TY JOUR TI Deep-water part of the Middle Caspian Sea as a promising area of operation of the Caspian sprats KW sonar studies KW Caspian sprats KW Middle Caspian KW density of clusters of sprats KW fishing density of sprats KW echogram JO Fisheries AU Pomogaeva, T.V. PY 2021 IS 2021 PB FSBI "TSUREN"