Rapid Changes in Salinity and Cyanobacterial Exposure Influence condition of Young of the Year (YOY) Perch (Perca fluviatilis) : A Field Study in the Curonian Lagoon(Lithuania)

Two decades ago the recruitment of YOY perch (Perca fluviatilis) started to decline along the Swedish east cost of the Baltic Sea. Factors that influence recruitment are e.g. eutrophication that causes habitat losses and overfishing of cod (Gadus morhua) which causes cascading effects in the food web. Filamentous cyanobacterial blooms are often toxic and has increased in the Baltic Sea and its coastal waters. The aim of this field study was to evaluate the effects of salinity and cyanobacterial exposure on fitness related parameters of young of the year (YOY) perch (Perca Fluviatilis) in a natural environment. Our study was performed in the Curonian Lagoon (Lithuania) in August 2009. The lagoon offers a temporary salinity gradient (wind induced influxes from the Baltic Sea) ranging from 7 psu in the north to 0 psu in the south. Submerged enclosures containing YOY perch were set up at three different locations along the salinity gradient in the Lagoon (referred to as North, Middle, South). The duration of the experiment was 21 or 27 days, depending on treatment. Measurements of perch condition were specific growth rate, somatic condition index (SCI) and whole fish lipid and protein content. Average chl a values for the three stations during the experimental time were: north 180 ± 70 µg/l chl a, middle 133 ± 36 µg/l chl a and south 180 ± 52 µg/l chl a. The North and the Middle stations experienced two different salinity influxes reaching a maximum salinity of 6.5 psu at the northern station. The duration of each saline influx was approximately 4-6 days. The saline water did not reach the Southern station at any time. Results show that perch from the southern station were in best condition in terms of specific growth rate and contents of total lipids. Compared to the South the perch condition declined to the Middle station and was lowest at the Northern station which experienced the highest degree of fluctuation in terms of salinity and cyanobacterial exposure. Examination of the abundance of the main food resource at the different stations revealed no statistical differences, which suggest that availability of food was not a factor in explaining the differences in growth.  The results possibly indicate that a changing environment with the potential synergistic negative effects of salinity and cyanobacteria has a higher negative impact on YOY perch condition compared to constantly high concentrations of cyanobacteria.