Toxic cyanobacterial blooms are an important problem worldwide. Cyanobacteria may negatively impact young-of-the-year (YOY) fish directly (toxin production, turbidity, decrease in water quality) or indirectly (trophic toxin transfer, changes in prey species composition). Here we test whether there are any differences in cyanobacterial tolerance between four geographically distinct populations of European perch (Perca fluviatilis). We show that P. fluviatilis may develop tolerance against cyanobacteria demonstrated by the ability of individuals from a marine site (exposed to annual cyanobacterial blooms) to increase their detoxification more than individuals from an oligotrophic site (rarely exposed to cyanobacteria). Our results also revealed significant interaction effects between genotypes within a population and response to cyanobacterial exposure in terms of absolute growth and detoxification activity. This genotype by treatment interaction may result in local adaptations to cyanobacterial exposure in P. fluviatilis. Hence, the sensitivity against cyanobacterial exposure may differ between within species populations increasing the importance of local management of fish populations.

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.  

AbstractThis study reveals that both cyanobacterial toxicity and turbidity have the potential to reduce the growth and energy storage of young-of-the-year (YOY) perch and thereby influence survival rates. During the 1990's a reduction in recruitment of YOY perch (Perca fluviatilis) occurred along the Swedish East coast. Concurrently, large blooms of filamentous cyanobacteria have increased in the Baltic Proper and in coastal waters. This study examined whether extended exposure to toxic and non-toxic filamentous cyanobacterium Nodularia affect YOY perch growth and feeding behavior under simulated bloom conditions (30 days at 50 μg Chl a L−1). Specific growth rate (SGR), the somatic condition index (SCI) and the lipid content of YOY perch (10–12 weeks old) were significantly lower in perch exposed to Nodularia compared to fed controls (no Nodularia). YOY perch exposed to non-toxic Nodularia displayed a higher attack rate than perch living in Nodularia free controls in 2 out of 3 trials. Reductions in growth and energy storage, mediated by cyanobacteria, increase the risk of starvation and predation and could locally influence recruitment of YOY perch.

Highlights► We investigate the effects of toxic and non-toxic cyanobacterial (Nodularia sp.) on young-of-the-year (YOY) perch (Perca fluviatilis). ► Endpoints are specific growth rate (SGR), lipid content and feeding behavior (feeding and attack rate). ► Results show that both non-toxic and toxic Nodularia reduce SGR and lipid content of YOY perch. ► Reduced growth and energy storage may locally influence recruitment of YOY perch. 

The brackish, bloom-forming cyanobacterium Nodularia spumigena produces a peptide called nodularin, which may induce liver damage in fish. In the summer of 2007, nodularin was detected in liver tissue of European flounder caught in Swedish waters of Öresund, within the upper salinity limit for N. spumigena. Nodularinconcentrations ranging between 22 and 557 μg kg⁻¹ liver (d.w.) were detected in fish liver. Nodularin was not detected in blue mussels (Mytilus edulis). Although N. spumigena blooms can occur in the area, the cyanobacteria were only present in very small amounts in 2007. Results suggested that nodularin accumulated inflounder livers during the summer of 2006, when vast N. spumigena blooms were observed in Öresund, and persisted over several months. Nodularin has previously been shown to induce oxidative stress in mice, crustaceans and mollusks but work on the potential negative effects of nodularin on fish is still scarce. To examine the dynamics of nodularin induced oxidative stress in liver tissue of flounder, the differential responses of the antioxidant enzymes glutathione-S-transferase catalase(CAT) and the formation of malondialdehyde (MDA) were monitored during 14 days in flounder exposed to an intraperitoneal injection of nodularin (0, 2, 10 and 50 μg nodularin kg⁻¹ body weight). The activities of GST and CAT in the liver decreased significantly in the 50 μg nodularin kg⁻¹ exposure after 7 days, but were restored to control levels after an additional 10 days of recovery. The results suggested that nodularin induced oxidative stress in terms of decreased GST and CATactivity, which can result in increased vulnerability of the cell to reactive oxygen species (ROS). No significant changes could be found in MDA levels between the treatments. Thus, the antioxidant defense system presumably managed to prevent oxygen mediated toxicity as seen by the unchanged levels of MDA. Alteration of the enzymatic defense system may increase energetic costs, thus reducing fish growth and survival. The present study also suggests that oxidative stress biomarkers can be used in fish to detect early responses to nodularin.