Methodological Aspects of Using the Frequency of Occurrence Indicator for Analysis of Ichthyocenoses with the Ichthyofauna of the Baltic Sea Coastal Zone Taken as an Example

The specificity of utilizing the frequency of occurrence indicator in relation to the study of the species structure of a fish community is considered. Unlike hydrobiological studies, fishing is carried out with gear of varying selectivity (nets with different mesh sizes, a fry drag with a 5 mm mesh size in the belly, or a cast seine with a 12 mm mesh size in the belly), which provides a set of values for the species occurrence coefficient and determines the use of a special approach to interpreting the results. The coastal ichthyocenosis of the Baltic Sea within Kaliningrad oblast was used as the object of study. Fishing was carried out in the period 1990–2022 at depths of up to 20 m using a set of fixed gillnets with a mesh size of 12 to 80 mm. In total, data on 894 catches were summarized, in which 11 800 fish were caught. It is shown that the frequency of occurrence of a species can be presented in several ways: (1) as the average for the catches of the entire set of mesh, (2) as the maximum for the mesh size optimal for a given species, (3) as a structural characteristic that determines the nature of organization of the ichthyocenosis of a species. In general, in the catches of a collection of gillnets, the coastal ichthyocenosis was found to be represented by 29 species of fish, among which the river flounder, Baltic herring, round goby, and smelt dominate in terms of occurrence. In the interannual aspect, the occurrence rate is demonstrated to show certain stability and is characterized by a confidence interval ranging within ±10–20%. Taking into account that the indicator of occurrence is quite simply determined just through the capture record of a species, it is recommended to use it in addition to the traditional indicators of the relative abundance and biomass of the species. Using multi-cell fishing gear for the assessment of the structure of a fish community is prerequisite.