Influence of cyanobacterial blooms on coastal fish recruitment

Sammanfattning: Cyanobacteria are the oldest oxygen-producing organisms on Earth and can be found in almost every terrestrial and aquatic habitat. Their long evolutionary history has enabled them to develop diverse adaptations in order to increase their survival in environments subjected to natural and anthropogenic changes. The frequency and intensity of cyanobacterial blooms in the Baltic Sea have increased during the last century. Summer blooms consisting of the filamentous cyanobacterial genera Nodularia, Aphanizomenon and Anabaena may cover up to 50 % of the Baltic Sea surface, which are the largest cyanobacterial blooms in the world. Simultaneously, recruitment of spring spawning fish such as perch (Perca fluviatilis) and pike (Esox Lucius) have decreased along the Baltic Sea coast. Temporal variations in adult fish abundance have been linked to recruitment success, which is dependent on growth of juvenile fish. Generally, low growth rates affect survival of juvenile fish by causing an increase in the time spent in stages prone to predation and by increasing winter mortality which is negatively size selective. Since growth is a crucial factor determining fish recruitment, all parameters that have the potential to influence fish growth could affect fish recruitment dynamics. Cyanobacteria negatively influence fish growth directly (toxicity, turbidity and changes in water quality) and indirectly (toxin transfer, changes in zooplankton community structure). Cyanobacterial toxins i.e. nodularin accumulate in common coastal fish species (flounders, perch and roach) resulting in an energetic cost associated with detoxification. Cyanobacteria, toxic or non-toxic, also affect the behavior of fish (prey capture) further increasing energetic costs. In nature, spatial variations of both cyanobacteria and salinity are a deadly combination for juvenile fish leading to increased detrimental effects of cyanobacteria on juvenile fish in brackish waters compared to freshwater. However, different fish populations react differently to cyanobacteria i.e. a marine population had higher tolerance to cyanobacteria compared to an oligotrophic population. At the coastal ecosystem level, cyanobacteria cannot explain the decline of juvenile fish. Nevertheless, at the local scale cyanobacteria certainly influence the recruitment of juvenile fish.