In-stream habitat restoration effectiveness studies primarily rely upon the observation of occupancy of restored habitat relative to unrestored habitat. In these observational studies, the effects of restoration can be inconsistent and limited, as we have observed in a multi-year study of restoration in the Entiat River, Washington. Furthermore, quantification of distribution patterns does not address whether traits correlated with the overall fitness of individuals (e.g., growth, survival) are enhanced by their occupancy of restored habitat. We used mark-recapture assays in multiple study years to measure growth of individual young-of-the-year Chinook Salmon (Oncorhynchus tschawytscha) and Steelhead Trout (O. mykiss) in restored and unrestored habitat in the Entiat River. However, comparison of raw growth rates was limited by low recapture rates in unrestored habitat, particularly for Chinook. Thus we developed a mechanistic size-over-time model that we applied to the within-season capture and recapture data with a Bayesian fitting routine. For steelhead, we had enough data in each habitat type to calculate size-corrected growth rates, in an effort to validate the model. Finally, we compared findings of growth patterns to previous work showing occupancy variation between restored and unrestored habitat in this study system.
Across five sampling years (2009, 2010, 2012, 2013 and 2016), estimation of model parameters demonstrated annual variability in growth, but that Chinook showing fidelity (15-60 d) to restored habitat reached a larger size earlier in the season than individuals not recaptured (2009, 2010 and 2013 only). By the end of the growing seasons, individuals did not differ in total length regardless of habitat occupancy. When we compared steelhead recaptured in restored habitat to those recaptured in unrestored habitat, model parameters indicated a more rapid increase in size of individuals in restored habitat, but only in 2009 and 2010. Again, there were no differences associated with habitats in the total size reached by individuals at the end of each season. Size-corrected raw growth rates (mm/day) corroborated the model results in 2009 and 2010 and that no other year showed a difference. Data for each study year showed that occupancy favored restored habitat except for steelhead in 2010; thus, growth results did not always corroborate occupancy patterns. 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 in occupancy data.