Snorkeling and Electrofishing: Comparing Cutthroat Population Estimate Methodologies at the Basin Scale

We compared abundance and distribution estimates based on snorkel count and multiple pass-removal electrofishing methods of coho and 1+ steelhead and cutthroat trout in Smith River, Oregon.  In each of six years, sampling was conducted during the summer low flow period.  Sample reaches were selected using a randomized, spatially balanced design.  Independent basin-wide estimates were based on electrofishing at an average of 24 sites/year and snorkeling at an average of 33 sites/year, representing 0.6% and 2.9% respectively of total salmonid rearing habitat.  Electrofishing was conducted in all habitat types in wadeable streams; snorkeling was limited to pools ≥6 m² area and ≥40 cm maximum depth in streams of all sizes.  Basin-wide cutthroat abundance estimates from electrofishing were an order of magnitude higher than estimates from snorkeling, mostly because cutthroat were often present outside of snorkeled pool habitats units and cutthroat have lower detection probabilities when snorkeling.  Snorkel detection probabilities compared to electrofishing were lowest for cutthroat (24%), moderate for steelhead (43%), highest for coho (74%).  Correcting for snorkel detection probability produced estimates that were not statistically different between methods.  We also investigated the variance in repeat sampling of survey reaches.  Correlation between counts in two independent surveys at the same site indicate repeatability is lower for cutthroat (R²=0.47) than steelhead (R²=0.80) or coho (R²=0.95).  Snorkeling has the ability to survey pool habitats too large for electrofishing, but these areas contributed only a small amount to the overall population estimate in this study.  This may be because areas with large pools in Smith River are bedrock dominated, lack complexity, and have high summer temperatures.  Cutthroat in Smith River were more evenly distributed than steelhead; cutthroat pool occupancies were higher for electrofishing (66%) than snorkeling (33%) but site occupancies were similar (75% and 78%).  Site occupancies for steelhead were higher for snorkeling than electrofishing (58% and 32%).  Snorkeling had advantages over electrofishing for steelhead because of their patchy distribution; the shorter electrofishing surveys were more likely to miss steelhead.  Lowering the criteria for snorkeled pools to ≥20 cm maximum depth increases consistency for all species.  Snorkel surveys for cutthroat could be effective if the detection probability is known.  However, issues with low detection probabilities and low repeatability when snorkeling outweigh the advantages of higher sample size and larger sampled area and we recommend electrofishing for cutthroat monitoring.