CASE STUDY #36
Introduction and spread of the “Russian-Doll Salmon Killer” in Norway following salmon transplant
Since its introduction the monogenean ectoparasite Gyrodactylus salaris has had devastating effects in Norwegian Atlantic salmon (Salmo salar) populations over the past four decades. In this period, the in- fection has spread to some 46 rivers. The parasite lives and reproduces in fresh- or slightly brackish water only, and the major mechanism for spread among geographi- cally separate regions has been directly related to movement of live fish between rivers or hatcheries. Today’s routines regulated by adjusted local legislation as well as EU legislation aims to preserve local populations and thus avoid mixing between geographically isolated strains as well as it aims to avoid spread of dis- ease and pathogenic organisms. Unfor- tunately, a slip in these principles during the 1970’ies, led to the sad story of the “Russian-Doll Salmon Killer” in Norway.
Gyrodactylus salaris is found parasitiz- ing several populations of salmon in the Baltic, a geographic region to which it is considered endemic. It was first observed in Norway in 1975. Later it has been con- cluded that it was introduced from the Baltic on several occasions, and that at least three of these introductions led to persisting epidemic outbreak (for recent literature, see e.g. Hansen et al. 2003 and Bakke et al. 2007). In Norway, the parasite generally reduces affected salmon popula- tions by 80–90%. In six populations, the parasite is considered having eliminated the salmon by driving the populations below sustainable density levels. Norwe- gian management authorities spend vast amounts every year on control measures
Gyrodactylus salaris has been referred to as the “Russian-Doll Salmon Kill- er” due to its mode of reproduction. Throughout their lifespan these her- maphroditic flukes they switch from asexual to sexual reproduction. A first- born of an individual is always asexual- ly derived. Furthermore, any newborn individual carries a nearly fully devel- oped offspring inside, again carrying a developing embryo – thus the reference to a Russian-Doll. In the picture, the nearly fully developed attachment or- gan (the opisthaptor consisting of two large hooks; the hamuli and 16 smaller hooks; marginal hooks) of the daugh- ter and the developing opisthaptor of the granddaughter can be seen.
in attempts to eliminate the parasite and prevent further spread. It has been es- timated that without control measures, reduction in Norwegian salmon fisheries would sum up to 15% (Johnsen and Jen- sen 2003).
All known introductions of G. salaris to Norway are results of transport of live fish. One of these transports was destined a central hatchery and farming facility from which salmon parr were distributed to several local hatcheries. In a recapture of the Gyrodactylus story in Norway, Johnsen and Jensen (1986) pointed out that of 14 geographically isolated regions in which G. salaris was found by 1985, 11 had received fish from hatcheries that were secondarily infected from the central one. After estab- lishment in new regions, further spread has in most cases probably taken place by fish movement through brackish water. Thus, the combination of lacking knowledge in the 1970’ies concerning the potential risks of moving even closely related fish stocks and the entering-point in a centralized hatchery turned out catastrophic.
The story of the “Russian-Doll Salmon Killer” in Norway has led to massive fo- cus from parasitologists and freshwater biologists over the past four decades (see Bakke et al. 2007). Despite the lesson learned and increased knowledge gained, the occasional surprising discovery of the parasite on new hosts and in new locali- ties gives an uncomfortable hint as to how little is really known about this and other potentially emerging diseases and patho- genic organisms.
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