124
T.F. Cross et al.
fecundity (from thousands of eggs per female in Atlantic salmon to millions per
female in cod) and, in the natural situation, mortality is very high (with only two
progeny needing to survive to reproduction, assuming equal sex ratio, to maintain a
stable population size). In the controlled and protected conditions of culture, mortality is usually reduced by one or more orders of magnitude, so a few parents can produce many progeny, often sufficient to entirely fill limited culture facilities. However,
such practises ignore the fact that genetic variability can be exponentially lost as
parental number decreases. In order to minimise such loss of variability or to keep it
within reasonable limits (less than 1% per generation), it is recommended that at least
50 individuals of each sex are utilised (assuming that the animals are not hermaphroditic) (Cross and King 1983). In addition, departure from an equal sex ratio can
greatly increase rate of loss of variability and in this case a larger number of parents
should be used. Aside from genetic depletion and, in contrast to wild populations
where temporal stability in allele frequency between cohorts predominates, different year-classes of reared strains often demonstrate large differences in allele
frequencies.
4.2.1.1 Molecular Studies
Early allozyme studies on Atlantic salmon revealed reduced genetic variability and
inter-cohort differences in allelic composition, in ranched and farmed strains
compared to wild populations, in both European and North American studies (Cross
and King 1983; McElligott et al. 1987; Verspoor 1988; Cross and ni Challanain
1991; Cross et al. 1993). More recent studies using microsatellite loci have
demonstrated similar effects (e.g., Norris et al. 1999). Many of the native marine
species that are currently farmed in Europe (such as cod, halibut, turbot, sea bass,
sea bream, lobster and scallop) also show similar differences from wild populations.
Stefansson et al. (2001), using several microsatellite loci, demonstrated these
effects in certain strains of halibut Hippoglossus hippoglossus but not in others, and
there were similar findings, in relation to reduced genetic variability, in turbot
Scophthalmus maximus (Coughlan et al. 1998). This reduction in genetic variability
can also be the case with non-native introduced aquaculture species, though here an
additional factor can be the small number of individuals in the initial introduction.
An example is the abalone species Haliotis discus hannei that is farmed in Ireland.
This species originates in Japan, and a comparison of wild Japanese individuals
with Irish broodstock shows substantial reduction in the number of alleles in the
latter group, at three microsatellite loci (Coughlan, Burnell and Cross-unpublished).
Since the mussel Mytilus edulis/galloprovincialis and flat oyster Ostrea edulis
farming industries in Europe have almost completely relied on wild-collected
“seed” (juveniles), up to the present, rather than hatchery intervention, there is
unlikely to be a similar problem with these species. However, in the case of
mussels, since a complex and extensive hybrid zone occurs along the western
European coastline from Portugal to the Faeroe Islands (Gosling 2003), there may
be fitness differences between individuals reared in different locations from their
T.F. Cross et al.
fecundity (from thousands of eggs per female in Atlantic salmon to millions per
female in cod) and, in the natural situation, mortality is very high (with only two
progeny needing to survive to reproduction, assuming equal sex ratio, to maintain a
stable population size). In the controlled and protected conditions of culture, mortality is usually reduced by one or more orders of magnitude, so a few parents can produce many progeny, often sufficient to entirely fill limited culture facilities. However,
such practises ignore the fact that genetic variability can be exponentially lost as
parental number decreases. In order to minimise such loss of variability or to keep it
within reasonable limits (less than 1% per generation), it is recommended that at least
50 individuals of each sex are utilised (assuming that the animals are not hermaphroditic) (Cross and King 1983). In addition, departure from an equal sex ratio can
greatly increase rate of loss of variability and in this case a larger number of parents
should be used. Aside from genetic depletion and, in contrast to wild populations
where temporal stability in allele frequency between cohorts predominates, different year-classes of reared strains often demonstrate large differences in allele
frequencies.
4.2.1.1 Molecular Studies
Early allozyme studies on Atlantic salmon revealed reduced genetic variability and
inter-cohort differences in allelic composition, in ranched and farmed strains
compared to wild populations, in both European and North American studies (Cross
and King 1983; McElligott et al. 1987; Verspoor 1988; Cross and ni Challanain
1991; Cross et al. 1993). More recent studies using microsatellite loci have
demonstrated similar effects (e.g., Norris et al. 1999). Many of the native marine
species that are currently farmed in Europe (such as cod, halibut, turbot, sea bass,
sea bream, lobster and scallop) also show similar differences from wild populations.
Stefansson et al. (2001), using several microsatellite loci, demonstrated these
effects in certain strains of halibut Hippoglossus hippoglossus but not in others, and
there were similar findings, in relation to reduced genetic variability, in turbot
Scophthalmus maximus (Coughlan et al. 1998). This reduction in genetic variability
can also be the case with non-native introduced aquaculture species, though here an
additional factor can be the small number of individuals in the initial introduction.
An example is the abalone species Haliotis discus hannei that is farmed in Ireland.
This species originates in Japan, and a comparison of wild Japanese individuals
with Irish broodstock shows substantial reduction in the number of alleles in the
latter group, at three microsatellite loci (Coughlan, Burnell and Cross-unpublished).
Since the mussel Mytilus edulis/galloprovincialis and flat oyster Ostrea edulis
farming industries in Europe have almost completely relied on wild-collected
“seed” (juveniles), up to the present, rather than hatchery intervention, there is
unlikely to be a similar problem with these species. However, in the case of
mussels, since a complex and extensive hybrid zone occurs along the western
European coastline from Portugal to the Faeroe Islands (Gosling 2003), there may
be fitness differences between individuals reared in different locations from their