Hankin and Reeves’ Approach to Estimating Fish Abundance in Small Streams: Limitations and Alternatives

Abstract.—Hankin and Reeves ’ (1988) approach to estimating fish abundance in small streams has been applied in stream fish studies across North America. However, their population estimator relies on two key assumptions: (1) removal estimates are equal to the true numbers of fish, and (2) removal estimates are highly correlated with snorkel counts within a subset of sampled stream units. Violations of these assumptions may produce suspect results. To determine possible sources of the assumption violations, I used data on the abundance of steelhead Oncorhynchus mykiss from Hankin and Reeves ’ (1988) in a simulation composed of 50,000 repeated, stratified systematic random samples from a spatially clustered distribution. The simulation was used to investigate effects of a range of removal estimates, from 75 % to 100 % of true fish abundance, on overall stream fish population estimates. The effects of various categories of removal-estimates-to-snorkel-count correlation levels (r 5 0.75–1.0) on fish population estimates were also explored. Simulation results indicated that Hankin and Reeves ’ approach may produce poor results unless removal estimates exceed at least 85 % of the true number of fish within sampled units and unless correlations between removal estimates and snorkel counts are at least 0.90. A potential modification to Hankin and Reeves ’ approach is the inclusion of environmental covariates that affect detection rates o