Epsilon Archive for Student Projects

Abstract

Populations of one of the most important predatory fish species within Baltic Sea coastal ecosystems, the Eurasian Perch (Perca fluviatilis), have declined in some coastal areas during recent decades. This has caused negative effects on an ecological, commercial, and cultural scale, and protection and restoration of recruitment areas (perch spawning grounds) are believed to be an important player in supporting future stocks.
Freshwater systems close to coastal areas have previously been noted as important spawning grounds for other fish species resident in coastal areas, but relatively little is known of this for perch. In this study, the main aim was to assess the importance of freshwater systems for the recruitment of coastal perch along the Swedish Baltic Sea coast, using otolith micro-chemistry. Otoliths were removed from perch specimens and analysed for strontium (Sr) and calcium (Ca) ratios via particle induced x-ray emissions techniques (μPIXE). In order to determine birth origin of the coastal perch in the study areas, a freshwater baseline was established using other freshwater recruited coastal perch, as well as lake perch specimens.
The results showed that the majority of perch juveniles in coastal areas were recruited in brackish water, albeit with variation across areas. In one area however, freshwater systems appeared to be extremely important for perch recruitment in the coastal area.
Notable changes in habitat between resident maternal environment, and juvenile birth environment were also assessed (via changes in salinity) to speculate maternal spawning preferences. This study showed that on average 30 % of juvenile fish caught in coastal areas had a ‘pronounced dip’ in their Sr:Ca profiles, which suggests that adult perch migration into waters with notably lower salinity for spawning is not uncommon in the Baltic Sea.
Often otolith strontium concentrations are suggested to be positively correlated to the respective salinity found in the resident habitat of a fish. This study found no correlation, and no apparent trends linking ambient salinity and Sr:Ca ratios in otolith cores.
In order to restore and maintain future perch stocks in the Baltic Sea it seems that the most important habitats to protect are those in the coastal areas, but freshwater systems in some areas must also be considered. Once these important recruitment sites are identified, both land and water management measures should be employed in order to conserve these sensitive coastal ecosystems, or return them to a natural state.