Effectiveness of Night Snorkeling for Estimating Steelhead Parr Abundance in a Large River Basin

The refinement of methodologies for estimating the abundance of juvenile salmonids in small streams has been an important area of fisheries research, but the development of methods suitable for larger streams has received insufficient attention. Using a novel approach for obtaining marked populations of fish, we evaluated the effectiveness of night snorkeling counts for estimating the abundance of steelhead Oncorhynchus mykiss parr in streams within a large river basin. Sampled streams ranged from headwater tributaries with wetted widths of less than 10 m to a large, main-stem river with a wetted width approaching 100 m. Estimates of snorkeling detection probability for steelhead parr were consistently high (overall mean = 0.65) and exhibited moderately low variability among sites (coefficient of variation = 0.24). Detection probability exhibited a dome-shaped relationship to fork length and declined with increasing cross-sectional site area. The high and relatively consistent detection probabilities we estimated indicate that night snorkeling can be an effective technique for estimating the basinwide abundance of steelhead parr.