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

	Author: Atis Minde

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Key Message

Temperature and Secchi depth are important factors in structuring coastal fish communities. In the Daugavgriva monitoring area these factors have varied but show no trend over the years. Daugava South shows signs of greater eutrophication and has higher total fish biomass and lower average trophic level than Daugavgriva North.

Background and oceanographic information

Daugavgriva area is located in the Southern part of Riga Gulf near the mouth of Daugava river. The sea bottom is mainly covered with sand and mud and is generally plain. Water temperature regime in monitoring area during summer season strongly depends on wind direction and strength. Wind-driven upwellings when the water temperature in the shallow coastal zone drops well below 10°C are common phenomenon during summer. Water salinity in the monitoring area depends of movement of freshwater masses coming from Daugava and Lielupe rivers and differs between 2-5 psu.

Figure 1. Mean, minimal and maximal water temperature °C and Secci depth in Daugavgriva area during fish monitoring.

Local pressure

The close vicinity of Riga and Jurmala cities and inlets of two biggest rivers of Latvia - Daugava and Lielupe ensures a wide range of sources of anthropogenic impacts to the area. Most significant of them are: water discharge from wastewater treatment plant of Riga city in the Daugavgriva South, nutrient rich waters of Daugava and Lielupe rivers, as well as pollution coming from Riga port.

There is also a relatively intensive coastal fishery in the monitoring area.

Results and assessment

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 freshwater, marine and migratory fish species in the monitoring area. Freshwater species dominate in the site with no trend over the study period. Number of marine and migratory fish mainly depends on water temperature and weather conditions in nearest pre-sampling period. Lower water temperature attracts more marine and migratory fish to shallow coastal waters.

Total biomass

Figure 3. Total biomass (mean per station in kg) in the monitoring area. Total biomass fluctuates strongly between years but without apparent trend. It is strongly affected by perch biomass (see below). Generally greater biomass in Daugavgriva South indicates increased nutritional conditions, apparently due to strong impact of wastewater treatment plant discharge and nutrient-rich waters of Lielupe river.

Slope of size spectrum

Figure 4. Slope of size spectrum in the outer Daugavgriva area both sides combined. Slope of size spectrum reflects the length distribution of the fish community. Values have been relatively stable since 1995 without any trend. Changes could be explained mainly by recruitment differences as well as fishing intensity.

Species diversity

Figure 5. Shannon – Wiener species diversity index in Daugavgriva area. It represents species richness and evenness of the fish community. Increasing values show increasing numbers of species with even proportion in the catch. No significant change in both sides of Daugava mouth through the study period. 

Average trophic level of catch

Figure 6. Average trophic level of coastal fish community in Daugavgriva area. Lower values indicate greater proportion of bethos-feeding species (roach, white bream) and less top predators (pikeperch and perch). No significant changes were observed throughout the study period in Daugavgriva area.

Key species indicators

Species biomass

Figure 7. Total biomass (mean per station in kg) of key species (perch) in the monitoring area. Variability over time represents fluctuations in recruitment and appearance of strong year classes of perch (see also Figure 3 above). No significant changes occurred during study period.

Mean age

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Mortality

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Slope of size spectrum, mean length

Figure 8. Slope of size spectrum and mean length of Perch in Daugavgriva area. Represents the length distribution of the perch population in the monitoring area. No significant trend was observed during study period. Changes could be explained mainly by recruitment differences as well as fishing intensity.

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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: 28.4.2008