about the effects of climate change may be misleading if fishing mortality is not considered. Predictions from ecosystem models are sensitive to the relative strengths of indirect and direct physical effects on middle and upper trophic levels. 1175 Résumé : Un modèle de l’écosystème pélagique du Pacifique oriental tropical nous a permis d’explorer comment des variations à l’échelle d’El Niño et de l’oscillation australe (ENSO) peuvent affecter les animaux des niveaux trophiques moyens et supérieurs. Nous avons imaginé deux scénarios de forçage physique : (1) les effets physiques sur la biomasse du phytoplancton et (2) les effets physiques simultanés sur la biomasse du phytoplancton et le recrutement des prédateurs. Nous avons simulé les effets des pulsations des anomalies climatique, de cycles climatiques et du réchauffement global. Les pulsations engendrent des oscillations qui se propagent dans l’écosystème, les cycles modifient la forme des oscillations et le réchauffement global engendre des tendances. Nos conclusions sont les suivantes : les trajectoires de la biomasse de populations isolées des niveaux trophiques moyens ou supérieurs ne peuvent servir à déceler les effets ascendants; les effets physiques qui affectent directement le recrutement des prédateurs peuvent être la source principale de la variation inter-annuelle dans les écosystèmes pélagiques; de tels effets directs peuvent tamponner le contrôle descendant exercé par la pêche commerciale; les prédictions sur les effets des changements climatiques peuvent être trompeurs, si on ne tient pas compte de la mortalité des poissons due à la pêche. Les prédictions des Can. J. Fish. Aquat. Sci. 60: 1161–1175 (2003) doi: 10.1139/F03-100 © 2003 NRC Canada 1161 Received 2 August 2002. Accepted 19 July 2003. Published on the NRC Research Press Web site at http://cjfas.nrc.ca on 21 October 2003. J17023 G.M. Watters1,2 and R.J. Olson. Inter-American Tropical Tuna Commission, 8604 La Jolla Shores Drive, La Jolla, CA 92037, U.S.A. R.C. Francis. School of Aquatic and Fisheries Sciences, University of Washington, Box 355020, Seattle, WA 98195, U.S.A. P.C. Fiedler and S.B. Reilly. NOAA Fisheries, Southwest Fisheries Science Center, P.O. Box 271, La Jolla, CA 92038, U.S.A. J.J. Polovina and C.H. Boggs. NOAA Fisheries, Honolulu Laboratory, 2570 Dole Street, Honolulu, HI 96822, U.S.A. K.Y. Aydin. School of Aquatic and Fisheries Sciences, University of Washington, Box 355020, Seattle, WA 98195, U.S.A., and NOAA Fisheries, Alaska Fisheries Science Center, 7600 Sandpoint Way NE, Seattle, WA 98115, U.S.A. T.E. Essington. NOAA Fisheries, Honolulu Laboratory, 2570 Dole Street, Honolulu, HI 96822, U.S.A., Center for Limnology, University of Wisconsin, 680 North Park Street, Madison, WI 53706, U.S.A., and Marine Sciences Research Center, Stony Brook University, Stony Brook, NY 11794, U.S.A. C.J. Walters. Fisheries Centre, University of British Columbia, 6660 NW Marine Drive, Building 022, Vancouver, BC 6VT 124, Canada. J.F. Kitchell. Center for Limnology, University of Wisconsin, 680 North Park Street, Madison, WI 53706, U.S.A. 1Corresponding author (e-mail: [email protected]). 2Present address: NOAA Fisheries, Pacific Fisheries Environmental Laboratory, 1352 Lighthouse Avenue, Pacific Grove, CA 93950, U.S.A. J:\cjfas\cjfas60\cjfas6009\F03-100.vp October 15, 2003 3:08:07 PM Color profile: Disabled Composite Default screen