Comparison of Day Snorkeling, Night Snorkeling, and Electrofishing to Estimate Bull Trout Abundance and Size Structure in a Second-Order Idaho Stream

Abstract

Biologists lack sufficient information to develop protocols for sampling the abundance and size structure of bull trout Salvelinus confluentus. We compared summer estimates of the abundance and size structure of bull trout in a second-order central Idaho stream, derived by day snorkeling, night snorkeling, and electrofishing. We also examined the influence of water temperature and habitat type on day and night counts of bull trout. Electrofishing yielded the largest estimates of abundance of age- 1 and older bull trout. Day snorkeling counts accounted for a mean of 75% and night snorkeling counts a mean of 77% of the fish estimated by electrofishing. Numbers of age- 1 and older bull trout observed during day counts did not differ from numbers observed at night. Water temperatures during underwater surveys were 9–13.5°C. Counts were not influenced by temperatures in this range; however, statistical power of the tests was low. The three sampling techniques yielded similar estimates of the size structure of the bull trout population. When compared with electrofishing, underwater estimates recorded fewer small fish and overestimated the size of some fish. We detected no significant interaction between the type of snorkeling count (day or night) and fish densities observed in different habitat types. Densities of bull trout observed during both day and night were similar, regardless of the habitat type. Under the conditions of water temperature, conductivity, visibility, and habitat in which we sampled, day snorkeling surveys were suitable for estimating the relative abundance and size structure of the bull trout population. Our results conflict with those of other studies that suggest night snorkeling is more effective than day snorkeling for censuring bull trout. Explanations for this discrepancy may include differences in stream temperature and stream channel features.