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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
Introduction and spread of the “Russian-Doll Salmon Killer” in
Norway following salmon transplant
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.
CASE STUDY #36
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.