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Features of kaluga sturgeon Acipenser dauricus and amur stergeon Acipenser schrenckii juvenile feeding in the Lower Amur

The aim of the work is to study the specifics and nutritional spectrum of juvenile kaluga and Amur sturgeon in the Amur on the halfway from the sturgeon hatcheries to the main feeding place — the Amur Estuary. There was Mysidacea in the food spectrum of juvenile kaluga from the Lower Amur, in 350–390 km from the mouth, in the first decade of July 2021. Mysids were found in 70% of the studied gastrointestinal tracts and their mass fraction was almost 44%. Fish played a secondary role in the diet of kaluga. The diet of the Amur sturgeon consisted mainly of Trichoptera (83.33%) and Chironomidae (42%) larvae. Mollusks were not found in the gastrointestinal tract of juveniles of both species feeding in the Amur riverbed. Both in diversity and the index of filling the gastrointestinal tracts, the diet of sturgeons in the Amur at the beginning of July 2021 was significantly inferior to the diet described by previous studies. The average number of identified objects in the gastrointestinal tracts of kaluga and sturgeon was 1.2 and 1.4, respectively; average gastric filling index — 41.1 and 28.8; the average value of the fatness coefficient for kaluga was 0.62, for sturgeon — 0.51. The different type of feeding and the range of food objects show that kaluga and Amur sturgeon juveniles are not food competitors in this section of the Amur. The high content of sand in the stomachs of sturgeons, 2.5 times higher by weight than the mass of the food in bolus, indicates the poverty of the sturgeon feed base on this section of the river in the conditions of abnormal flooding of the spring-summer season of 2021. In such hydrological conditions, food organisms — small fish, insect larvae — dispersed over large areas of the river floodplain, that made them hard-to-get for sturgeons living in the main channel and permanent channels.