The nature of the Kupiškis Valsčius a rural district
Kupiškio Marios is one of the largest river-dammed lakes in Lithuania. It is a shallow eutrophic water body. The investigations on fish stocks had been performed in August and September of 2012 according to the methodology approved by the Environment Minister of the Republic of Lithuania. Fishes of nine species, i.e., roach, rudd, bleak, bream (Blicca bjoerkna), carp bream (Abramis brama), pike-perch, ruffe, perch and pike, had been caught during the investigations. Roach prevailed in this lake both by abundance and the biomass. Compared to the data of the research done in 2007, the 2012 results show an increase in the biomass of roach, but that of carp beam declined. The predominant prey species in Kupiškio Marios is pike-perch (Sander lucioperca), followed by a subdominant species of pike. Among other fishes, a relative mass of carp bream is slightly higher and reaches nearly 21 percent. Although the relative abundance of prey fishes in the Kupiškis Marios is not high and makes just 16 percent of the total fish abundance, the biomass of prey fish corresponds to the indices of the balanced fish communities and reaches nearly 30 percent. The relative abundance of the rest fish species and their biomass changed insignificantly; only rudd should be considered a rare fish in this lake. Of the fishes forming facultative populations, the ruffe (Gymnocephalus cernuus) and bleak (Alburnoides bipunctatus) have been caught; their abundance is effectively regulated by prey fishes. In 2012, the total fish biomass in Kupiškio Marios reached 133 kg/ha, and the abundance of fishes except for young ones, was 2650 fish per hectare. Thus this lake is one of the most productive water bodies in Lithuania. These fish stocks are widely used by anglers, therefore, in order to ensure the stability of fish community, a stocking should be applied: 7501000 yearly pikes should be let into the lake each year, as well as European catfish (Silurus glanis) each 35 years. During the investigations, a high abundance of narrow-clawed crayfish (Astacus leptodactylus) was observed in Kupiškis Marios, but in 2015 due to epizootic, they died. Nevertheless there are hopes the stocks of the crayfish to restore in the future.