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

 

	Author: Kaj Ådjers, Antti Lappalainen

 

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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. Geometrical mean age and lenght of the key species perch have decreased during the period 1991 - 2004. This indicate increased mortality of adult perch, decreased growth rate and/or increased fishing pressure. Adult individuals may also emigrate from the area. Biomass of roach has increased, which indicates enhanced eutrophication.

 

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.

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 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 diversity

Total biomass and diversity show no significant trends during the assessment periods (Figure 3).

 

 Figure 3. Total biomass and diversity at Brunskär monitoring area.

 

Average trophic level and slope of size spectrum of catch

Average trophic level and slope of size spectrum of catch show no trends during the assessment periods (Figure 4). The development of trophic level shows negative correlation to diversity (Figure 3), 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. Average trophic level and slope of size spectrum at Brunskär monitoring area.

 

Key species indicators, perch

Roach, (Rutilus rutilus) is together with perch an important key species in the area. Roach biomass is hence included as an additional indicator.

 

Perch and roach biomass

The biomass of perch has remained unchanged during the assessment period, while roach has increased significantly (regression, p = 0.0001; Figure 5). Increased occurrence of roach is regarded as an indicator of increased eutrophication.

 

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

 

Perch mean length and mortality

Mean age decreased significantly (regression, p=0.037). Mortality appear to increase, but not significantly (Figure 6). The decreased mean age indicates that the proportion of young individuals have increased, which may indicate higher mortality from fishing or emigration. It may also indicate decreased growth rate.

 

Figure 6. Perch mean age and mortality at Brunskär monitoring.

 

 Perch mean length and slope of size spectrum

Geometrical mean length has decreased during the assessment period (regression, p = 0.006; Figure 7). The decrease shows a larger proportion of small and younger individuals. This is an indication of higher fishing pressure or decreased growth rate. This dercease is in accordance with the decrease of geometrical mean age.

 

 Figure 7. Geometrical mean length and slope of size spectrum 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.

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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: 13 February 2008; Kaj Ådjers