Sammendrag
The aquaculture industry in Norway produced 940 000 metric tons salmon and trout to a value of 22 billion NOK in 2009 and is the world’s largest producer of farmed salmon. Ichthyotoxic algae may however cause large economic losses for fish farmers in Norway. Harmful algal blooms has been a threat to fish farms in Norway ever since its early era in the early70-ies. In the period 1974-1991 the dinoflagellate Karenia mikimotoi (=Gyrodinium aureolum) formed blooms almost yearly that at times discolored the water brown and caused recurrent fish kills in fish farms along the Skagerrak coast. In 1988 the haptophyte Chrysochromulina polylepis formed the possibly most extensive and toxic bloom ever recorded, killing not only farmed fish but also feral fauna and flora in the Skagerrak and Kattegat off the coasts of Norway, Sweden and Denmark. While this bloom appeared to be a unique event, the related haptophyte Prymnesium parvum formed recurrent blooms in a brackish-water fjord system in Ryfylke, W Norway that caused fish kills in the period 1989-1995. The fish farms were then moved from the most affected fjord arms and when they started up again in 2006 Prymnesium parvum again formed ichthyotoxic blooms. The haptophyte Chrysochromulina leadbeateri similarly killed farmed fish during several years (1991, 2000, 2003, and 2010) in the Lofoten area in N Norway. During the last decade another algal group has caused problems for fish farmers in Norwegian waters, the dictyochophytes, represented by Pseudochattonella and recently Dictyocha species. Pseudochattonella was for the first time recorded in bloom concentrations in N European waters in 1998, and it has been speculated whether it may have been introduced from Asia by ballast water. In 2001, Pseudochattonella formed a massive ichthyotoxic bloom in the Skagerrak and Kattegat, this time in cold waters (2-6 °C). Cultures were isolated, examined and found to represent a new species, Pseudochattonella farcimen, differing in rDNA sequence, ultrastructure and growth preferences from the previously known P. verruculosa (=Chattonella verruculosa), first reported from Japan. Since then Pseudochattonella has been recorded in the Kattegat and Skagerrak yearly and some years in bloom concentrations (e.g. 2006, 2011). The most recent ichthyotoxic algal flagellate to be recorded in Norwegian waters is the silicoflagellate Dictyocha fibula. It may occur in a naked form without the typical silica-skeleton (speculated to be identical to ‘Chattonella globosa’), in bloom concentrations together with the skeleton-bearing form. Fish-kills have occurred since 2007 along the W and S coast of Norway in waters dominated by this species. All these ichthyotoxic species have several features in common. They are nanoplanktonic (size range approx. 5-30 µm in diameter) flagellates with yellow-brown plastids of a haptophyte origin. They usually form blooms after the diatom spring bloom in stratified waters. Some have been shown to be mixotrophic. The toxin structure, production and toxicity are not well understood. Despite ample studies to elucidate factors influencing toxin production and growth we do still not fully understand what triggers the toxicity and bloom formation. Functional genomics may contribute to fill some of these gaps.