River Restoration Effects on Steelhead Populations in the Manistee River, Michigan: Analysis Using an Individual-Based Model

Abstract

The Manistee River, Michigan, watershed includes two dams as well as residential and agricultural development, and the river itself contains a sizeable population of steelhead Oncorhynchus mykiss that supports a valuable recreational fishery. Restoration of the Manistee River and its flow regime may improve steelhead habitat and the fishery. We developed an individual-based model of steelhead in the Manistee River to assess the population effects of changes in the number of spawners, water discharge from Tippy Dam, and water temperature. The model follows steelhead from spring spawning to the end of the growing season in early fall and depicts the river environment as a series of cells that vary in dimension, water velocity, and substrate. Simulated water discharge, temperature, and prey availability changed daily based on observations from Tippy Dam. Empirically based models describe individual steelhead fry and parr foraging and growth. In the model, steelhead select habitats and maximize individual fitness while accounting for dominance and the availability of feeding territories. We calibrated the model to replicate fish growth, mortality, and population size. Simulation experiments manipulated the number of spawning females, water discharge, and water temperature. The results suggest that Manistee River steelhead incur density limitations in the fry and parr stages and that water discharge and temperature changes affect the number and biomass of parr. Increasing river discharge negatively affected parr numbers and weight. Decreasing maximum midsummer temperature increased parr numbers and weight when the change was large, but otherwise had little effect. These results indicate that restoration of the natural flow regime in the Manistee River will probably increase the quality of the habitat for steelhead but that density limitations in the fry and parr stages may ultimately limit population growth.