Accuracy of Underwater Counts of Juvenile Chinook Salmon, Coho Salmon, and Steelhead

Underwater observers counted juvenile salmonids in study areas on mid-Columbia River tributaries, and the accuracy of these counts was examined. We compared counts taken by two to six experienced snorkelers with counts obtained after use of sodium cyanide in the same sites at stream temperatures that ranged from 5.6 to 18.3°C. We found no significant relationships between the accuracy of visual counts of juvenile salmonids and discharge, surface area, or time of day that we snorkeled. However, the accuracy of visual counts of age-0 chinook salmon Oncorhynchus tshawytscha, age-0 steelhead O. mykiss, and age-1 and older (age-1+) steelhead, but not of age-0 coho salmon O. kisutch. declined significantly with decreased temperatures. At temperatures above 14°C, snorkelers counted about 70% of the juvenile salmonids present. Below 14°C, most counts included under half the fish present, and below 9°C, snorkelers observed less than 20% of the fish present. At temperatures above 14°C, accuracy of snorkel counts among the three species varied, apparently because of differences in fish size and behavior. Over 90% of the salmon were counted if they occurred in clusters of fewer than 40 fish, but coho and chinook salmon were undercounted by 50% or more if they resided in mixed groups of more than 40 fish. Snorkelers underestimated the numbers of small steelhead (<100 mm in fork length) by 40% or more if these fish occupied stations in water shallower than 15 cm. In stations with abundant cover, we underestimated age-1 + steelbead numbers by 60%.