Using the Gill Nets Survey for Assessment of Fish Stock and Allowable Catch in the Vistytis Lake, Kaliningrad Oblast, Russia

Abstract

The paper describes a method to estimate the abundance of benthic fishes in the Vistytis Lake (area of 1.8 thousand ha, maximum depth of 52 m) using bottom gill nets with 12–60 mm mesh size. Annual 10-day surveys were performed from 2007 to 2020 at 30 stations located throughout the water area. The gill nets are not suitable for assessment of absolute number of fishes since it is unknown catching square of the net and catchability coefficient. However, the integration of catches per unit effort (in number of fishes or in kilograms per 25 m of net length per fishing day) and the area of the respective depth ranges and mesh size, can be used to calculate the relative abundance and biomass of the stock as well as the species composition of ichthyocenosis. The net mesh size can be considered as an identifier of the fish size group. This gives an opportunity to analyse the length structure of the population. The catches per unit effort for each mesh size can be interpreted as a total allowable catch (TAC), provided each hectare of the water body is fished once a year with a 25 m long net with each mesh size. The average stock abundance estimated with application of this approach gives the number of fishes 681.9 ± 144.8 thousand specimens and 38.44 ± 7.61 tons correspondingly, and the interannual variation coefficient for different species ranges from 50 to 100%. With regard to the water body area, the fish population density is 378 specimens/ha and 21.7 kg/ha, which is comparable to the ichthyofauna characteristics of the oligotrophic lakes in northwestern Russia. The method can be applied to any water body inappropriate for fishing by gears such as a trawl to assess the numbers of fish.