A strategy for simulating brown trout population dynamics and habitat quality in an urbanizing watershed

Abstract

A mathematical model was developed to simulate the population dynamics of an allopatric brown trout Salmo trutta population in the Paint Branch of Montgomery and Prince Georges counties, Maryland. The model relates land disturbing activities and imperviousness in the watershed to trout habitat quality through generalized relationships involving sediment transport, hydrology, and water temperature. Habitat quality then influences population dynamics. The model operates in three sectors: watershed, habitat, and population. The sectors are replicated at six sites along the longitudinal gradient of the stream. Watershed effects related to hydrology and sediment transport are accumulated from one sub-watershed to the next. The habitat sector is the Habitat Suitability Index (HSI) for brown trout (Raleigh et al., 1986, Habitat suitability index models and instream flow suitability curves: brown trout. US Fish Wildl. Ser., Biol. Rep. 82 (10.124)). The author collected field measurements on 18 HSI parameters for input to the model. The HSI values then are dynamically adjusted through time according to impacting factors from the watershed sector. These adjusted HSI values determine population growth by affecting probabilities of reproduction, recruitment through the life stages, and mortality. The model was verified by comparing modelled and actual population data from a single site over the past 17 years. Simulations represent various watershed development strategies for the next 20 years. General trends in the modelled adult population suggest that population recovery after a construction event is achieved in watersheds of moderate imperviousness (<15%) but not in more densely developed watersheds.