4 Interactions of Wild and Reared Fish and Invertebrates
133
4.4 Genetic Interactions with Wild Populations/Species
Direct and indirect genetic interactions are possible between reared and wild individuals in nature. Direct interactions occur with the same or closely-related species by
interbreeding (producing intra-specific or inter-specific hybrids). Indirect genetic
interactions may occur where either ecological competition or disease introduction by
the reared strain (which may lead to modification of immune-response genes) causes
reduction in size of wild populations, leading to much reduced population size (and
effective population number, N e ) and thus increased genetic drift, which can both
decrease genetic variability and alter genetic composition.
In shellfish populations where disease has substantially reduced population size,
breeding programmes to increase resistance have been undertaken with a view to
supplementing or replacing the wild stocks, e.g., for increased resistance to
Bonamia ostreae in Ostrea edulis (Naciri-Graven et al. 1998, 1999; Culloty et al.
2004), Haplosporidium nelsoni in Crassostrea virginica (Ford and Haskin 1987)
and Marteilia sydneyi and Bonamia roughleyi in the Sydney Rock oyster Saccostrea
glomerata (Nell and Perkins 2006). In a study of hatchery propagated populations
of the flat oyster with increased resistance to Bonamia ostreae, although heterozygosity was still high in the resistant and control populations, the number of alleles
in the selected population was significantly reduced compared with the control
population, which appeared to be mostly due to a loss of rare alleles (Launey et al.
2001). As a result of this loss in variability a decrease in performance, for both
growth and survival, was predicted from the second generation onwards.
4.4.1 Direct Genetic Interactions
Reared animals may breed among themselves in the wild (assortative mating) but
it is likely they will also breed with wild conspecifics where these occur. In the case
of Atlantic salmon, it is clear that interbreeding takes place between reared and wild
fish, and F 1 hybrid offspring are produced. Subsequent generations have been
produced in the hatchery and then reintroduced into experimental situations in a
field experiment (F2 hybrids and parental ´ F1 backcrosses) (McGinnity et al.
2003). Early Norwegian experiments on genetic tagging in cod using an allozyme
locus, indicate that some interbreeding occurs between wild and ranched fish, when
the latter were released into fjords (Jorstad et al. 1994). These animals, which are
evidence of further introgression, have been rarely identified in unmanipulated
situations. The extent of interbreeding of reared and wild conspecifics (or assortative
mating of reared animals when introduced into the wild), needs to be quantified for
all aquaculture species with substantial production from farming, stocking or
ranching, although Youngson et al. (2001) suggest that there will be far less
introgression in sea bass and sea bream than in Atlantic salmon. Fleming et al.
(2000) have shown, with Atlantic salmon, that spawning success of farm fish is
considerably lower than wild fish, and that the success of farmed males is
particularly low. These results have been obtained from detailed observational work
133
4.4 Genetic Interactions with Wild Populations/Species
Direct and indirect genetic interactions are possible between reared and wild individuals in nature. Direct interactions occur with the same or closely-related species by
interbreeding (producing intra-specific or inter-specific hybrids). Indirect genetic
interactions may occur where either ecological competition or disease introduction by
the reared strain (which may lead to modification of immune-response genes) causes
reduction in size of wild populations, leading to much reduced population size (and
effective population number, N e ) and thus increased genetic drift, which can both
decrease genetic variability and alter genetic composition.
In shellfish populations where disease has substantially reduced population size,
breeding programmes to increase resistance have been undertaken with a view to
supplementing or replacing the wild stocks, e.g., for increased resistance to
Bonamia ostreae in Ostrea edulis (Naciri-Graven et al. 1998, 1999; Culloty et al.
2004), Haplosporidium nelsoni in Crassostrea virginica (Ford and Haskin 1987)
and Marteilia sydneyi and Bonamia roughleyi in the Sydney Rock oyster Saccostrea
glomerata (Nell and Perkins 2006). In a study of hatchery propagated populations
of the flat oyster with increased resistance to Bonamia ostreae, although heterozygosity was still high in the resistant and control populations, the number of alleles
in the selected population was significantly reduced compared with the control
population, which appeared to be mostly due to a loss of rare alleles (Launey et al.
2001). As a result of this loss in variability a decrease in performance, for both
growth and survival, was predicted from the second generation onwards.
4.4.1 Direct Genetic Interactions
Reared animals may breed among themselves in the wild (assortative mating) but
it is likely they will also breed with wild conspecifics where these occur. In the case
of Atlantic salmon, it is clear that interbreeding takes place between reared and wild
fish, and F 1 hybrid offspring are produced. Subsequent generations have been
produced in the hatchery and then reintroduced into experimental situations in a
field experiment (F2 hybrids and parental ´ F1 backcrosses) (McGinnity et al.
2003). Early Norwegian experiments on genetic tagging in cod using an allozyme
locus, indicate that some interbreeding occurs between wild and ranched fish, when
the latter were released into fjords (Jorstad et al. 1994). These animals, which are
evidence of further introgression, have been rarely identified in unmanipulated
situations. The extent of interbreeding of reared and wild conspecifics (or assortative
mating of reared animals when introduced into the wild), needs to be quantified for
all aquaculture species with substantial production from farming, stocking or
ranching, although Youngson et al. (2001) suggest that there will be far less
introgression in sea bass and sea bream than in Atlantic salmon. Fleming et al.
(2000) have shown, with Atlantic salmon, that spawning success of farm fish is
considerably lower than wild fish, and that the success of farmed males is
particularly low. These results have been obtained from detailed observational work