Modelling carrying capacity dynamics for the conservation and management of territorial salmonids

Inherent in the carrying capacity notion is the basic idea of a maximum population a particular level of resources can support over a period of time. Knowledge of carrying capacity is essential for wildlife conservation since it is intrinsic in determining how much habitat must be conserved to maintain healthy populations. Further, this concept has been the cornerstone of the management of exploited animal and plant populations. Yet the question about what determines carrying capacity for territorial species and how it can be quantified has been long neglected by ecological research. We propose a novel method to model carrying capacity dynamics for territorial salmonids, which can be further applied to any territorial species as long as they are principally limited by habitat conditions. In our model, maximum abundance is limited by environmentally induced fluctuating habitat conditions and regulated through territorial behaviour. Carrying capacity is estimated as the amount of habitat available divided by the expected individual territory area for a given life stage. We tested whether the model was capable of explaining the annual fluctuations in densities of brown trout Salmo trutta from 12 Mediterranean populations for a 12-year study period. We observed not only that density of the different life stages tracked carrying capacity dynamics, but also that the eventual cohort performance was affected by both intercohort competition and intensity of intracohort competition experienced in the previous year. Likewise, recruitment depended on the levels of carrying capacity saturation experienced by adult stock the year before. In any case, resilience decreased with carrying capacity. Such results suggest that restoration measures attempting to increase population abundance through stocking, increased breeding dispersion or cohort survival may reduce the performance of both the enhanced and competing cohorts. Further, high exploitation rates may lead populations occurring at low carrying capacities to extinction.