Factors regulating brown trout populations in two French rivers: Application of a dynamic population model

A dynamic population model was developed to study the impact of biotic and abiotic environmental factors on changes in trout populations. The model is based on the Leslie Matrix and simulates population change by age class in terms of biological parameters (i.e. fish survival, fertility, growth rate s), which are dependent on environmental conditions. Changes in physical ha bitat, expressed as Weighted Usable Area, cause displacement of fish and in creased mortality. Calculations were made at 1-month intervals to account f or the effect of climatic variations on the population. The model was used to analyze the dynamics of two trout populations, quite different in terms of their biological characteristics: one in Lower Norman dy in the Oir watershed and the other in the Pyrenees Mountains in the Nest e d'Oueil watershed. Application of the model to those populations revealed two types of stabilizing mechanisms. The first was a capacity for populati on restoration, which is well-represented by the model through the phenomen on of density-dependent mortality in the first months of life. The second w as adjustment of the adult population to the carrying capacity of the envir onment. The two applications demonstrate the utility of this type of model for unde rstanding and simulating the dynamics of different cohorts of a population. Coupling habitat models and dynamic population models facilitates the iden tification of key periods during which carrying capacity-related to the hyd rology-becomes a limiting factor for fish. This brings new perspectives to water management and may facilitate analysis of instream flow requirements related to water development projects, such as hydropower plants. Copyright (C) 2001 John Wiley & Sons, Ltd.