Methodological Aspects of using the frequency of occurrence Indicator for an analysis of Ichthyocenoses, 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. The coastal ichthyocenosis of the Baltic Sea within the Kaliningrad Region was used as a study object. Fishing was carried out at up to 20 m depths using a set of collection of gillnets with a mesh size of 12 to 80 mm in the period 2006–2022. In total, data on 894 catches were summarized, in which 11.8 thousand 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 an ichthyocenosis’ species organization. In general, in the catches of a collection of gill nets 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.