Estimating Winter Salmonid Abundance in Small Western Washington Streams: A Comparison of Three Techniques

Abstract

We compared the relative efficiency of day snorkeling, night snorkeling, and multiple-removal electrofishing at estimating abundance of juvenile coho salmon Oncorhynchus kisutch and trout Oncorhynchus spp. during winter in four small western Washington streams. Salmonid abundance was estimated within 32 individual habitats at low to moderate winter flows and low water temperatures (<7.0°C). Day snorkel counts were significantly less than both night snorkel counts and multiple-removal electrofishing estimates. Night snorkel counts were not significantly different from electrofishing estimates, although the percentage of electrofishing estimates accounted for by night snorkeling varied by stream from 50% to 175% for coho salmon and from 75% to 82% for trout. The difference between electrofishing estimates and night snorkel counts was positively correlated with fish density for coho salmon, indicating that night snorkel counts were most similar to electrofishing estimates at densities less than 0.5 fish/m2. Fish length frequency distributions from night snorkeling and electrofishing were significantly different. However, when only fish 160 mm or shorter were examined, there was no significant difference in length frequency distributions. Night snorkeling was less labor intensive, taking only 26.8% of the time required by electrofishing. Night snorkeling appears to represent an accurate, cost effective, and benign method of enumerating juvenile salmonids during winter months in coastal streams that are less than 12 m in bank-full width and that are characterized by low fish densities, moderate flows, and low temperatures. In contrast, day snorkeling appears inadequate under most winter conditions because of daytime concealment behavior of salmonids at low temperatures.