Use of a mechanistic growth model in fish habitat restoration: juvenile Chinook salmon and steelhead as a test case

Abstract

Abstract Few studies of in-stream habitat restoration effectiveness address whether traits that affect fitness of individuals (e.g., growth) are altered by restoring habitat. We used mark-recapture studies to measure growth of sub-yearling Chinook salmon and steelhead in restored and unrestored habitat in the Entiat River, Washington, USA. In each of five sampling years (2009, 2010, 2012, 2013, 2016), we compared growth rates between habitat types, using a mechanistic growth model which we fit to our data using Hamiltonian Monte Carlo (HMC) techniques. Modeling strategies differed for the two species: For Chinook, we compared growth patterns of individuals recaptured in restored habitat for 15-60 d with those not recaptured regardless of initial habitat at marking. For steelhead, we had enough recaptured fish in each habitat type to use the model to directly compare growth between habitats. Chinook juveniles remaining in restored habitat reached a larger size earlier in the season than transient individuals with all study years combined, but this pattern varied slightly among years. Steelhead showed more rapid growth in restored habitat in 2009 and 2010 only, but were of a larger size earlier in the season in unrestored habitat. We explain these patterns in the context of spatio-temporal partitioning habitat. Despite this, there was no case where fish performed better overall in unrestored habitat. Size-corrected raw growth rates for steelhead (mm · day−1) supported the model in that growth rate was higher in restored habitat in the same years as the corresponding model parameter. Finally, we compared growth patterns with the relative density of these species in each habitat type and found that growth did not always correspond to observed density differences. Chinook abundance was higher in restored habitat in each study year, but no growth parameter differences were found in 2012 or 2016. Steelhead growth and density both favored restored habitat in 2009, whereas growth was higher in restored habitat in 2010 when there was no density difference between habitats. Thus, measurement of growth rate complements restoration efficacy studies because it can both prevent overconfidence in density data and detect benefits when density data are inconclusive.