Response of trout, sculpins, and salamanders to experimental manipulation of large wood in Cascade Mountain streams

Response of aquatic vertebrates to increased pool habitat complexity due to abundance of large wood was evaluated experimentally in three streams in the Cascade Mountains of Oregon. No difference in aquatic vertebrate density was detected among treatments, though there was a trend of increasing aquatic vertebrate density with increasing large wood abundance. The small change in density of aquatic vertebrate species may have been due to the overriding influence of the high habitat heterogeneity associated with cobble/ boulder substrate in these streams. Large wood may have a stronger influence in simpler streams with smaller particle size, in differing environmental regimes, or at spatial scales larger than channel unit. Mark-recapture estimates, catchability-based estimates, and maximum likelihood multiple removal estimates were compared in all three streams. Physical heterogeneity influenced efficiency of capture. The single pass catchability-based estimates were larger than mark-recapture estimates and maximum likelihood multiple removal estimates. Mark-recapture estimates were larger than maximum likelihood multiple removal estimates. The mark-recapture method gives more reliable and accurate estimates than maximum likelihood multiple removal method, but both methods may substantially underestimate the population. Taxon, fish length, and habitat variables influenced catchability for the population estimators examined. Probability of capture of vertebrates for the catchability-based estimator consistently decreased with increasing habitat complexity.
Understanding factors that affect the accuracy of population estimators and the impact that habitat may have on catchability will assist fisheries scientists and managers in making informed decisions regarding the selection of a population estimation method. Research on habitat restoration and habitat relationships must recognize limitations of population estimators and catchability. Recognition of the limitations of population estimators and catchability should allow resource scientists to design more reliable monitoring and evaluation of habitat restoration projects.