Analysis of Individual Fish Growth Reveals Variable Efficacy of in-Stream Habitat Restoration

Multi-year data in the Entiat River, Washington, USA, show increased occupancy by young-of-the-year Steelhead Trout and Chinook Salmon in pools formed by restoration structures compared with unrestored pools. This pattern is temporally variable and spatially localized; thus, we used mark/recapture assays to compare individual growth in restored vs. unrestored habitat during the summer rearing season in multiple study years and to compare growth with previous observations of occupancy. Steelhead growth was higher in restored pools in two study years, including one in which there was no occupancy difference among habitats. In a third, steelhead were more abundant in restored pools without a difference in growth. Growth was strongly size-dependent: after reaching 55-60 mm in length, steelhead grew at the same rate in both habitat types, resulting in similarly sized fish. Chinook recaptures in unrestored habitat were insufficient to compare habitats; however analysis of size data through the season showed a similar pattern to steelhead in which the primary growth benefit of restored habitat occurred at smaller sizes. Growth analysis is important to the evaluation of restoration efficacy because it can occasionally detect benefits when abundance data are inconclusive and can also prevent overconfidence when occupancy differences among habitats are observed.