A Baltic pelagic fish community revisited: Indications of profound changes in species composition in the Stockholm Archipelago

While the central Baltic herring (Clupea harengus) stock biomass is estimated to have been reduced by similar to 80 per cent since the 1970 s, the local effects of this decline remain unassessed. Coastal commercial fishermen have witnessed a widespread depletion at the spawning grounds and generally low herring abundance in inshore areas. We carried out hydroacoustic surveys, supported by midwater trawling in August 2021, to test whether herring abundance and recruitment have changed over time in the inshore areas. We used assessed recruitment success, measured as young-of-the-year (YOY) abundance, as a proxy for herring abundance in several areas of the southeast Stockholm Archipelago and compared the results with previous investigations in this area (hydroacoustics 2000-2001 and midwater trawling 2002-2004 monitoring). We found that the composition of the fish community had undergone significant changes. While acoustic backscattering (sA) representing the whole fish community were similar to previous estimates, species composition was significantly changed. YOY herring, previously dominant, had declined from 55% to 4% in numbers, whereas three-spined stickleback (Gasterosteus aculeatus) increased from no representation to 76%. The observed changes makes it imperative that further investigation concerning herring population structure, biomass, and recruitment are conducted