4 Interactions of Wild and Reared Fish and Invertebrates
129
1997) and of cod within the North Sea and English Channel in the eastern Atlantic
(Hutchinson et al. 2001) and off eastern Canada (Ruzzante et al. 1999). Although
presently unclear, these population groupings or local populations may be locally
adapted.
As mentioned earlier, there is evidence from Atlantic salmon of adaptational
differences between major population groupings. Bakke et al. (1990) demonstrated
that salmon from the Baltic are resistant to the monogenetic trematode Gyrodactylus
salaris, whereas western European salmon are highly susceptible to infestation,
which usually results in mortality. Thus, it is recommended that for Salmo salar,
there be no movement between major population groupings, at least for stocking
(movement of salmon for farming has taken place from Europe to North America,
but this is now strongly discouraged by relevant Governments). As a precautionary
approach, the same stricture should be applied to other cultured species (e.g., when
sea bass are moved from the Atlantic to the Mediterranean as broodstock, see
recommendations). Less effort has been applied to investigating population
structure in other species cultured in Europe than with salmon, cod and lobsters,
and further studies are urgently required.
4.2.3 Marine Translocated Species
The native population structure of species such as Pacific oyster, Crassostrea gigas
and Manila clam, Ruditapes philippinarum, translocated from other regions is not
relevant in the present context, except that it is important to know the provenance and
pre-translocation history of these reared strains (whether they were taken directly
from the wild, in what numbers and at what life stage; what pathogens occur in a particular area; whether a hatchery generation was used prior to translocation, etc.). It is
also important to know whether individuals originated from one or more natural populations. If the latter (or if a new translocation is planned from a different wild population) there may be problems with outbreeding depression, resulting in decreased
fitness of progeny of crosses between genetically different populations, which may
have performance implications in the area of introduction.
The Pacific oyster, Crassostrea gigas, is now farmed throughout the world with
an annual production of about 3.6 million tonnes. This ubiquitous species was
imported from Japan into British Columbia, Canada in the 1950s where it was
naturalised and from there it was moved to France in the 1970s where, once again
it established breeding populations (Gosling 2003). In 1964 Dutch oyster farmers
imported it from British Columbia to augment native Ostrea edulis stocks. This
introduction was carried out on the premise that these oysters would not reproduce
at the latitude of Dutch coastal waters. However by 1980s C. gigas was able to
extend naturalised populations from the Oosterschelde estuary to the Wadden Sea
area near Texel where it replaced native O. edulis and M. edulis on the intertidal
beds (Rajagopal et al. 2005). It is now posing both a severe ecological and economic
threat to native species and traditional fisheries, respectively, in this region.
129
1997) and of cod within the North Sea and English Channel in the eastern Atlantic
(Hutchinson et al. 2001) and off eastern Canada (Ruzzante et al. 1999). Although
presently unclear, these population groupings or local populations may be locally
adapted.
As mentioned earlier, there is evidence from Atlantic salmon of adaptational
differences between major population groupings. Bakke et al. (1990) demonstrated
that salmon from the Baltic are resistant to the monogenetic trematode Gyrodactylus
salaris, whereas western European salmon are highly susceptible to infestation,
which usually results in mortality. Thus, it is recommended that for Salmo salar,
there be no movement between major population groupings, at least for stocking
(movement of salmon for farming has taken place from Europe to North America,
but this is now strongly discouraged by relevant Governments). As a precautionary
approach, the same stricture should be applied to other cultured species (e.g., when
sea bass are moved from the Atlantic to the Mediterranean as broodstock, see
recommendations). Less effort has been applied to investigating population
structure in other species cultured in Europe than with salmon, cod and lobsters,
and further studies are urgently required.
4.2.3 Marine Translocated Species
The native population structure of species such as Pacific oyster, Crassostrea gigas
and Manila clam, Ruditapes philippinarum, translocated from other regions is not
relevant in the present context, except that it is important to know the provenance and
pre-translocation history of these reared strains (whether they were taken directly
from the wild, in what numbers and at what life stage; what pathogens occur in a particular area; whether a hatchery generation was used prior to translocation, etc.). It is
also important to know whether individuals originated from one or more natural populations. If the latter (or if a new translocation is planned from a different wild population) there may be problems with outbreeding depression, resulting in decreased
fitness of progeny of crosses between genetically different populations, which may
have performance implications in the area of introduction.
The Pacific oyster, Crassostrea gigas, is now farmed throughout the world with
an annual production of about 3.6 million tonnes. This ubiquitous species was
imported from Japan into British Columbia, Canada in the 1950s where it was
naturalised and from there it was moved to France in the 1970s where, once again
it established breeding populations (Gosling 2003). In 1964 Dutch oyster farmers
imported it from British Columbia to augment native Ostrea edulis stocks. This
introduction was carried out on the premise that these oysters would not reproduce
at the latitude of Dutch coastal waters. However by 1980s C. gigas was able to
extend naturalised populations from the Oosterschelde estuary to the Wadden Sea
area near Texel where it replaced native O. edulis and M. edulis on the intertidal
beds (Rajagopal et al. 2005). It is now posing both a severe ecological and economic
threat to native species and traditional fisheries, respectively, in this region.