Assessing the Benefits of Valley‐Bottom Restoration for Salmonids Using Spatially Explicit, Individual‐Based Modeling

ABSTRACT As attention to valley‐bottom or Stage‐0 restoration increases, direct attempts to evaluate its effects on highly valued salmonid fishes remain rare. We evaluated a valley‐bottom restoration project at Whychus Creek, Oregon, by simulating its effects on the abundance of both resident trout (native Rainbow Trout Oncorhynchus mykiss and introduced Brown Trout Salmo trutta ) that currently occupy the site and anadromous Chinook Salmon O. tshawytscha that historically occupied the site. Specifically, we compared results from a spatially explicit, individual‐based model for pre‐restoration versus post‐restoration conditions in a 650‐m reach of stream within the larger restoration area. For both pre‐ and post‐restoration conditions, we explicitly represented the entire areas inundated by the highest streamflow that occurred during a 15‐year simulation period. The model represented the entire lifecycle of resident trout and the part of the lifecycle of Chinook Salmon that would occur at the study site, from the arrival of adults to the emigration of juvenile fish; simulations concurrently included all three salmonid species. The simulations indicated large benefits of restoration for all three species; model results for trout biomass corresponded with single empirical estimates of abundance before and after restoration. However, simulations also yielded year‐class failures for both species of resident trout under both pre‐ and post‐restoration conditions. This last result highlights the ability of mechanistic individual‐based models to not only estimate the effects of restoration on populations of special concern, but also to place those effects in the context of the broad array of factors affecting populations.