Within the last 30 years the number of recorded harmful algal blooms in coastal regions has increased on a worldwide basis. A number of reasons have been put forward to explain this, e.g. increased nutrient load to marine waters due to anthropogenic activities and global warming due to increased burning of fossil fuels. Similarly, blooms of single-celled harmful algae also seem to have increased in Danish waters where they form a recurring problem with extensive impact on both caged and wild fish. Toxic blooms have on a number of occasions eradicated entire fish populations in just a few days. This of course has detrimental economic consequences for the implicated fish farmers and the general public that use the marine environment for recreational and commercial fishing.

In Danish waters, fish farming has the potential to become a profitable business but in order for this to occur we need to establish modern means to cope with blooms of fish-killing algae. It is speculated that fish are killed by either toxins leaked directly into the open water column or through clogging of fish gills by toxic algae. Yet our understanding of how fish are affected, what are the responsible toxins and how best to quantify them is still at an early stage. The algae that are implicated in fish-killing events in Denmark belong to a variety of different algal groups and they tend to cause problems in spring and summer. However, within the last 10 years, we have also witnessed winter blooms with huge impact on local fish populations. The most important species in Danish waters associated with fish kills are Pseudochattonella farcimen (a dictyochophyte), Prymnesium parvum and Chrysochromulina polylepis (haptophytes), and Karenia mikimotoi (a dinoflagellate).

Prymnesium parvum. Photo: Gert Hansen/SCCAP

On top of this we have algal species with ichthyotoxic potential, which have not yet been associated with fish kills in Danish waters, but which have been implicated in fish kills in other parts of the world (i.e. the dinoflagellates Alexandrium spp. and Karlodinium spp. Most of these algal species are not readily identified and quantified with conventional microscopical techniques used by consultancies. In addition, no chemical protocols are available for authorities to monitor for the toxins of these algae. Both these facts may lead to improper warnings of the fish production industry and of the public.

The majority of literature on the effects of ichthyotoxic algal species on other organisms come from bioassay tests in which the ability of the toxic algae to kill other algae and grazers (often termed “allelopathy”). In some cases blood cells (mammalian and fish blood cells) have been used as bioassays. The typical effect of the algae observed in these tests is cell lyses of target cells. Cell lysis can in many cases be induced using cell free filtrates of dense cultures of the toxic alga, indicating that the toxins are leaked to the surroundings, and that the effect is not caused by a cell-to-cell contact. In a few cases (i.e. Karlodinium spp. and maybe Pseudochattonella farcimen) toxins are not leaked to the medium, and a cell-to-cell contact is required to obtain an effect. Very few papers have actually studied the effects of ichthyotoxic algae on fish directly under controlled conditions and in most cases the mechanism of action of the toxic algae is unknown. The typical effects observed on wild and caged fish that have been exposed to a bloom of these ichthyotoxic algae include loss of normal swimming behaviour and huge secretion of mucus on the fish gills. Fish collected during blooms typically show necrosis of the gills with separation of the outer layer of gill epithelial cells. At present no data are available on what actually causes death of fish. Do they die from oxygen deficiency due to heavy mucus production as a response to the toxic algae or do they die from gill epithelial destruction leading to either lower levels of toxins entering the fish or from cell lysis of red blood cells – or a combination of these effects?

Alexandrium pseudogonyaulax. Photo: Gert Hansen/SCCAP

Very little is known about the exact chemical nature of the ichthyotoxic substances produced by the algae associated with fish kills in Danish waters (or worldwide). Thus, monitoring the toxicity of algal blooms made by these algae is currently not possible. Many different compounds, typically polyethers and fatty acids isolated from ichthyotoxic algae have been proposed as the active compounds responsible for fish kills, but very few of these have actually been tested on fish or fish fry in relevant concentrations. Recent work on Prymnesium parvum has documented that the toxins "Prymnesin 1 & 2" (polyoxy-polyene-polyethers) usually thought to be ichthyotoxic, in fact are not those that are ichthyotoxic. In some preliminary work we have shown that trout can be acclimated to the ichthyotoxins produced by P. parvum, but the mechanism behind the acclimatization is unknown. Other haemolytic toxins like the “Haemolysins” produced by Chrysochromulina polylepis and Karenia mikimotoi are structurally characterized, but it is still unknown if these are the responsible toxins for fish kills. In some cases the toxins (Karlotoxins) are not leaked into the medium, like in Karlodinium spp. and the fish kills may be due to “swarming attacks” from the algae that feed directly on the fish.