120
T.F. Cross et al.
2. The monogenetic trematode, Gyrodactylus salaris, was introduced to Norway
on infested salmon parr used in a stocking exercise. These parr came from the
Baltic area (a different genetic grouping-see below), where salmon are relatively
resistant, and this has led to massive mortality and drastic population reductions
in many Norwegian rivers.
Here, the likely severity of these effects in other major marine and anadromous fish
and invertebrate species, used in both contemporary and emerging aquaculture
ventures, is discussed with consideration being given to how differences in genetic
composition and life cycle in various species may influence these effects.
Consideration is then given to experiments that have investigated the extent of these
problems with different species and methods to reduce detrimental genetic effects
are discussed. Most examples are of eastern North Atlantic and Mediterranean
native species or of species introduced into these areas (e.g., Pacific oyster
Crassostrea gigas), but since ecological analogues occur in other areas it is felt that
the principles discussed will have a wider applicability. Recommendations are then
presented which, it is hoped, will be of particular interest to Governmental and
regional policy makers and to environmental managers.
4.2 Genetic Composition of Cultured Strains Compared
with their Wild Progenitors
4.2.1 Within Population Intra-Specific Comparisons
Reared (cultured) strains* often differ genetically from their wild progenitor populations*, both in levels of genetic variability (usually reduced) and in genetic composition (usually different from progenitors and often temporarily unstable between
reared cohorts). These effects have been demonstrated in numerous species over the
last three decades, using an array of molecular techniques (see Box 4.1).
Genetic variability is usually expressed as heterozygosity (proportion of heterozygotes at a polymorphic gene locus) or as allelic richness (an estimate of the
number of alleles at a specific gene locus). The latter can be a more sensitive indicator of loss of genetic variability and, is thus, more commonly invoked. However,
loss of genetic variability in terms of heterozygosity may result in poor growth and
performance. The most utilised measure of genetic composition is allele frequency
(the proportion of each allele at a specific polymorphic locus in a wild or reared
sample). One of the main causes of reduced genetic variability in reared strains is
the use of much smaller numbers of parents as broodstock than are common in wild
*In this chapter the term “wild population” is used for genetically-distinct statistically-defined
sub-specific groupings, usually reproductively isolated from one another. Reared groupings
derived from wild populations are referred to as “reared strains”.
T.F. Cross et al.
2. The monogenetic trematode, Gyrodactylus salaris, was introduced to Norway
on infested salmon parr used in a stocking exercise. These parr came from the
Baltic area (a different genetic grouping-see below), where salmon are relatively
resistant, and this has led to massive mortality and drastic population reductions
in many Norwegian rivers.
Here, the likely severity of these effects in other major marine and anadromous fish
and invertebrate species, used in both contemporary and emerging aquaculture
ventures, is discussed with consideration being given to how differences in genetic
composition and life cycle in various species may influence these effects.
Consideration is then given to experiments that have investigated the extent of these
problems with different species and methods to reduce detrimental genetic effects
are discussed. Most examples are of eastern North Atlantic and Mediterranean
native species or of species introduced into these areas (e.g., Pacific oyster
Crassostrea gigas), but since ecological analogues occur in other areas it is felt that
the principles discussed will have a wider applicability. Recommendations are then
presented which, it is hoped, will be of particular interest to Governmental and
regional policy makers and to environmental managers.
4.2 Genetic Composition of Cultured Strains Compared
with their Wild Progenitors
4.2.1 Within Population Intra-Specific Comparisons
Reared (cultured) strains* often differ genetically from their wild progenitor populations*, both in levels of genetic variability (usually reduced) and in genetic composition (usually different from progenitors and often temporarily unstable between
reared cohorts). These effects have been demonstrated in numerous species over the
last three decades, using an array of molecular techniques (see Box 4.1).
Genetic variability is usually expressed as heterozygosity (proportion of heterozygotes at a polymorphic gene locus) or as allelic richness (an estimate of the
number of alleles at a specific gene locus). The latter can be a more sensitive indicator of loss of genetic variability and, is thus, more commonly invoked. However,
loss of genetic variability in terms of heterozygosity may result in poor growth and
performance. The most utilised measure of genetic composition is allele frequency
(the proportion of each allele at a specific polymorphic locus in a wild or reared
sample). One of the main causes of reduced genetic variability in reared strains is
the use of much smaller numbers of parents as broodstock than are common in wild
*In this chapter the term “wild population” is used for genetically-distinct statistically-defined
sub-specific groupings, usually reproductively isolated from one another. Reared groupings
derived from wild populations are referred to as “reared strains”.