Water‐Level Fluctuation Effects on Centrarchid Reproductive Success in Reservoirs: A Modeling Analysis

An individual‐based model of nesting smallmouth bass Micropterus dolomieu and white crappie Pomoxis annularis is used to compare reproductive success in a deep reservoir and a shallow reservoir under different amplitudes and periods of water‐level fluctuations. The model simulates nest site selection, egg deposition, and the subsequent development and survival of young from eggs through dispersal in a two‐dimensional spatial grid of habitat cells representing the reservoir bottom. All simulations are for May through September using a daily time step. Nest site (cell) selection is determined from species‐specific habitat suitability indices based on substrate, slope, depth, and structure. The development of the young is temperature dependent. Mortality occurs via attrition, abandonment, and whole‐nest catastrophe. Model corroboration was performed with observations of nesting white crappies and smallmouth bass in Brownlee Reservoir, Idaho–Oregon. The predicted egg‐to‐dispersal survival declined with increasing amplitude similarly for both species and more sharply in the shallow reservoir. The minimum survival across periods tended to occur at a period of about 5 d for white crappies but at about 10 d for smallmouth bass, which coincided with the early life stage developmental times for both species. The highest survival for both species occurred at a low amplitude and short period in the deep reservoir, while white crappie survival generally increased and smallmouth bass survival decreased with increasing period and amplitude in the shallow reservoir. We generalize our modeling results to centrarchids and suggest that the duration of key early life stages should be matched with the period and amplitude of water‐level fluctuations to maximize survival and that in some situations, trade‐offs between species survival rates will complicate management.