Accuracy of snorkelling counts in assessing spawning stock of Atlantic salmon,Salmo salar,verified by radio‐tagging and underwater video monitoring

Abstract The accuracy of underwater snorkel counts of adult Atlantic salmon, Salmo salar L., was analysed in a small tributary of the River Teno before and during the spawning period using two independent verification methods (video counts and radio‐tagged fish) with known numbers of fish. The observation efficiency of snorkelling crews was generally high, but that of an inexperienced crew (65–72%) was significantly lower than an experienced crew (81–82%). Habitat type affected the accuracy of snorkelers, especially those of the inexperienced crew. The efficiency of snorkelers was high in pool‐type habitats (75–100%) but decreased considerably in faster flowing turbulent habitats (43–82%), where the inexperienced crew provided the lowest estimates. Thus, snorkel counting is a valid method for estimating Atlantic salmon spawning population densities in small rivers with adequate underwater visibility and with a sufficiently experienced snorkelling crew.