Temporal development of the coastal fish community in Brunskär (Finland), Archipelago Sea

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1 Temporal development of the coastal fish community in Brunskär (Finland), Archipelago Sea Authors: Outi Heikinheimo, Jens Olsson and Eevi Suleva Key Message There was no trend in the water transparency and temperature in the Brunskär area between. Perch dominated in the monitoring catches, and therefore the changes in the indicators largely reflected the fluctuations in the perch stock. The size-related indicators showed a decreasing trend in the period 3, which was mainly due to lowering abundance of large whitefish and ide, and the strong pikeperch year class 997, which was declining in the catches during this period. Small roach individuals were exceptionally abundant in, most probably due to warm summers that produced strong year classes. 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 m within the island groups, except in narrow ravines cutting into the area. Water depth outside the island group may go down to m. The mean surface temperature has been rather high during the assessment period, about C in several years (Figure ). Transparency, measured as Secchi depth, was around -7 m. Nutrient concentrations indicate oligotrophic conditions, which is confirmed by the high water transparency. Salinity in this sea area is -7 PSU.

2 Secchi depth (m) Temperature ( C) N-tot, μg/l P-tot, µg/l Local pressure Human settlements are few and environmental impact is hence small. The fishing pressure in the National Park area is low. There was no change in the environmental variables during the monitoring period (Figure ) Secchi depth Temperature Tot-N Tot-P 8 Figure. Temporal development of Secchi depth and surface temperature during the gillnet monitoring (August), and nutrient concentrations in the sea area near Brunskär (Korppoo) in. No trend was seen in the variables. Results and assessment The indicators used below and their relevance for conveying developments in the environment are derived from HELCOM (), and described in the main indicator fact sheet on Temporal development of Baltic coastal fish communities and key species. Overall state During this relatively short time period the changes in the indicators reflect largely the abundance and size distribution of perch. The number of Large Individuals has decreased, which was due to high abundance of whitefish and ide in the beginning of the period. The PCA-analysis indicates a change in the fish community in Brunskär, in /7. Between - the fish community was characterised by high values for the size related indicators. After 7 the abundance of Large Perch and Large Individuals was lower and that of Marine Species (herring) was higher. At the same time there was an increase in the Non-piscivores.

3 State PC PC Diversity Index Large perch Large individuals Marine species Mean Maximum Length Nonpiscivores Mean Trophic Level Cyprinids Perch abundance Piscivore abundance Total abundance PC PC Figure. Result of an analysis of all indicators together for the Brunskär area (PCA, years -). In the left hand plot, years located close to each other have similar values for the studied indicators. The black line connects the years in a chronological order. The right hand plot shows the indicators mainly characterizing the different years, by pointing in the direction of that year. Long vectors indicate a strong relationship between the indicator and the years studied. Only the first two ordination axes of the PCA is presented, the first axis (PC-) explained 8.% of the total variation observed in the data set, and the second axis (PC-) explained 8. %. There seems to have been a change in the fish community in Brunskär, in /7. Between - the fish community was characterised by high values for the size related indicators. After 7 the abundance of large perch and large individuals was lower and that of marine species (herring) was higher. At the same there was an increase in the nonpiscivores, mainly cyprinids. - Species composition Total Abundance Figure 3. Temporal development of the Total Abundance (CPUE) in the Brunskär area. The area was monitored using Nordic nets in the years -. There is no clear trend in the Total Abundance in the Brunskär area. The indicator is strongly affected by changes in perch abundance and is used as an index of the size of the fish community. Increased abundance indicates increased nutrition conditions and production potential.

4 CPUE Perch Abundance Figure. Temporal development of the Perch Abundance (CPUE) in the Brunskär area. The area was monitored using Nordic nets in the years -. There is no clear trend in the Perch Abundance in the Brunskär area. The indicator is strongly affected by changes in year class strengths and is used as an index of the size of the standing stock of the species. In the long term, increased abundance or perch indicates increased nutrition conditions, high summer temperatures and production potential. Cyprinids 3 8 Figure. Temporal development of Cyprinids (CPUE cm) in the Brunskär area. The area was monitored using Nordic nets in the years -. There is inter-annual variation in this indicator but no significant trend. There seems to be generally less cyprinids in Brunskär than in the other Finnish areas (Tvärminne, Helsinki and Kaitvesi). The peak in was caused by the abundance of small roach ( cm). Cyprinids is an indicator of the abundance of species from carp family, and is influenced by prevailing nutrient and temperature conditions. Increasing cyprinid abundance indicates increased nutritional conditions, production potential and water temperatures.

5 CPUE Marine Species 8 8 Figure. Temporal development of Marine Species (CPUE) (all length classes) in the Brunskär area. The area was monitored using Nordic nets in the years -. There is inter-annual variation in the indicator values but no significant trend. Indicator is sensitive to events of upwelling of cold and saline water. The value of Marine Species indicator in the Brunskär area is mainly dependent on the occurrence of herring (Clupea harengus). Generally, decreased abundance of marine species indicates lowered salinity levels and increased water temperature. Size structure Large Individuals Figure 7. Temporal development of Large Individuals (CPUE) in the Brunskär area. The area was monitored using Nordic nets in the years -. There is a decreasing trend in this indicator (R =.8, P=.). Large Individuals is an indicator of perch, pike and pikeperch, but also large cyprinid species like common bream. In the Brunskär area ide, and Baltic whitefish were the most numerous species of large individuals ( 3). The indicator is dependent on the temperature conditions and fishing pressure, but to lesser extent also on nutrient status. Decreased values of this indicator are associated with decreased water temperature, increased fishing pressure and decreased nutrient conditions, but also reflect year-class strengths of the species.

6 cm Mean Maximum Length Figure 8. Temporal development of Mean Maximum Length (cm) in the Brunskär area. The area was monitored using Nordic nets in the years -. There seems to be a slight decreasing trend in the indicator (R =., p=.). Mean Maximum Length is an indicator of the size structure in the fish community, and is a measure of the relative abundances of large and small species in the community. As such it also to some extent considers recruitment of different species, in this case especially perch. The indicator is dependent on the temperature conditions and fishing pressure, but to a lesser extent also on the nutrient status. Decreased values of this indicator are associated with decreased water temperature, increased fishing pressure and decreased nutrient conditions. Large Perch Figure 9. Temporal development of Large Perch (CPUE cm) in the Brunskär area. The area was monitored using Nordic nets in the years -. There is a decreasing trend in this indicator (R =.78, p<.). Large Perch indicator is dependent on the temperature conditions and fishing pressure, but to lesser extent also by nutrient status. Decreased values of this indicator may be associated with decreased water temperature, increased fishing pressure or decreased nutrient conditions, but the indicator is largely affected by occasional strong year classes born in warm summers (in this case 997).

7 Index Trophic structure Mean Trophic Level,,,3,,, 3,9 3,8 3,7 3, 3, 8 Figure. Temporal development of Mean Trophic Level in the Brunskär area. The area was monitored using Nordic nets in the years -. There is no clear trend in the indicator values. Mean Trophic Level reflects the trophic status of the fish community, where increasing values indicate that a higher proportion of species are at higher trophic levels, e.g., piscivorous fish and/or lower levels of the plankton and benthos feeding species. This is well seen in the indicator values; in the year 3 there was a high value in the Mean Trophic Level index and also high value in the Piscivores index. In the year there was a low value in the Mean Trophic Level index and a high value in the Cyprinids index. Mean Trophic Level was generally higher in Brunskär than in the other Finnish areas (Tvärminne, Helsinki and Kaitvesi). Generally, the indicator is influenced by fishing pressure and nutrient levels. Piscivore Abundance Figure. Temporal development of Piscivore Abundance (CPUE) in the Brunskär area. The area was monitored using Nordic nets in the years -. There is inter-annual variation in the indicator values but no significant trend. Piscivore Abundance reflects the abundance of piscivorous fish in the community and as such also its trophic status. Generally, the indicator has a positive response to decreased fishing pressure.

8 Index CPUE Non-piscivores Figure. Temporal development of Non-piscivores (CPUE) in the Brunskär area. The area was monitored using Nordic nets in the years -. There is a slight increasing trend in the indicator (R =., p=.). Non-piscivore Abundance reflects the abundance of fish of a low trophic level (below ) in the community and as such also its trophic status. Generally, the indicator has a positive response to increased fishing pressure and nutrient level, and is connected to the abundance of cyprinids. Species diversity Diversity Index,,8,,, 8 Figure 3. Temporal development of Diversity Index (Shannon Wiener Diversity Index) in the Brunskär area (all length classes). The area was monitored using Coastal survey nets in the years -. There is inter-annual variation in this index but no significant trend. The values of this indicator are generally lower than in the other Finnish areas (Helsinki, Tvärminne and Kaitvesi). This might be associated with the dominance of perch in the catch. The Shannon- Wiener index reflects the species richness and equitability of the community. Decreasing values indicate community changing towards few dominating species and a small number of species. Calculations are based on the biomass proportion of each species.

9 Methods used Fishing with Nordic survey nets (mesh sizes,,, 9,, 3, 38, 8 and mm from knot to knot) was performed annually once in August at 39 randomly selected stations, divided into four depth intervals. This method was introduced in and has since then been used. The former method with coastal survey nets (mesh sizes 7,,, 33 and mm from knot to knot) was used annually (99-) at nine stations. The stations were repeatedly fished for six times in early August. The time period for the assessment of fish community status in the Brunskär monitoring area include the years. Monitoring was performed with Nordic coastal multi- mesh gillnets and the analyses are based on relative abundance of fish (CPUE). Fish smaller than cm were excluded from the analyses (with the exception of Mean Trophic Level and Diversity Index). A different monitoring method using coastal survey nets was performed during 99, and the results are not directly comparable as they were based on relative biomasses (WPUE) ( A different set of indicators were assessed and only a few indicators were common for both assessment periods. References HELCOM. Indicator based assessment of coastal fish community status in the Baltic Sea -9. Balt. Sea Environ. Proc. No. 3B For reference purposes, please cite this Baltic Sea environment fact sheet as follows: [Author's name(s)], [Year]. [Baltic Sea environment fact sheet title]. HELCOM Baltic Sea Environment Fact Sheets. Online. [Date Viewed], Last updated: September 3