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

 

	Author: Kaj Ådjers, Antti Lappalainen

 

Return to main indicator fact sheet

Key Message

Temperature and water transparency have not changed since 1998 and 1991, respectively. Few fish indicators show significant changes and the fish community has hence remained similar over the investigation period. Only geometrical mean age and lenght of the key species perch have decreased during the period 1991 - 2004. Also mortality of perch show a tendency to increase over the same time period, however not significantly. This indicate increased mortality of adult perch, decreased growth rate and/or increased fishing pressure. Adult individuals may also emigrate from the area. The number of marine species decreased during the period 1996 - 2004 indicating that the environment develop towards less marine conditions and higher water temperatures.

Background and oceanographic information

The monitoring area of Brunskär is located in the southern part of the Archipelago Sea and within the borders of the National Park of the Archipelago Sea. The archipelago is made up of alternating areas of open firths and dense groups of islands. The islands are rocky and the bedrock is often exposed. Water depth seldom exceeds 5 m within the island groups, except in narrow ravines cutting into the area. Water depth outside the island group may go down to 50 m.

 

The mean surface temperature is ca 14 C showing no trend (Figure 1). Transparency, measured as Secchi depth, is good around 6 m. Nutrient concentrations indicate oligotrophic conditions, which is confirmed by the high water transparency. Salinity is 6 - 7 PSU.

 

Figure 1. Oceanographic parameters at Brunskär monitoring area.

Local pressure

Human settlements are few and environmental impact is hence small.

Results and assessment

The presented time period for the local fact sheet of Brunskär monitoring area include the years 1991 - 2004. Significant changes over time are indicated with the line of best fit. The period 1996 - 2004 is also assessed.

Community indicators

Number of species

The number of freshwater species was low during in the beginning of the assessment period 1991 - 2004 (Figure 2). Higher numbers followed from 1994 and has since then remained high. Few marine species were found in the beginning of the investigation period, higher number was found in the middle of the 1990's and few again during the 2000's. The number of marine species has decreased significantly during the period 1996 - 2004 (regression, p = 0.011). The recent decline of marine species may be connected to a possible longterm increase of temperature and possible decrease of salinity.

 

 Figure 2. Number of species at Brunskär monitoring area.

 

Total biomass and slope of size spectrum

Total biomass and slope of size spectrum show no significant trends during the assessment periods (Figure 3).

 

 Figure 3. Species diversity and average trophic level of catch at Brunskär monitoring area.

 

Species diversity and average trophic level of catch

Species diversity and average trophic level of catch show no trends during the assessment periods (Figure 4). The parameters show negative correlation to each other, which is dependent on the perch catches. Large perch catches induces high values of trophic levels and low values of diversity and vice versa.

 

Figure 4. Diversity and trophic level at Brunskär monitoring area.

 

Key species indicators, perch

Species biomass and mean age

The biomass of perch has remained unchanged during both assessment periods, while mean age has decreased significantly during both periods (period 1991 - 2004, regression, p = 0.037; period 1996 - 2004, p = 0.039). A decrease of mean age indicate increased mortality or emigration of large individuals, which in turn may indicate increased fishing pressure (Figure 5).

 

 Figure 5. Biomass of perch and mean age at Brunskär monitoring area.

 

Mortality

No significant trends appear in perch mortality, although an increased tendency may be detected (Figure 6). The tendency is probably related to the decresed mean age and length.

 

Figure 6. Perch mortality at Brunskär monitoring.

 

Geometrical mean length

Geometrical mean length has decreased during the assessment period 1991 - 2004 (regression, p = 0.006), but no significant trend during the period 1996 - 2004. The decrease indicate decreased growth rate or increased mortality of adult perch (Figure 7). This dercease is in accordance with the decrease of geometrical mean age.

 

 Figure 7. Geometrical mean length of perch in Brunskär monitoring area.

Methods used

Fishing with coastal survey nets (mesh sizes 17, 22, 25 33 and 50 mm from knot to knot) at nine stations was annually performed. The stations were repeatedly fished for six times (three times 2002 - 2004) in early August. This method was carried out until year 2004.

 

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). 39 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

Ådjers., K., J. Andersson, P. Böhling, M. Mölder, E. Neuman and O. Sandström, 1996. Monitoring in Baltic Coastal Reference Areas. TemaNord 1996:627. 38 pp.

Ådjers., K., M. Appelberg, R. Eschbaum, A. Lappalainen, A. Minde, R. Repecka and G. Thoresson, 2006. Trends in coastal fish stocks of the Baltic Sea. Boreal Environ. Res. 11: 13-25.

HELCOM, 2006. Assessment of Coastal Fish in thr Baltic Sea. Balt. Sea Environ. Proc. No. 103 A.

HELCOM, 2006. Changing Communities of Coastal Fish. Balt. Sea Environ. Proc. No. 103 B.

Return to main indicator fact sheet

 

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: 13 February 2008; Kaj Ådjers