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

 

	Author: Kaj Ådjers

 

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Key Message

Temperature has increased, salinity and transparency have decreased in the waters at Finbo monitoring area. These are important oceanographic parameters structuring the coastal fish community and several coastal fish indicators have changed significantly during the 18 years investigation period. Number of species and species diversity have decreased and growth rate and productivity have increased. The changes of oceanographic parameters favours warm water species and freshwater species such as perch, pike, pikeperch, ruffe and different species of cyprinids, while marine species such as, flounder, turbot, eelpout and different species of sculpins, are suppressed.

 

Background and oceanographic information

Finbo monitoring area is located in the north western part of Åland. The archipelago is sheltered from south and opens gradually towards north. The depths seldom exceed 20 m. The shores are mainly cliffs and rocks, but sandy shores are common in creeks. The bottoms are sand mixed with clay, gravel and mud.

The mean water temperature during May – October was in the beginning of 1990’s between 13 and 14 ºC (Figure 1). The temperature has since then increased (regression; p=0.003). A peak of 16.8 ºC was noted in 2006. Upwellings of cold water are common during summer. Temperature drops in bottom waters of up to 6ºC from one day to another have been recorded. Salinity has decreased from 6.2 to 5.7 PSU during a 18 years period (regression; p=0.002, Figure 1). Transparency, recorded as Secchi depth, detected in August, was in the beginning of 1990’s mostly below 5 m, but has since 1999 decreased to between 4 and 5 m (regression; p=0.003, Figure 1). The nutrient content in the surface water is on a moderate level. Phosphorus content during May – September was 13 – 15 µg/l in 2006 and nitrogen was between 250 - 300µg/l. There were no obvious changes of the nutrient content during the last 15 years (data from Husö biological station).

 

Figure 1. Oceanographic parameters at Finbo monitoring area.

 

Local pressure

The surrounding land areas are sparsely inhabited and pollution sources are lacking. A few professional fishermen are active in the area, but sport fishing and household fishing are common.

 

Results and assessment 

The presented time period for the local fact sheet of Finbo monitoring area include the years 1991 - 2008. Significant changes over time are indicated with the line of best fit.

 

Community indicators

Number of species

The number of freshwater species has declined since 1991 (regression, p=0.012). There was no trend for marine species despite the salinity decrease. The significant decrease of number of freshwater species is probably a result of catches of few rare species in the early years of the investigation period.

 

Figure 2. Number of species in Finbo monitoring area.

 

Total biomass

A significant increase of total biomass has occurred during the investigation period (regression, p=0.001; Figure 3). High temperature resulted in increased growth rate and strong year classes for the dominating warm water species, which in turn generated increased total biomass. Total biomass is dominated by the key species perch, and any change in perch biomass affects the total biomass (see below biomass of key species indicators).

 

Species diversity

Species diversity has significantly decreased during the investigation period (regression, p=0.0001; Figure 3). The decrease is related to catches of a few individuals of rare species in the beginning of the period and to increased dominance of perch and roach at the end of the period.

 

 

Figure 3. Total biomass and Shannon-Wiener diversity index at Finbo monitoring area.

 

Slope of size spectrum

The slope of size spectrum increased (regression, p=0.049; Figure 4) during the investigation period 1991 - 2006, but this trend is absent when the results from the two last years are included. This indicates that fishing pressure, growth rate and productivity has remained unchanged.

 

Average trophic level of catch

 No trends in the average trophic level has occurred during the investigation period (Figure 4). A peak appeared in 2003, which was caused by large share of perch. Perch, as a carnivore, possesses a high value for trophic level, compared with  low value species such as roach. The stable trend indicates that fishing pressure has not changed during the investigation period.

 

Figure 4. Slope of size spectrum and trophic level of coastal fish at Finbo monitoring area.

 

Key species indicators, perch

The key species at Finbo monitoring area is perch, a warm water, freshwater species that dominates the catches in the area. Roach may be regarded as an other important key species in the area. The development of roach biomass is added as an additional indicator.

 

Perch and roach biomass

Perch biomass has increased significantly since 1991 (Figure 5; regression, p=0.0003). As there is no trend in the mean age development (Figure 6), the biomass increase indicate increased growth rate and productivity, which could be induced by higher temperatures. Roach biomass has also increased during the investigation period (Figure 5; regression, p=0.001). The increasing development of roach is probably explained by the changed oceanographic conditions in the area. The species is regarded as a indicator of increased eutrophication. A catch peak appeared in 2005 for both species. Favourable catch conditions were partly the reason.

 

Figure 5. Perch and roach biomass at Finbo monitoring area.

 

Mean age and mortality

Mean age of perch shows no trend (Figure 6), which indicate that the fishing pressure have not changed during the investigation period. Mortality has remained unchanged during the 17 years period (data from 2008 is under preparation) indicating a stable fishing pressure and mortality of other causes such as predation and diseases (Figure 6).

 

 Figure 6. Perch mortality at Finbo monitoring area.

 

Mean length and slope of size spectrum

The two indicators are related and both tend to increase over time. However, mean length is not significant for the investigation period while the slope of size spectrum shows a significant increase (regression, p=0.005). This indicates that perch have become larger over time and as already discussed, the mean age has not changed. It indicates that growth rate and productivity has increased.

 

Figure 7. Mean length and slope of size spectrum at Finbo monitoring area.

Methods used

Fishing with coastal survey nets (mesh sizes 17, 22, 25 33 and 50 mm from knot to knot) at eight stations was annually performed. The stations were repeatedly fished for six times (three times 2005 - 2006) 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

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: 3 February 2009; Kaj Ådjers