PARENT SESSION
Contributed Oral Session 149: Evolutionary Ecology: Modeling
Friday, August 12, 8:00 AM - 11:30 AM, Meeting Room 511 C, Level 5, Palais des congrès de Montréal

Harvesting physiologically structured populations with density-dependent development.

van Kooten, Tobias ^{*,1, 2}, Persson, Lennart¹, de Roos, Andre², ¹ Department of Ecology and Environmental Science, Umea, Sweden² Institute for Biodiversity and Ecosystem Dynamics, Amsterdam, The Netherlands

ABSTRACT- Sustainable exploitation of natural populations is based on the assumption that at some intermediate population density density-dependence is relaxed and both population growth rate and biomass production are maximized. The excess, harvestable production is referred to as the maximum sustainable yield (MSY). The direct coupling between population density and population production is, however, not accurate. High densities and thus low food availabilities primarily affect the life history of individuals, and not the population growth rate. We study a size structured population model, which allows us to explicitly incorporate density dependent physiological development of individuals, rather than an approximation of the population level effects. This approach simultaneously allows us to explicitly incorporate the size dependence of most exploitation strategies (e.g. mesh size or bait size in fisheries). We show that size selective harvesting of a population with resource dependent life history strongly affects the life history both of the target species and of other species it interacts with. Our results further indicate that due to the buffering properties of resource dependent individual growth, the total biomass yield always increases with increased harvesting intensity, up to a critical intensity where the buffering effect fails and the population suddenly goes extinct, suggesting that management aiming at MSY may jeopardize the survival of the exploited population. More generally, we show that the total biomass yield or the catch per unit effort, two commonly used predictors in fisheries management, are highly unreliable indicators of the state of the exploited population. Fishermen often argue against fishing regulations stating that the catch per unit effort is not declining with increased effort. We provide a possible explanation for this observation. To correctly assess the risk of overexploitation, detailed knowledge about the size distribution of both the exploited population and about populations with which it has strong trophic interactions are a necessity.

Key words: harvesting, structured population dynamics, density-dependent development