Nikolaj Gedsted Andersen:
The original idea with this project was to study the susceptibility of fish towards fish diseases during sublethal concentrations of harmful algae. Harmful algal blooms in the sea can have deleterious impacts directly on fish in mariculture. But very little is known about fish health during exposure to sublethal concentrations of harmful algae. In Danish waters Pseudochattonella spp. currently pose the greatest problem for mariculture of fish. Since 1998, Pseudochattonella spp. has formed early spring blooms off the coasts of Scandinavia, causing massive kills of cultured and wild fish. Knowledge about their biology, bloom formation and ichthyotoxic mechanism is very scarce, but it seemed to be the relevant alga for studying susceptibility of fish towards harmful algae and diseases.
We isolated several strains of Pseudochattonella farcimen from a bloom in 2009, and did some initial growth experiments to test the best growth conditions for studying their effects on fish. We show that P. farcimen is adapted to successfully compete and grow in a low light and temperature regime, which it is exposed to in the late winter and early spring. We could not demonstrate the ichthyotoxic effect of P. farcimen in the laboratory on rainbow trout (Oncorhynchus mykiss) using cultured strains of P. farcimen from Danish waters. The exact reason is unknown at present, but could be related to environmental conditions before and during the fish trials, i.e. temperature and pH. Furthermore, we could conclude that the shape of P. farcimen cells could serve as an indicator of the growth condition and probably also of the degree of ichthyotoxicity.
In 2011, we established a field experiment during an early spring bloom to study the ichthyotoxic effect of the alga under controlled natural bloom conditions. We managed to eliminate the ichtyotoxic effect of the bloom water by continuous removal of the Pseudochattonella farcimen cells with a sand filter. Nevertheless, we were unable to work with this organism as a model organism for studying susceptibility of fish towards fish diseases. We therefore selected the well-known ichthyotoxic alga Prymnesium parvum for these studies.
Initially, we tested the long-term mortality of rainbow trout when exposed to whole cultures of Prymnesium parvum. This resulted in a LC50 based on fish mortality after 5 days of 1.11×105 cells ml-1 and most surprisingly, an observed short term acclimation of the fish towards the algae. Thereafter, we tested if sublethal algal concentrations increased the susceptibility of the fish to viral haemorrhagic septicaemia virus (VHSV), one of the most significant viruses in mariculture. The outcome of these studies turned out to be dependent upon how the exposure was carried out. If fish were exposed to the algae for 6 hours prior to the addition of virus, the fish were less susceptible to VHSV compared to fish given the control treatment without algae. If virus and algae were added simultaneously, inclusion of the algae significantly increased the mortality by approximately 50 % compared to fish exposed to virus only. Therefore, depending on the local exposure conditions, sublethal concentrations of P. parvum can affect susceptibility of the fish to infectious agents like VHSV.