Investigations of spawning migration and assessment of abundance of the Kamchatka steelhead (<Emphasis Type="Italic">Parasalmo mykiss</Emphasis>) from the Utkholok River by means of Didson dual-frequency identification sonar

Quantative estimates of abundance of anadromous Kamchatka steelhead Parasalmo mykiss in the period of its maximum spawning migration in the shallow Utkholok River by the instrumental method using Didson dual-frequency identification sonar are presented. Equipment and methods are described for the stationary observation of spawning run of Kamchatka steelhead through the river cross section and mobile surveys of the water area of the Utkholok R. As a result of stationary observations, the abundance and size composition of Kamchatka steelhead migrating to the upper reaches of the river are determined. Peculiarities of the diurnal dynamics of migration of Kamchatka steelhead characterized by two sharp peaks of run intensity in the morning and evening were revealed. During the period of observations from September 22 to October 20, 2007, in the seasonal dynamics of migration two peaks of intensity of run of Kamchatka steelhead depending on an abrupt decrease in water temperature by 2°C were also registered. Mobile surveys supplied data on the abundance and size composition of Kamchatka steelhead in the surveyed water area of the river, on its spatial distribution, and dynamics of accumulation of fish in the river. The abundance of Kamchatka steelhead in the river almost doubled every three-four days and the spatial distribution was aggregated, producing maximum concentration mainly in areas of increasing depth and channel meandering before and after riffles. Comparison of the results of assessment of abundance of Kamchatka steelhead that arrived for spawning in 2006 and 2007 showed significant fluctuations in population abundance.