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

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.