BS10 Marine fish stocks

>> Background information

Development

Perch and Cyprinids. Test fishing with multi-mesh gillnets in the three permanent monitoring sites along the southern coast of Finland indicates no trend in either European Perch or Cyprinid (Roach, Bream and White bream) catches since 2005. However, differences in the Perch/Cyprinid ratio between the monitoring areas imply local differences in the level of eutrophication.

In the coastal waters of Helsinki, the fish community has been Cyprinid-dominated, whereas in the Archipelago Sea (Brunskär), Perch have clearly dominated the catch in relation to Cyprinids. In Tvärminne the populations of Perch and Cyprinids have been at the same level on average. The low Perch catch in weight in relation to Cyprinid catch in the region off Helsinki is in addition partly explained by high fishing pressure on large Perch.

The fluctuations in the Perch/Cyprinid-ratio, most prominent in the Archipelago Sea over the years, mainly reflect the dependency of the year-class strength of perch on water temperature in the Baltic Sea, warm temperatures causing strong year-classes.

Cod and Baltic Herring. Spawning stock estimates based on long-term catch statistics and catch composition of the eastern Baltic cod indicate a notable increase in stock biomass in the late 1970?s, mainly owing to favourable environmental conditions for successful spawning. The peak was reached in 1982 and after a relatively steady state for a few years, the size of the spawning stock collapsed. Since 1990, the eastern Cod stock has stayed at a low level.

The rapid decline of the Cod stock was partly caused by the lowered water salinity and oxygen level in the Baltic Sea, which impaired reproduction. The negative trend was enhanced by heavy exploitation. The unfavourable environmental conditions for reproduction and extensive fishing pressure have hindered stock recovery.

The decline of the Cod stock has favoured the stocks of the important prey species for Cod, Baltic Herring and Sprat. This is clearly indicated by an increase in spawning stock biomass of Sprat in the whole of the Baltic Sea area. In the Bothnian Sea, also favourable conditions for reproduction and practically a lack of Sprat and other competitors have influenced the increase in Baltic Herring stock biomass. The decline in herring spawning stock biomass in the Baltic Proper and Gulf of Finland is partly explained by a decrease in individual growth and size due to changes in the plankton community and intensified intra- and inter-specific competition with sprat owing to an increase in abundance.

Fish stocks as indicator

The fishes inhabiting the Baltic Sea represent a mixture of marine and freshwater species that have adapted to live in the brackish water. They make a significant contribution to the Baltic Sea ecosystem by occupying various positions in the food chain. The abundance and diversity of the Baltic Sea fish are influenced by both anthropogenic and natural factors, including fisheries impact, water quality (e.g., pollution, toxic waste) and productivity (eutrophication) as well as salinity, temperature and species interactions. The health of the Baltic Sea is thereby reflected in its fish assemblages.

Fishing is one of the main factors globally threatening the biodiversity of fish stocks by reducing their abundance and, potentially, also population structure and life history traits. In order to preserve the biodiversity and good environmental status of the Baltic Sea, the long-term viability of the fish stocks should be maintained by sustainable harvesting.

Eutrophication, on the other hand, causes increased production of fish biomass but also changes in fish community structure and function in the Baltic Sea. A slight eutrophication may favour the abundance of Percids, whereas more heavily eutrophicated waters usually show an increase in Cyprinid abundance, at the expense of percids. A decline in the Perch/Cyprinid ratio implies a development towards a more cyprinid-dominated community, and when reflecting an increase in the abundance (or biomass) of the cyprinids, it can be regarded as an indication of increased amounts of nutrients (eutrophication), warmer water temperatures, and in consequence, strong Cyprinid year-classes.