Comparison of snorkeling and electrofishing for estimating abundance and size structure of brook trout and brown trout in two southern New England streams

Abstract

Abundance and size structure of brook trout Salvelinus fontinalis and brown trout Salmo trutta determined by snorkeling were compared with estimates determined by electrofishing depletion in 30 habitat units of two Connecticut streams. Snorkel counts were significantly correlated ( P<0.001) to electrofishing depletion estimates for two length groups of each species, although the strength of correlations varied between species, streams, and length groups of trout. Some regression equations that predicted electrofishing depletion estimates from snorkel counts were significantly improved with the addition of independent variables describing large woody debris. Regression parameters indicated that snorkel counts were underestimated in habitat units that contained substantial amounts of large woody debris. Overall, length–frequency distributions obtained by snorkeling were similar to those obtained by electrofishing. In one case, the frequency of brown trout less than 15 cm determined by snorkeling was slightly overestimated relative to electrofishing, while the frequency of 15.0–22.9 cm brown trout was slightly underestimated by snorkeling relative to electrofishing. These results suggest that snorkel counts may be a reliable alternative to electrofishing depletion for estimating trout abundance and size structure in habitat units of small- to mid-size Connecticut streams. However, the consideration of woody debris variables may significantly improve the predictive ability of electrofishing depletion estimates based on snorkel counts.