Temporal development of Baltic coastal fish communities and key species in Råneå 

	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 Råneå monitoring area, these parameters show no significant trends in 1994-2004. No significant changes were observed in community indicators over the years. The mean length of Perch decreased during the period of investigation. A tendency of increasing Perch density and no change in mortality rates suggest that the smaller mean size primarily is a result of increasing recruitment.

Background and oceanographic information

The Råneå monitoring area is located in the northern part of Botnian bay. The monitoring area comprises the inner and middle part of the archipelago. The inner parts are sheltered and shallow, with depths below 10 m. The middle parts consist of groups of islands with water depths that can reach 20 m. The granite bedrock is plain and mostly covered by moraine deposits, which create a stony shoreline. The bottom consists mostly of mud and clay. The salinity in the surface water is 0–2 PSU. The level of nutrients is very low and there is no lack of oxygen.

Figure 1. Temperature and Secchi depth in the monitoring area at Råneå. No significant changes occurred during the period.

Local pressure

The area is no major recipient. Seal appear regularly in the 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 monitoring area at Råneå. No significant changes occurred during the period. 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 monitoring area at Råneå. Total biomass fluctuates strongly during the monitoring period and no significant changes occur. The 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 monitoring area at Råneå. 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 monitoring area at Råneå. No significant changes occur during the period.  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. Decreasing values indicate decreasing numbers of larger individuals.

Average trophic level of catch

Figure 6. Average trophic level of coastal fish in the monitoring area at Råneå. 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 decreases.

Key species indicators

The key species in the Råneå 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 monitoring area at Råneå. No significant changes occur during the period, but the records were high in recent years. 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 female Perch in the monitoring area at Råneå. 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 monitoring area at Råneå. No significant changes occur during the period. Mortality of Perch is expressed as percent annual decrease, based on the relative abundance in consecutive years of female individuals, 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 monitoring area at Råneå. The mean length of Perch decreased significantly in 1994-2004. Mean length in Perch reflects the length distribution in the fish stock. The decrease could depend on increasing recruitment, extensive size selective fishing and/or changed individual growth rate.

Slope of size spectrum 

Figure 11. Slope of size spectrum of Perch in the monitoring area at Råneå. No significant changes occurred during the period. Slope of 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 seven stations. The stations were repeatedly fished for six 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