Winter Dynamics of Drifting Invertebrates’ Vertical Distribution in a Small Salmon River

The composition of the winter syrton of a small salmon river was dominated by mayfly larvae (Ephemeroptera), dipteran larvae (Diptera), and adult water mites (Hydracarina). The presence of a significant linear dependence of the number of drifters entering the nets on the volume of water filtered by the nets has not been proven, so we used not the syrton density (ind./m³), but the data of actual catches. The vertical distribution of drifting invertebrates of different taxonomic affiliation had its own characteristics. Under conditions of good illumination of the water column, the drift intensity of invertebrates increased in the daytime. This fact is probably indirectly related to the low water temperature, which limits the swimming ability of drift-feeding fish, which because of this become more vulnerable to fish-feeding warm-blooded predators, and therefore in winter during daylight hours lead a predominantly inactive and secretive way of life. In December, most daytime drifters migrated near the surface of the water column, and in January, on the contrary, near the bottom. The latter may be due to the overwhelming effect of lower daytime air temperatures, since until mid-February the riverbed was not completely covered with ice. After a heavy February snowfall, the upper layers of the river flow turned out to be isolated from the effect of air temperature, and the level of illumination of the water column during the day began to correspond to the period of late twilight. As a result, fish became less accessible to warm-blooded semiaquatic predators, the intensity of invertebrate drift in the daylight hours decreased sharply, and the distribution of drifters throughout the water column vertical became fairly uniform.