Fish Distribution in Lacustrine and Riverine Biotopes of Bratsk Reservoir Based on the Results of Hydroacoustic Surveys

Abstract Fish distribution in different areas of Bratsk Reservoir has been studied using hydroacoustic equipment. The average abundance of fish decreases ( r = –0.70; p < 0.05; R 2 = 0.49) from the near-dam reach (214 ± 34 ind./ha) to the water area in the upper reaches of the reservoir (113 ± 43 ind./ha). A similar relationship is observed in the dynamics of the abundance of early juveniles and the immature fish population. However, such relationship has not been found in fish with a size of more than >100 mm, which with a low abundance are relatively evenly distributed along the longitudinal axis of the reservoir (10.5 ± 6.6 ind./ha). The common perch ( Perca fluviatilis Linnaeus, 1758) is the most abundant species in the reservoir, its highest concentrations are recorded in the lower lacustrine part of the reservoir (139 ± 22 ind./ha). Less abundant cyprinids (Cyprinidae) (41 ± 15 ind./ha) and whitefishes (Salmonidae, Coregoninae) (12 ± 4 ind./ha) are more evenly distributed along the longitudinal axis of the reservoir. Temperature stratification of the water column is an important factor in the ecological differentiation of the fish population in the reservoir. The major part of the ichthyomass consisting of relatively warm-water fish species from the families Percidae and Cyprinidae (common roach ( Rutilus rutilus (Linnaeus, 1758) and common bream ( Abramis brama (Linnaeus, 1758)) is concentrated in the warming up epilimnion. The fish population in the cold hypolimnion consists of single individuals of the Baikal omul ( Coregonus autumnalis (Georgi, 1775)) and peled ( C. peled (Gmelin, 1789)). Transverse distribution of fish depends on the presence of a runoff flow. At relatively high flow rates, fish begin to avoid the midstream part in the studied sites of the reservoir and concentrate in the floodplain areas ( R 2 = 0.54; p < 0.0001).