Ichthyoplankton of the South-Eastern Baltic Sea in the summer season: modern changes in the distribution and abundance of eggs and larvae of mass species

Abstract

The aim of ichthyoplankton studies in July 2019 was to monitor the distribution and estimate the abundance of: 1) eggs and larvae of mass pelagophilous fish species (cod, sprat) in deep-water areas; 2) larvae of small-sized benthic fish species inhabiting the shallow coastal zone of the South-Eastern Baltic Sea (the exclusive economic zone of the Russian Federation). It was noted, the main concentrations of cod and sprat eggs were in the northern part of the area (southern slope of Gotland Deep). Aggregations with a noticeably lower abundance of eggs were observed in the Gdansk basin. In July 2019, the average abundance of cod eggs (3.4 ind./m2) was higher than in the years without strong North Sea advections according to observations in the 1990–2000. Although the reproduction of sprat was nearing completion, in July its eggs dominated the ichthyoplankton assemblage of the deep-water zone, tenfold exceeding the numbers of cod eggs. In the composition of the ichthyoplankton of the shallow-water zone, 7 species of fish larvae were found, among which the sand goby dominated in numbers. Its small-sized larvae were most abundant above depths from 20 to 40 m, which made it possible to identify this depth range as an area of intensive spawning of the species. The decrease in the number of sand goby larvae in the 2010s, compared to the beginning of the 2000s, could be associated with the spread of the Ponto-Caspian invader round goby in the coastal zone, small-sized fry of which were also found in July 2019 in the ichthyoplankton of the coastal zone.