Microalgae kill farmed fish in Northern Norway

Fig. 1. Chrysochromulina leadbeateri seen in the light microscope. Photo: Bente Edvardsen, UiO.

Blooms of microalgae has threatened the aquaculture in Northern Norway this spring. In May-June this year (2019), the microalga Chrysochromulina leadbeateri formed blooms in the sea outside Nordland and Troms in Northern Norway that killed more than 13,000 tonnes of farmed fish. This is the most extensive fish mortality caused by toxic algae that has ever been registered in Norway.

These algae are tiny, single-celled organisms that swim in the sea, a type of phytoplankton that belong to the group haptophytes. Chrysochromulina leadbeateri is microscopic, only 3-8 thousand millimeters (microns), but the number can be several millions in one liter of seawater (Fig. 1). They are photosynthetic and use light as energy and water, CO2 and inorganic nutrients to build up sugars and other photosynthetic products. Phytoplankton is food for zooplankton, which in turn is food for fish etc. But many haptophytes can also use organic nutrients and bacteria as food, they are mixotrophic.

We know of about 30 Chrysochromulina -like species in Norwegian coastal waters and only a few of them are known to be toxic to fish. We need to use an electron microscope to tell the species apart (Figs. 2-3).

Prymnesium parvum and Prymnesium polylepis (= Chrysochromulina polylepis) are related species that have caused massive fish kills several times along the Norwegian coast.

From environmental DNA-analyses we know that there is a large unknown diversity of these haptophytes and other microalgae in Norwegian coastal waters and in the Arctic. In the project TaxMArc we discover, isolate and described some of this unknown microalgal diversity.

This spring TaxMArc members Uwe John (AWI) and Luka Supraha (UiO) were on a cruise with the German research vessel Heincke along the Northern Norway coast and collected samples from the bloom. The plankton diversity will be examined by DNA-analyses at AWI, and cultures of Chrysochromulina leadbeateri are now in the process to be isolated at UiO. The cultures will be used in experiments to better understand under which conditions these algae thrive and when they become toxic.

We need to know the algae in order to develop better warning systems and suggest measures to protect farmed fish, that cannot escape, from massive fish kills in the future.  

By Professor Bente Edvardsen, University of Oslo