The research of salmon migration intensity in estuary part of Bolshaya River (west part of Kamchatka peninsula) using survey set-net and hydroacoustical method

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

The presence of survey set-nets on the pathways of fish migration leads to considerable changes in their migration behaviour. Besides of negative impact on the process of salmon migration, these methods of stock estimation give raise some doubts of the final results reliability. The aim of research was to compare the reliability of the data on the salmon migration intensity using the survey set-nets the hydroacoustic methods. We used echosounder Lowrance X-45 with wide-screen vibrator (120 deg.) which effectively works at the small depth. The studies were carried out in the nearshore waters in the immediate proximity to river mouth during the spawning migration of salmon. As a survey fishing gear the set-net of improved construction with verified parameters was used from July to August for the last three years. The data on the fish density obtained in the course of the echosounding surveys have shown direct relationship (r=0.78; F=24.8; p<0.01) with the catch amount as the intensity of migration varied from the lowest to medium values. In the cases of increasing migration intensity this relation took shape of the parabola with a relatively low determination index (R2= 41.2; F=3.05; p < 0.05). The analysis of the data collected through the 3-years studies shows a decrease in the correlation between the catches and the amount of fish entering the fishing area of the survey set-net as the migration intensity rises from its high values to the maximum. The reason of this effect is very low catching and holding capacities of the set-net in case of repletion of its playground and bag, which have been approved by the underwater observation. “The resistance' of the set-net to the flow of migration salmon leads to the replacement of the well-ordered unidirectional movement of the separate shoals, and to a considerable increase in number of salmons swimming round the set-net from the sea-side, and as a result to a decreasing representativeness of he catch data used for the stock estimation.