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

 

	Author: Jens Olsson and Jan Andersson

 

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Key Message

Temperature and secchi depth are important for the structure of coastal fish communities. In the waters at Kvädöfjärden inner monitoring area, there has been an increase in temperature and a decrease in secchi depth.

Several coastal fish indicators have changed significantly during the monitoring period. Slope of size spectrum of all species increased during the monitoring period indicating an increase of larger individuals in the fish community, which inturn suggests low fishing pressure and/or increased growth rate and productivity.

The number of species caught during the sampling period have increased, something also refelcted in the almost signinficant increase in diversity. This increase is likely goverened by the decreased secchi depth and increased water temperature leading to that warm-water species favoured by more eutrophic conditions like pikeperch, tench and common bream have appeared in the catches during the last half of the sampling period. Sprat is also a species that has entered the catches during recent years, probably as a result of its increased biomass in the Baltic Sea.

Background and oceanographic information

Kvädöfjärden belongs to the continuous archipelago of the Swedish east coast. The bedrock consists of granite, which is frequently exposed in the outer areas. In sheltered areas, soft bottom dominates, whereas exposed areas are composed of rocky and stony bottoms. The inner area inhabits sheltered bays, while parts of the outer area are exposed to the open sea. In the inner area, deep furrows, with depths down to 30 meter, exist.

Figure 1. Temperature and Secchi depth in Kvädöfjärden between 1962 and 2007.

 

Temperature and Secchi depth

The mean water temperature during May – September was in the beginning of the 1960’s between 13 and 14 ºC. Since then temperatures have increased, and during the last five years the mean temperature has been 16.1 ºC. Secchi depth has decreased from around 7 to about 3.5 meter during a 40-year-period. The surface current is most frequently flowing southwards and the salinity in the area is normally between 6 and 8 psu.

Local pressure

Kvädöfjärden represents a region with very limited impact of local discharge.

Results and assessment

The presented time-period for the local fact sheet of Kvädöfjärden monitoring area include the years 1987 - 2007. Significant changes over time are indicated with the line of best fit.

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, at Kvädöfjärden inner monitoring area. Both the total number of species (not shown here) and the number of freshwater species have increased during the monitoring period. During the period 18 different species have been caught in the area.

 

Total biomass

 

Figure 3. Total biomass (mean per station in kg) at Kvädöfjärden inner monitoring area. Total biomass increased from 1987 to 1994. Thereafter it has decreased somewhat but steadily fluctuates between years. This indicator is strongly influenced by the biomass of the key species perch (see below biomass of key species indicators).

Species diversity

 

Figure 4. Shannon-Wiener diversity index at Kvädöfjärden inner monitoring area. There is a slight trend (not statistically significant) of an increased diversity during the period.

Slope of size spectrum

 

Figure 5. Slope of size spectrum of all species at Kvädöfjärden inner monitoring area. This indicator increased during the monitoring period which indicates an increase of larger individuals in the fish community, suggesting low fishing pressure and/or increased growth rate and productivity. 

 

Key species indicators

  The key species at Kvädöfjärden monitoring area is perch, a warm water, freshwater species.

 

Species biomass

 

 

Figure 6. Perch biomass (mean per station, kg) at Kvädöfjärden inner monitoring area. Perch biomass shows no significant trend during the sampling period, but peaked in 1994 and 2002. In 1994, the total community biomass also reached its peak suggesting that perch is the dominating species of this fish community.  

Mean length

 

Figure 7. Mean length of perch at Kvädöfjärden inner monitoring area. Mean length shows no significant trend during the monitoring period. Mean length reflects the length distribution of the fish stock. 

 

Slope of size spectrum

 

 

Figure 8. Slope of size spectrum of Perch at Kvädöfjärden inner monitoring area. This indicator shows no significant trend during the period. Slope of size spectrum reflects the length distribution of the fish population and is affected by e.g. recruitment success and/or size selective fishing. A flat negative slope indicates occurrence of high number of large individuals in relation to small individuals.

Methods used

Fishing with coastal survey nets (mesh sizes 17, 21, 25 33 and 50 mm from knot to knot) at six stations was annually performed. The stations were repeatedly fished for three nights in early August.

A new method was introduced in 2002, which included a new survey net, coastal Nordic nets (mesh sizes 10, 12, 15, 19, 24, 30, 38, 48 and 60 mm from knot to knot). 45 randomly selected stations, divided into four 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, Nationell och regional fiskövervakning.

 

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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: 2008-09-19