Gallagher and Gallagher, 2005

Gallagher, S. P., and C. M. Gallagher. 2005. Discrimination of chinook salmon, coho salmon, and steelhead redds and evaluation of the use of redd data for estimating escapement in several unregulated streams in northern California. North American Journal of Fisheries Management 25:284-300.

We developed and evaluated a stratified index redd area method to estimate Chinook salmon Oncorhynchus tshawytscha, coho salmon O. kisutch, and steelhead O. mykiss escapement in several coastal streams in northern California based on the assumption that redd size is related to the number of redds a female builds. Sources of error in redd counts were identified, including the use of logistic regression to classify redd species (necessary due to temporal overlap in the spawning of these species in coastal northern California). Redd area escapement estimates were compared with estimates from more conventional methods and releases above a counting structure. Observer efficiency in redd detection ranged from 0.64 (SE = 0.10) to 0.75 (SE = 0.14) and was significantly associated with streamflow and water visibility (analysis of variance [ANOVA]: F = 41.8; P < 0.001). Logistic regression reduced uncertainty in redd identification. Redd area and date observed were significant in predicting coho salmon and steelhead redd species (Wald's z = 11.9 and 18.09, respectively; P < 0.001). Pot substrate and redd area were significant in classifying Chinook and coho salmon redds (Wald's z = 5.88 and 4.03; P = 0.015 and 0.04, respectively). Stratified index redd area escapement estimates and estimates based on capture-recapture experiments, area-under-the-curve estimates, and known releases above the counting structure (coho salmon only) were not significantly different (ANOVA: F < 13.6; P > 0.06). Escapement estimates assuming one redd per female were only significantly different from other methods for steelhead (ANOVA: F = 13.11; P = 0.006). Redd counts were significantly correlated with escapement estimates (r > 0.82; P < 0.04). Reduction of counting errors and uncertainty in redd identification, biweekly surveys throughout the spawning period, and the use of redd areas in a stratified index sampling design produced precise, reliable, and cost-effective escapement estimates for Chinook salmon, coho salmon, and steelhead.