Features of the omul (Coregonus autumnalis migratorius) distribution in different parts of Baikal Lake by acoustic data
Pelagic omul Coregonus autumnalis migratorius is the main commercial species of Baikal Lake. Its biomass was estimated for different bathymetric zones in different parts of the Lake on acoustic data obtained in the period from May 27 to June 9, 2003 by the echo-sounder Symrad EY-500 with towed antenna Split Beam (frequency 70 kHz, output power 610 W). Three bathymetric zones were examined: > 400 m; 200400 m; < 200 m in four parts of the Lake: southern, middle, and northern Baikal and the area at the Selenga River mouth. In total, 70 longitudinal and cross-lake transects were made with minimal depth 1020 m that was about 80% of the whole lake area. Echointegration analysis was made for the upper water layer (300 m) or for the whole water column at shallows using the Echoview 3.10 software with 0.5 mile interval (totally 2010 estimations). Bathymetric distribution of the omul biomass was similar for the southern, middle and northern Baikal areas: 6876 % was in the zone above the depths > 400 m, 1326 % - above the depths 200400 m, and 411 % - in the shallow zone. The distribution was completely different in the area off the Selenga: only 10 % was in the deep-water zone, 18 % in the zone 200400 m, and 72 % - in the shallow zone. These features reflected complex patterns of redistribution the fish belonged to different populations and age groups of omul had migrating from wintering places to feeding grounds. To be more realistic, acoustic surveys in the deep zone must be conducted in detail in short terms. Usual May-June surveys limited by 400 m isobath underestimate considerably the stock of omul in Baikal Lake.