Temporal development of the coastal fish community in the Curonian Lagoon (Lithuania) Authors: Justas Dainys and Linas Ložys Key Message Overall, in the fish community in the Curonian Lagoon there has rather stable between -. The fish community has rather been exhibiting annual fluctuations with respect to the majority of the indicators assessed. The years of, and do, however, to some extent differ from the other years in the time-series. During these three years the abundance of perch and piscivores was relatively high, as were the diversity, trophic level and proportion of piscivores. Background and oceanographic information Monitoring in the Curonian Lagoon is performed once a year in July at two sites (Atmata and Dreverna). Average depth at Atmata is 4.5 m and about 2 m at the Dreverna site. The Atmata site is characterized by silty bottoms and is located in the area of direct influence of the Nemunas river; as a result the water is always fresh. The Dreverna site is characterized by sandy bottoms and the water is fresh. The area is, however, occasionally affected by brackish water inflows from the Baltic Sea. As a result the salinity in the area can temporarilly increase. Both areas are affetcedby commercial and to some extent recreational fishing, as well as by predation from cormorants. Local pressure Commercial fisheries exhibits one of the main pressures on Curonian Lagoon fish communityies. The commercial fishery have got compensations to leave the sector, and the fishing efforts have been reduced by more than a third (Figure 1). The landings are, however, still rather high suggesting increased efficiency of the fishermen remaining in the sector. It is also known that cormorants consume large numbers of fish (on average 653,3±59,4 tonnes during -) in
Landings (tonnes) Secchi depth (m) Temperature summer ( C) Tot-N (mg/l) Tot-P (mg/l) the Curonian Lagoon (Pūtys a). About 6 breading birds is found on the Curonian spit in Juodkrante (in about 7 km from Dreverna monitoring site). First cormorants started breading at the colony in 1989 and fast growth of the colony continued until ; after number of breading birds fluctuated about 3 pairs (Pūtys b). Nutrient runoff (total phosphorous) to the Curonian Lagoon from land, as a potiential pressure, has a tendency to decrease over the assesement period -, and the secchi depth increased during these years (Figure 1; data provided by the Marine Research Department). Surface water temperature in the Curonian Lagoon have increased by.6 C during the period 1961, however, no trend has been observed during the assesment period (-; Figure 1). Water temperature appears to be the key factor determining the seasonal and long-term variability of primary production and the abundance of phytoplankton, and therefore also the level of biological production and trophic status in the area. The rise in temperatures has also fostered the ongoing eutrophication and hyperblooms of cyanobacteria in the Curonian Lagoon, despite the reduction in external nutrient loading from land in the form of fertilisers and industrial products. Increased incidence of more westerly winds in combination with sea level rise and reduced river discharge has also resulted in a long-term increase in annual mean salinity in the area (Dailidienė et al. ). 1,9,8,7,6,5,4 Secchi depth Temperature summer 22 2 18 16 14 12 1 2 1,8 1,6 1,4 1,2 1,8,6,4,2 Tot-N Tot-P,25,2,15,1,5 16 14 12 1 8 6 4 Figure 1. Temporal development in variables potentially affecting the state of fish communities in the Curonian lagoon. A decreasing trend was seen in commercial landings of fish (R2 =.1, p <.5) and total phosphorous (R2 =.45, p <.5). Increasing trend was seen in Secchi depth (R2 =.34, p <.5). No trend was seen in the variables of total nitrogen and summer temperature (p >.5).
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 Monitoring data cover the period -. There is no overall temporal trend for the calculated indicators (Figure 3-13). Total Abundance is correlated with Cyprinids as a result of that the fish community in the area is dominated by freshwater species from the carp family. Total Abundance, is also correlated to Piscivore Abundance and Abundance of Perch, mainly reflecting a dominance of perch in all years of monitoring (Figure 2). Mean length perch and Mean Maximum Length correlates negatively to Diversity Index and Abundance of Perch. Mean Trophic Level corelates to Diversity Index and Abundance of Perch, and correlates negatively to Mean Length Perch. Some of the indicators e.g. Mean Trophic Level and Piscivore Proportion are negatively correlated to Mean Length Perch. Overall, the fish community in the Curonian Lagoon has not gone through any significant change in its structure since the monitoring program started in (Figure 2). Figure 2. Result of an analysis of all indicators together for the Curonian Lagoon 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-1) explained 46.6% of the total variation observed in the data set, and the second axis (PC-2) explained 21.2%.
Species composition Total Abundance 7 6 5 4 3 2 1 Figure 3. Temporal development of the Total Abundance (, Catch Per Unit Effort) in the Curonian Lagoon. There is no overall temporal trend for the indicator. Total Abundance is used as an index of the relative size and productivity of the fish community. Increased abundance indicates increased nutritional conditions and production potential, whereas decreased abundance indicates the opposite. Perch Abundance 18 16 14 12 1 8 6 4 2 Figure 4. Temporal development of the Perch Abundance () in the Curonian Lagoon area. There is no overall temporal trend for the indicator. Increased abundance indicates increased nutritional conditions and production potential, whereas decreased abundance indicates the opposite.
Cyprinids 4 35 3 25 2 15 1 5 Figure 5. Temporal development of Cyprinids () in the Curonian Lagoon. There is no overall temporal trend for the indicator. Cyprinids is an indicator of the abundance of species from the carp family, and is influenced by prevailing nutrient and temperature conditions. Increased cyprinid abundance indicates increased nutritional conditions and production potential, but also increased water temperatures. Marine Species 1,4 1,2 1,,8,6,4,2, Figure 7. Temporal development of Marine Species () in the Curonian Lagoon. There is high interannual variation of this indicator over time. There is no overall temporal trend for the indicator. The indicator is sensitive to cold and brackish water inflow into lagoon from Baltic sea. Increased abundance of marine species generally indicates increased salinity and lower temperature.
Cm Size structure Large Individuals 7 6 5 4 3 2 1 Figure 8. Temporal development of Large Individuals () in the Curonian Lagoon. There is no overall temporal trend for the indicator. Large Individuals is an indicator of the abundance of individuals in the fish community over 3 cm total length. As such it generally reflects the abundance of mainly large Roach and Pikeperch. The indicator is dependent on the temperature conditions and fishing pressure, but to a lesser extent also by the nutrient status. Generally, increased values of this indicator are associated with increased water temperature, lower fishing pressure and increased nutrient conditions. Mean Maximum Length 39 37 35 33 31 29 27 25 Figure 9. Temporal development of Mean Maximum Length (cm) in the Curonian Lagoon. There is no overall temporal trend for the indicator. 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. The indicator is dependent on the temperature conditions and fishing pressure, but to a lesser extent also by the nutrient status. Generally, increased values of this indicator are associated with increased water temperature, lower fishing pressure and increased nutrient conditions.
Cm Mean Length Perch 22 21 2 19 18 17 16 15 Figure 1. Temporal development of Mean Length Perch (cm) in the Curonian Lagoon. There is no overall temporal trend for the indicator. Mean Length Perch is an indicator of the demographic characters and size structure in the key species Perch. The indicator is affected by variation in recruitment, extensive size selective fishing and changed individual growth rate. As such it is dependent on the temperature conditions and fishing pressure. Generally, increased values of this indicator are associated with increased water temperature and lower fishing pressure.
INDEX Trophic structure Mean Trophic Level 3,6 3,5 3,4 3,3 3,2 3,1 3 2,9 2,8 2,7 Figure 11. Temporal development of Mean Trophic Level in the Curonian Lagoon. There is no overall temporal trend for the indicator. Mean Trophic Level reflect the trophic status of the fish community, where increasing values indicate that a higher proportion of species at higher trophic levels, e.g., piscivorous fish and/or lower levels of the plankton- and benthos-feeding species. Generally, the indicator is influenced by fishing pressure and nutrient levels. Since the indicator is based on proportions of different species, it should preferably be interpreted in association with other indicators such as Cyprinids and Piscivore Abundance. Piscivore Abundance 18 16 14 12 1 8 6 4 2 Figure 12. Temporal development of Piscivore abundance () in the Curonian Lagoon. There is no overall temporal trend for the indicator. High indicator values in is related with Piscivore abundance reflect the abundance of piscivorous fish in the community but to some extent also the trophic status. Generally, the indicator has a positive response to decreased fishing pressure.
Index Species diversity Diversity Index 1,8 1,7 1,6 1,5 1,4 1,3 1,2 1,1 1,9,8 Figure 13. Temporal development of Diversity Index (Shannon-Wiener Diversity Index) in the Curonian Lagoon. There is no overall temporal trend for the indicator, but pronounced interannual variation is seen. The Shannon-Wiener index reflects the species richness and equitability of the community. Increasing values indicate increasing numbers of species with even proportion in the catch. Decreasing values indicate few dominating species and a community with a small number of species. Calculations are based on the proportion of each species. Methods used Fishing with coastal survey nets (mesh sizes 17, 22, 25, 3 mm from knot to knot) was performed annually from at two stations. The stations were repeatedly fished for three nights in the middle July. References Dailidienė I., Davulienė L., Kelpšaitė L. and Razinkovas A.. Analysis of the Climate Change in Lithuanian Coastal Areas of the Baltic Sea. Journal of Coastal Research 28 (3): 557 569. HELCOM.. Indicator based assessment of coastal fish community status in the Baltic Sea -. Balt. Sea Environ. Proc. No. 131B. Pūtys Ž. a. Great cormorant Phalacrocorax carbo sinensis diet and its effect on fish populations and their community in the eutrophic Curonian Lagoon ecosystem. Summary of doctoral dissertation, Vilnius, 48 pp. Pūtys Ž. b. Great cormorant Phalacrocorax carbo sinensis diet and its effect on fish populations and their community in the eutrophic Curonian Lagoon ecosystem. Doctoral dissertation, Vilnius, 151 pp. (In Lithuanian) 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], http://www.helcom.fi/baltic-seatrends/environment-fact-sheets/. Last updated: 26 September 213