4 Interactions of Wild and Reared Fish and Invertebrates
135
lower reproductive fitness. For other aquaculture species, there are undoubtedly
examples of major ecological effects of translocated species (see above in relation
to Crassostrea gigas and Ruditapes phillipinarum). However, whether this is also a
problem with native finfish and invertebrate aquaculture species other than salmon,
remains to be investigated. Effective population size (N e ) of wild populations of
species such as cod is likely to be exponentially larger than for salmon (assuming
healthy populations that have not been severely reduced by overexploitation- but
see Hutchinson et al. (2003) concerning cod), but this has not been established for
all the major aquaculture species. Furthermore, current aquaculture production is
much lower than for salmon and it seems unlikely that N e of wild populations of
these species will be reduced sufficiently to make genetic drift a major factor in
reducing variability or altering genetic composition. However, a very rapid rise in
production is anticipated and it should be noted that where wild conspecifics occur
and interbreeding is going on, it may be difficult to identify or distinguish indirect
from direct genetic effects.
Introduced diseases are a major concern in marine biology in general and can
have profound ecological effects in the present context (as mentioned earlier, there
are examples in salmon of the major effects of furunculosis and Gyrodactylus in
Norway). Translocated species or sub-species may be more likely to cause
damaging effects in this respect, since the diseases they carry are likely also to be
genetically different or exotic to local taxa. Exotic diseases may be carried by
introduced species, which have a relatively minor effect on their normal host, but
can have a serious impact on naïve and often highly sensitive native species. In
Bonamia ostreae infection of the oyster Ostrea edulis, it appears that prior to initial
exposure, all naïve oysters are susceptible to infection resulting in heavy mortalities
(Culloty et al. 2004). This has recently been demonstrated, with an extension of the
range of this parasite from North America into Canada (Marty et al. 2006), within
Europe to Scotland (http://www.scotland.gov.uk/News/Releases/2006/07/27154609),
and from Europe into Morocco (http://www.oie.int/eng/info/hebdo/AIS_43.HTM).
For Dermo disease in Crassostrea virginica (caused by Perkinsus marinus), the
pathogen is now ubiquitous along the Atlantic and Gulf coasts of the USA, having
recently extended its range to the Maryland portion of Chesapeake Bay and northward along the Atlantic coast from New Jersey to Maine. In addition, it has been
found that different regions can possess unique assemblages of genetic strains of
the parasite (Reece et al. 2001). Furthermore, comparisons of clonal and parental
culture genotypes indicates that cultures initiated from a single oyster can be
polyclonal, showing that an individual can be infected with multiple strains, thus
making any control measures more difficult. Recently, in the study of bivalve diseases, methods such as suppression subtractive hybridisation have been used to
look at gene expression in susceptible and resistant bivalves to such pathogens as
Perkinsus marinus in Crassotrea virginica and Crassostrea gigas (Tanguy et al.
2004) and to bacteria-challenged Crassotrea gigas (Gueguen et al. 2003), to determine the role of particular immune components in response to infection. One limitation to investigating gene expression in invertebrates is that for a number of
species, the full complement of immune components has still to be determined.
135
lower reproductive fitness. For other aquaculture species, there are undoubtedly
examples of major ecological effects of translocated species (see above in relation
to Crassostrea gigas and Ruditapes phillipinarum). However, whether this is also a
problem with native finfish and invertebrate aquaculture species other than salmon,
remains to be investigated. Effective population size (N e ) of wild populations of
species such as cod is likely to be exponentially larger than for salmon (assuming
healthy populations that have not been severely reduced by overexploitation- but
see Hutchinson et al. (2003) concerning cod), but this has not been established for
all the major aquaculture species. Furthermore, current aquaculture production is
much lower than for salmon and it seems unlikely that N e of wild populations of
these species will be reduced sufficiently to make genetic drift a major factor in
reducing variability or altering genetic composition. However, a very rapid rise in
production is anticipated and it should be noted that where wild conspecifics occur
and interbreeding is going on, it may be difficult to identify or distinguish indirect
from direct genetic effects.
Introduced diseases are a major concern in marine biology in general and can
have profound ecological effects in the present context (as mentioned earlier, there
are examples in salmon of the major effects of furunculosis and Gyrodactylus in
Norway). Translocated species or sub-species may be more likely to cause
damaging effects in this respect, since the diseases they carry are likely also to be
genetically different or exotic to local taxa. Exotic diseases may be carried by
introduced species, which have a relatively minor effect on their normal host, but
can have a serious impact on naïve and often highly sensitive native species. In
Bonamia ostreae infection of the oyster Ostrea edulis, it appears that prior to initial
exposure, all naïve oysters are susceptible to infection resulting in heavy mortalities
(Culloty et al. 2004). This has recently been demonstrated, with an extension of the
range of this parasite from North America into Canada (Marty et al. 2006), within
Europe to Scotland (http://www.scotland.gov.uk/News/Releases/2006/07/27154609),
and from Europe into Morocco (http://www.oie.int/eng/info/hebdo/AIS_43.HTM).
For Dermo disease in Crassostrea virginica (caused by Perkinsus marinus), the
pathogen is now ubiquitous along the Atlantic and Gulf coasts of the USA, having
recently extended its range to the Maryland portion of Chesapeake Bay and northward along the Atlantic coast from New Jersey to Maine. In addition, it has been
found that different regions can possess unique assemblages of genetic strains of
the parasite (Reece et al. 2001). Furthermore, comparisons of clonal and parental
culture genotypes indicates that cultures initiated from a single oyster can be
polyclonal, showing that an individual can be infected with multiple strains, thus
making any control measures more difficult. Recently, in the study of bivalve diseases, methods such as suppression subtractive hybridisation have been used to
look at gene expression in susceptible and resistant bivalves to such pathogens as
Perkinsus marinus in Crassotrea virginica and Crassostrea gigas (Tanguy et al.
2004) and to bacteria-challenged Crassotrea gigas (Gueguen et al. 2003), to determine the role of particular immune components in response to infection. One limitation to investigating gene expression in invertebrates is that for a number of
species, the full complement of immune components has still to be determined.