Individual-based model of stream-resident rainbow trout and brook char: model description, corroboration, and effects of sympatry and spawning season duration

An individual-based model of the population dynamics of sympatric rainbow trout (Oncorhynchus mykiss) and brook char (Salvelinus fontinalis) is described and analyzed. The model simulates daily growth, mortality, movement, and spawning over the full life cycle of each species for 100 years in a compartmentalized, hypothetical stream configured for the southern Appalachian mountains, USA. Egg and alevin development is temperature-dependent with mortality having constant, spatial, and temperature-dependent components. Daily growth of fry, juveniles, and adults is based on bioenergetics and consumption of drift prey. Mortality rate of fry through adults decreases with length. Model predictions of densities, growth, age, and size structure were similar to those observed in southern Appalachian streams. Five different conditions were simulated to explore the population dynamics and competition between the two species: (1) sympatric populations (baseline), (2) allopatric brook char, (3) allopatric rainbow trout, (4) and (5) sympatric populations with reduced or increased spawning season durations. Results indicated that density-dependence mainly operated during the fry and juvenile stages. Brook char were more affected by interspecific competition than rainbow trout, and crowding of fry negatively affected brook char (with little effect on rainbow trout), whereas low fry density favored brook char.