Temporal development of Baltic coastal fish communities and key species

 

	Authors: HELCOM Coastal Fish Monitoring Experts

 

Key message

High water temperatures during growth seasons in recent years have possibly affected the fish communities in the area in southern Bothnian Sea. This is reflected as an increased recruitment and/or increased individual growth rate of dominating, warm water dwelling, species, like perch (Perca fluviatilis) and roach (Rutilus rutilus). Indications of a development towards more fresh water conditions appeared in Bothnian Sea and Western Baltic Sea. In northern Gulf of Riga a high exploitation rate of piscivorous fish during mid 1990’s was clearly recognised. Flounder (Platichthys flesus), the key species for the southern Baltic Proper, has increased during the period possibly due to population recovery after a previous high exploitation rate.

Results and assessments

Relevance of the indicators for describing developments in the environment

Fish community

Number of species, separated into freshwater and marine species (migratory species are classified into freshwater or marine species according to reproduction habitat)

The number of species caught depends on several factors such as salinity, habitat heterogeneity, temperature and exposure of the area sampled. Number of species also depends on the number of efforts used in the sampling, as the probability to catch a species increases with increased number of efforts and size of the sampled area. Number of species usually decreases with decreasing salinity as marine species disappear and the fish communities become more dominated by freshwater species.

Total biomass

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. Fishing pressure may also affect the total biomass. Total biomass, expressed as weight per unit effort (WPUE, kg), is estimated from actual biomass figures or calculated from the number of caught fish per length group in the length distribution.

Species diversity

Shannon Wiener index reflects the species richness and equitability of the community. Increasing values indicate increasing diversity, caused by a high number of species with even proportion in the catch. Decreasing values indicate low diversity with 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

Slope of size spectrum reflects the length distribution of the fish community. Recruitment variability, extensive size selective fishing, as well as change in species composition in the fish community could affect the slope. Decreasing values indicate a development towards smaller individuals.

Average trophic level of catch

Average trophic level of a fish community reflects both size structure and food web composition. Decreasing values indicate that the proportion of species at higher trophic levels, e.g., piscivorous fish, decreases and that the fish community is largely composed of plankton- and benthos-feeding species. The average trophic level also reflects environmental and anthropogenic pressures such as fishery. The trophic level of coastal fish communities was calculated based on the annual weight per unit effort (WPUE) for each species and area, adjusted by species-specific values for trophic level according to Fishbase (2004).

Key species

Species biomass

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 fishery, as well as changes in interspecific interactions among species. Total biomass, expressed as weight per unit effort (WPUE, kg), is estimated from actual biomass figures or calculated from number of caught fish per length group in the length distribution.

Mean age

Mean age of a species reflects the age distribution in the stock. It is affected by reproductive success and mortality rate (fishing pressure and predation).

Mortality

Mortality is expressed as percent annual decrease for each year of sampling, based on the mortality of individuals that are 3-7 year old. Increased mortality indicates increased fishing, predation or mortality due to other causes.

Mean length

Mean length reflects the length distribution in the fish stock and is affected by variation in recruitment, extensive size selective fishing and/or changed individual growth rate.

Slope of size spectrum

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 steep negative slope indicates presence of few large individuals, while a flat negative slope indicates occurrence of high number of large individuals in relation to small individuals.

Policy relevance and policy references

Coastal fish is a self-sustaining natural resource, which is important for economical, recreational and biodiversity purposes. The self-sustaining capability may be jeopardized by different human activities and global changes, demanding regular assessments of coastal fish communities and stocks. ICES have during a long time made assessments for the Baltic off shore species Baltic herring (Clupea harengus), cod (Gadus morhua) and sprat (Sprattus sprattus).

Assessment

The assessments are based on results presented in table 1 and 2 and on results presented in the local fact sheets listed with links in table 3.

Coastal fish community

Bothnian Bay

The community indicators show stable fish communities in the sampling areas. A decrease in the trophic level at Holmö outer was the only significant trend detected at community level during the sampling period. Increased proportions of plankton- and benthos-feeding species such as roach (Rutilus rutilus) and bleak (Alburnus alburnus) in the early 2000´s caused this trend.

Bothnian Sea, Archipelago Sea

Significant changes in oceanographic conditions, increased water temperature, such as decreased salinity and water transparency, cause important altered environmental conditions for fish. As a result a number of indicators show significant changes in the sampling areas. At Forsmark area, the slope of size spectrum of total catch increased. At Finbo, the number of freshwater species and diversity decreased and total biomass increased. High mean temperature over the growth season, as an important structuring factor, generates strong year classes and enhanced growth rate for fresh water species resulting in significant increasing trends in the fish indicators. No fish community trends appeared in the Brunskär sampling area.

Gulf of Riga, Eastern Baltic Proper

No significant long-term trends were noted in the areas located in the sampling areas in Gulf of Riga. Trophic levels are low in Daugavgriva indicating low proportion of piscivorous fish, which are the main target of the coastal fishery. The decrease in number of marine species at Jurkalne is a result of occurrence of few individuals of rarely caught species at the beginning of the sampling period, resulting in a decreasing number of marine species over time. The decline of fresh water species in Curonian lagoon has similar causes.

Western Baltic Proper, Southern Baltic Proper

Freshwater species have increased at Kvädöfjärden in Western Baltic Proper and in one area in Southern Baltic Proper, probably as a result of more freshwater and eutrophic conditions. Slope of size spectrum increased at Kvädöfjärden, suggesting increased size. At Vinö in Western Baltic Proper, total biomass, decreased mainly as a result of a biomass decrease of the key species perch.

 

Table 1. Indicators for coastal fish communities

 

 

Linear regression analyses were used to detect trends, indicated as below.

 

Linear regression analyses were used to detect trends, indicated as below.

Key species, perch or flounder

Bothnian Bay

Mean age for key species decreased at Holmön area, which may indicate increased growth rate, in combination with increased reproductive success in recent years. Mean length at Råneå decreased, which probably is a result of enhanced recruitment and thus increased intraspecific competition.

Bothnian Sea

The increasing trends of biomass, mean length and slope of size spectrum in the sampling area indicate enhanced growth rate and strong year classes as a result of high temperatures during growth season in recent years.

Archipelago Sea

The increase of biomass of both perch and roach (find Finbo local fact sheet from Table 3) at Finbo may indicate increased eutrophication. The decreasing trends of mean age and length at Brunskär indicate enhanced reproductive success by increased number of young and small individuals. Roach has increased also in this area indicating increased eutrophication (find Brunskär local fact sheet from Table 3).

Gulf of Riga, Eastern Baltic Proper

No significant trends were noted in the sampling areas in Gulf of Riga and Eastern Baltic Proper. High exploitation of piscivorous fish at Hiiumaa resulted in weak populations of perch during mid 1990’s (find Hiiumaa local fact sheet from Table 3). The population has since then recovered to some extent.

Western Baltic Proper, Southern Baltic Proper

At Vinö, located in Western Baltic Proper, a decrease of mean age and mean length indicates a shift towards smaller individuals, which may indicate increased mortality or decreased growth. In the sampling in southern Baltic Proper, the biomass of flounder increased over time in both sampled areas. This development possibly reflects a population recovery after that the high exploitation of the species in 1980’s ceased.

 

Table 2. Indicators for key species 1996-2005

 

 

Linear regression analyses were used to detect trends, indicated as below.

 

 

References

Fishbase, 2004. R. Froese, and D. Pauly (Eds) 2004. World Wide Web electronic publication. www.fishbase.org. version (12/2004).

Ljunggren, L., A. Sandström, G. Johansson, G. Sundblad and P. Karås, 2005. Rekryteringsproblem hos Östersjöns kustfiskbestånd. Finfo, 2005:5. (in Swedish with English summary).

Thoresson, G. 1996. Guidelines for coastal fish monitoring – fishery biology. Kustrapport 1996:2.

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

Data

Data on coastal fish communities are collected by annual test fishing with gill nets in the coastal zone. The test fishing is carried out in August and target species are demersal species large enough to be caught in gill nets. The data providing institutions are:

• The Estonian Marine Institute, University of Tartu, Estonia

• Degreeprogramme of Fisheries and Environment, Turku University of Applied Sciences, Finland
• Finnish Game and Fisheries Research Institute, Finland

• Palmenia Centre for Continuing Education, Helsinki University, Finland

• Institute of Ecology, Vilnius University, Lithuania

• Latvian Fish Resources Agency, Latvia

• National Environmental Research Institute, Aarhus University, Denmark

• Provincial Government of Åland Islands, Åland Islands

• Swedish Board of Fisheries, Sweden

• Sea Fisheries Insitute, Gdynia, Poland

Metadata

Eleven areas, some divided into two different sub areas, are investigated annually on long-term basis using multi mesh sized gillnets. Two more areas are on a similar basis investigated with trawl surveys. Five indicators on coastal fish community level and five on key species level have been evaluated.

 

Table 3. Presentation of areas, gear used and key species studied. Note: To access local indicator fact sheets, click on the area links.

 

Basin	Area	Gear	Key species
Bothnian Bay	Råneå	Coastal survey nets	Perch (Perca fluviatilis)
	Holmön (inner/outer)	Coastal survey nets	Perch (Perca fluviatilis)
Bothnian Sea	Forsmark (inner/outer)	Coastal survey nets	Perch (Perca fluviatilis)
Archipelago Sea	Finbo	Coastal survey nets	Perch (Perca fluviatilis)
	Brunskär	Coastal survey nets	Perch (Perca fluviatilis)
Gulf of Riga	Hiiumaa	Net series	Perch (Perca fluviatilis)
	Daugavgriva (north/south)	Net series	Perch (Perca fluviatilis)
Eastern Baltic Proper	Jurkalne	Net series	Flounder (Platichthys flesus)
	Curonian lagoon (Atmata/Dreverna)	Net series	Perch (Perca fluviatilis)
Western Baltic Proper	Kvädöfjärden (inner/outer)	Net series	Perch (Perca fluviatilis)
	Vinö	Net series	Perch (Perca fluviatilis)
Southern Baltic Proper	Poland	Trawl	Flounder (Platichthys flesus)

 

Figure 1. Map over the Baltic Sea and sampled areas. Click image to enlarge.

 

 

 

For reference purposes, please cite this indicator fact sheet as follows:
[Author’s name(s)], [Year]. [Indicator Fact Sheet title]. HELCOM Indicator Fact Sheets 2008. Online. [Date Viewed], http://www.helcom.fi/environment2/ifs/en_GB/cover/.

 

Last updated: 12 June 2009