Temporal development of Baltic coastal fish communities and key species in Inner Forsmark

	Authors: Kerstin Söderberg and Jan Andersson

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Key Message

Temperature and Secchi depth are important for the structure of coastal fish communities. In the Forsmark inner monitoring area, these parameters show no significant trends. Four coastal fish indicators have changed significantly during the monitoring period. Increasing values of slope of size spectrum of the fish community indicates a development towards larger individuals, mostly represented by Perch (Perca fluviatilis) and Bream (Abramis brama). Both species increased their biomass. In Perch this could be explained by a significantly increased growth rate. The mean length and the slope of size spectrum of Perch increased significantly, probably due to increased productivity in combination with a moderate fishing pressure.

Background and oceanographic information

Forsmark monitoring area is situated along an almost unexploited coastline, stretching from Skaten in the north to Forsmark nuclear power plant in the south. The coastal area is characterized by a sloping pine forest on moraine deposits and in the water there are frequent reefs, rocks and islets. The surface water in the northern half of the area has a moderate rate of exchange (5-10 days), while the surface water in the southern half has a slow rate of exchange (10-20 days). The salinity is 3-4 PSU.

Figure 1. Temperature and Secchi depth in the inner monitoring area at Forsmark. No significant changes occur during the period.

Local pressure

Öregrundsgrepen receives cooling water from three aggregates of the Forsmark nuclear power plant, approximately ten degrees warmer than the surrounding water. Process water from two of the aggregates discharges into the “biotest basin” (Biotestsjön), an artificial lake, and is then released into Öregrundsgrepen. Fertilizing substances from the outlet of the nuclear power plant sewage treatment plant has a minor impact on the monitoring area.

Results and assessment

The indicators used below and their relevance for conveying developments in the environment are described in the main indicator fact sheet on Temporal development of Baltic coastal fish communities and key species.

Community indicators

Number of species

Figure 2. Number of species, divided into freshwater, marine and migratory species, in the inner monitoring area at Forsmark. No significant changes occur during the period. Few marine and migratory species were caught. The number of species caught depends on many factors such as salinity, habitat heterogeneity, temperature and exposure to the open sea of the area sampled. The number of species usually decreases with decreasing salinity as marine species disappear and the fish communities become more dominated by freshwater species.

Total biomass

Figure 3. Total biomass (mean per station in kg) in the inner monitoring area at Forsmark. The total biomass fluctuates strongly during the monitoring period and no significant changes occur. Total biomass is strongly affected by changes in Perch biomass (see below biomass of key species indicators). Total biomass is used as an index of the size of the standing stock. Increased biomass indicates increased nutritional conditions and production potential, whereas decreased biomass indicates the opposite.

Species diversity

Figure 4. Shannon Wiener diversity index in the inner monitoring area at Forsmark. No significant changes occur during the period. The Shannon Wiener index reflects the species richness and equitability of the community. Increasing values indicate increasing numbers of species with even proportion in the catch. Decreasing values indicate few dominating species and a community with a small number of species. Calculations are based on the biomass proportion of each species.

Slope of size spectrum

Figure 5. Slope of size spectrum in the inner monitoring area at Forsmark. Increasing values indicate a development towards larger individuals. The slope of the size spectrum reflects the length distribution of the fish community. Recruitment variations, extensive size selective fishing, as well as a change in species composition in the fish community could affect the slope.  

Average trophic level of catch

Figure 6. Average trophic level of coastal fish in the inner monitoring area at Forsmark. No significant changes occur during the period. Increasing values indicate that the proportion of species at higher trophic levels, e.g., piscivorous fish increases and/or that the plankton- and benthos-feeding species decrease.

Key species indicators

The key species at Forsmark monitoring area is Perch (Perca fluviatilis), a warm water dwelling freshwater species that dominates the catches in the area.

Species biomass

Figure 7. Perch biomass (mean per station in kg) in the inner monitoring area at Forsmark. The biomass of Perch increase during the monitoring period. Perch biomass strongly affects the total biomass (see above biomass of community indicators). Species biomass is an index of the size of the standing stock. Biomass changes reflect changes in external driving forces such as productivity, temperature and in some cases fishery.

Mean age

Figure 8. Mean age of a female Perch in the inner and outer monitoring area at Forsmark. No significant changes occur during the period. The mean age reflects the age distribution in the stock. It is affected by reproductive success and mortality rate (fishing pressure and predation).

Mortality

Figure 9. Mortality of Perch in the inner and outer monitoring area at Forsmark. No significant changes occur during the period. Mortality of Perch is expressed as percent annual decrease, based on the mortality of individuals that are 3-5 years old. Increased mortality indicates increased fishing, predation or mortality due to other causes.

Mean length

Figure 10. Mean length of Perch in the inner monitoring area at Forsmark. The mean length of Perch increase over time. The mean length in Perch reflects the length distribution in the fish stock and is affected by variation in recruitment, extensive size selective fishing and changed individual growth rate.

Slope of size spectrum 

Figure 11. Slope of size spectrum of Perch in the inner monitoring area at Forsmark. Increasing values indicate a development towards larger individuals. The slope of the size spectrum reflects the length distribution of the fish population and is affected by e.g. individual growth rate, recruitment success and/or size selective fishing. 

Methods used 

Fishing with coastal survey nets (mesh sizes 17, 22, 25 33 and 50 mm from knot to knot) was performed annually at four stations. The stations were repeatedly fished for three times in early August.

A new method was introduced in 2002, which included a new survey net, Nordic coastal multi mesh gillnet (mesh sizes 10, 12, 15, 19, 24, 30, 38, 48 and 60 mm from knot to knot). 45 randomly selected stations, divided into several depth intervals are annually fished once in August. Data from the new method is not included in the indicator fact sheet.

References 

Söderberg, K., G. Forsgren och M. Appelberg 2004. Samordnat program för övervakning av kustfisk i Bottniska viken och Stockholms skärgård – utveckling av undersökningstyp och indikatorer. Finfo 2004:7, 1–90.

Links

Swedish Board of Fisheries; Institute of Coastal Research

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For reference purposes, please cite this indicator fact sheet as follows:

[Author’s name(s)], [Year]. [Indicator Fact Sheet title]. HELCOM Indicator Fact Sheets 2007. Online. [Date Viewed], http://www.helcom.fi/environment2/ifs/en_GB/cover/.

Last updated: 2007-12-13