The research of salmonids migrations in the lower and estuarine parts of Onega River and Northern Dvina River (White Sea basin) with the use of hydroacoustic method

No abstracts are to be cited without prior reference to the author.

Complicated hydrodynamic structure of the rivers, especially in the lower part if them, determined by rivers’ channel morphometry exerts significantly influences the fishes’ behaviour, which become apparent in distributional and displacement patterns of migratory fishes. Such factors as sharp changes of the depth and width of channel or in the degree of meandering alter the trajectories of fish movements causing the long delays before the obstacles and changes in velocity regime of migration. Presence of the survey set fishing gears in the river results in similar changes of migration behaviour that can be classified as “violations'. On the one hand, it brings the negative impact on the migration; on the other hand, it gives rise to some doubts about reliability of the data on abundance of the migrating fishes obtained by means of set fishing gears and may cause an increased level of elimination. The purpose of this work was to study the distribution and behaviour of migrating fishes within the affecting zone of set survey gears on the example of the North Dvina River and Onega River. For studying the distribution and density of the Atlantic salmon Salmo salar L. and the brown trout S. trutta L. we have used echosounder Lowrance X-45 with a wide-screen vibrator (120º) and satellite navigation module LGC-1. The fishing weir has been used as a survey fishing gear. The researches of fish distribution in the active zone around the fishing weir have shown that this area has smaller structural organization of fish. No significant dependence of fish distribution on the channel depth was registered here; the range of preferred depths was not evident; the density of fish aggregations and their migration speed notably differed. Certainly, the impairments of migratory behaviour caused by the set survey gears, uneven distribution of migrating fish in cross-section of the river intensified by the action of the gear, and species-specific peculiarities in fish response to the obstacles do not allow to estimate satisfactory the number of fish passing to their spawning grounds.