Ecology, 92(12), 2011, pp. 2175–2182 Ó 2011 by the Ecological Society of America Stage-specific biomass overcompensation by juveniles in response to increased adult mortality in a wild fish population JAN OHLBERGER, 1,5 ØYSTEIN LANGANGEN, 1 ERIC EDELINE, 2 DAVID CLAESSEN, 3 IAN J. WINFIELD, 4 NILS CHR.STENSETH, 1 AND L. ASBJøRN VøLLESTAD 1 1 Centre for Ecological and Evolutionary Synthesis (CEES), Department of Biology, University of Oslo, P.O. Box 1066 Blindern, 0316 Oslo, Norway 2 UPMC-Paris6, UMR (CNRS) 7618, Laboratoire Bioge ´ochimie et Ecologie des Milieux Continentaux, 46 Rue d’Ulm, F-75230 Paris Cedex 05, France 3 Environmental Research and Teaching Institute, Laboratoire d’Ecologie et Evolution (UMR 7625), Ecole Normale Supe ´rieure, 24 Rue Lhomond, F-75230 Paris Cedex 05, France 4 Centre for Ecology and Hydrology, Lancaster Environment Centre, Library Avenue, Bailrigg, Lancaster, Lancashire LA1 4AP United Kingdom Abstract. Recently developed theoretical models of stage-structured consumer–resource systems have shown that stage-specific biomass overcompensation can arise in response to increased mortality rates. We parameterized a stage-structured population model to simulate the effects of increased adult mortality caused by a pathogen outbreak in the perch (Perca fluviatilis) population of Windermere (UK) in 1976. The model predicts biomass overcompensation by juveniles in response to increased adult mortality due to a shift in food-dependent growth and reproduction rates. Considering cannibalism between life stages in the model reinforces this compensatory response due to the release from predation on juveniles at high mortality rates. These model predictions are matched by our analysis of a 60- year time series of scientific monitoring of Windermere perch, which shows that the pathogen outbreak induced a strong decrease in adult biomass and a corresponding increase in juvenile biomass. Age-specific adult fecundity and size at age were higher after than before the disease outbreak, suggesting that the pathogen-induced mortality released adult perch from competition, thereby increasing somatic and reproductive growth. Higher juvenile survival after the pathogen outbreak due to a release from cannibalism likely contributed to the observed biomass overcompensation. Our findings have general implications for predicting population- and community-level responses to increased size-selective mortality caused by exploitation or disease outbreaks. Key words: biomass model; compensatory response; infectious disease; life history; maturation; mortality; pathogen; perch (Perca fluviatilis); reproduction; stage structure. INTRODUCTION Increasing mortality is expected to decrease popula- tion density because individuals are removed from the population. However, besides the negative direct effect on population numbers, mortality also releases the surviving individuals from competition for resources. If growth, maturation, and/or reproduction are food- dependent processes, this indirect density-dependent effect may lead to higher growth rates, faster matura- tion, and/or increased adult fecundity. Such indirect effects of mortality may result in increased stage-specific biomass production thereby compensating for the removal of individuals from the population (Werner and Gilliam 1984, De Roos et al. 2007). Experimental studies on laboratory populations have shown that stage-specific biomass remains unchanged or increases if not all individuals are subjected to increased mortality rates, for instance in water fleas (Slobodkin and Rich- man 1956), blowflies (Nicholson 1957), soil mites (Cameron and Benton 2004), and fish (Schro¨der et al. 2009). Differences in mortality rates between size classes or life stages of natural populations are commonly observed in nature and may arise from size-specific predation (Brooks and Dodson 1965, Werner and Gilliam 1984), harvesting by humans (Law 2000, Fenberg and Roy 2008), or parasite infections (Ohl- berger et al. 2011b). Recently developed theoretical models of stage- structured consumer–resource systems have shown that compensation (unchanged biomass) or overcompensa- tion (increased biomass) in stage-specific biomass can arise in response to increased mortality rates (De Roos et al. 2007, 2008b). This phenomenon occurs in populations in which one or more of the life stages (e.g., juveniles and adults) are limited by intraspecific competition at high population densities. Biomass compensation or overcompensation emerges in the life Manuscript received 2 March 2011; revised 6 July 2011; accepted 13 July 2011. Corresponding Editor: M. A. Hixon. 5 E-mail: jan.ohlberger@bio.uio.no 2175 R eports