4 Interactions of Wild and Reared Fish and Invertebrates
125
natal site. In Bantry Bay, Ireland there is a natural hybrid population of mussels
(Mytilus edulis/galloprovincialis) and the local aquaculture industry collects seed
both on ropes and from intertidal rock surfaces. A molecular ecology study, using
allozyme markers, has shown that the seed collected on suspended rope collectors
was mainly the Mytilus edulis type and those scraped off rocks were predominantly
Mytilus galloprovincialis (Fischer 1995). However, when ropes originating from
both sources were tested just before harvest, the Mytilus galloprovincialis type
predominated. The reason would appear to be due to the latter morph’s stronger
byssal attachment, allowing it to survive better in longline culture. Approximately
2,000–3,000 t of mussel are farmed within this bay yearly, and it is reasonable to
assume that the culture operations are exerting a continual selection pressure in
favour of M. galloprovincialis.
Another example occurs in the Netherlands, where wild mussel seed (Mytilus
edulis) are transferred from one area of the Dutch Waddensee to more productive
sites in the same region. A comparable industry occurs in Ireland where the seed is
dredged from offshore sites and relocated to coastal areas within the Irish Sea. Until
recently it was assumed that indigenous mussel populations in both countries were
not exposed to new genetic material, however, shortages of seed in the Dutch
Waddensee (due to conservation measures) have resulted in Dutch importations of
Irish mussel seed. Although previous studies have shown that Irish Sea mussels (the
new source of the Dutch imports) are Mytilus edulis and not affected by hybridisation with Mytilus galloprovincialis, the effects of introducing conspecifics from
another geographic region is not known. There is some circumstantial evidence that
even within the Irish Sea there may be adaptations of mussel populations to local
ecology. A recent attempt to restore a N.E. Ireland estuarine mussel fishery with
sub-tidal mussel seed from the southern Irish Sea was only partially successful,
where despite importing a total of 3,700 t of mussels over 4 years, the restored stock
only reached about one fifth its original level (Burnell, unpublished data). It is
conceivable that the original estuarine population, which was removed to deepen a
shipping channel, was physiologically adapted to the fluctuating salinity of the
estuary. Hydrographic modelling has demonstrated the retention of larvae close to
the mouth of the estuary, which supports the hypothesis of a self-recruiting
population.
4.2.1.2 Breeding Programmes for Species Other than Salmonids
For cultured strains of European marine or anadromous species other than Atlantic
salmon, breeding programmes (where they exist), are at a much earlier stage. This
might be taken to indicate that there will be less genetic difference between wild
populations and reared strains (since they have been less generations in captivity and
therefore less accidental or deliberate anthropogenic selection has been applied).
Certain cultured species such as cod Gadus morhua and Atlantic halibut Hippoglossus
hippoglossus show substantially better survival of progeny following mass spawning
125
natal site. In Bantry Bay, Ireland there is a natural hybrid population of mussels
(Mytilus edulis/galloprovincialis) and the local aquaculture industry collects seed
both on ropes and from intertidal rock surfaces. A molecular ecology study, using
allozyme markers, has shown that the seed collected on suspended rope collectors
was mainly the Mytilus edulis type and those scraped off rocks were predominantly
Mytilus galloprovincialis (Fischer 1995). However, when ropes originating from
both sources were tested just before harvest, the Mytilus galloprovincialis type
predominated. The reason would appear to be due to the latter morph’s stronger
byssal attachment, allowing it to survive better in longline culture. Approximately
2,000–3,000 t of mussel are farmed within this bay yearly, and it is reasonable to
assume that the culture operations are exerting a continual selection pressure in
favour of M. galloprovincialis.
Another example occurs in the Netherlands, where wild mussel seed (Mytilus
edulis) are transferred from one area of the Dutch Waddensee to more productive
sites in the same region. A comparable industry occurs in Ireland where the seed is
dredged from offshore sites and relocated to coastal areas within the Irish Sea. Until
recently it was assumed that indigenous mussel populations in both countries were
not exposed to new genetic material, however, shortages of seed in the Dutch
Waddensee (due to conservation measures) have resulted in Dutch importations of
Irish mussel seed. Although previous studies have shown that Irish Sea mussels (the
new source of the Dutch imports) are Mytilus edulis and not affected by hybridisation with Mytilus galloprovincialis, the effects of introducing conspecifics from
another geographic region is not known. There is some circumstantial evidence that
even within the Irish Sea there may be adaptations of mussel populations to local
ecology. A recent attempt to restore a N.E. Ireland estuarine mussel fishery with
sub-tidal mussel seed from the southern Irish Sea was only partially successful,
where despite importing a total of 3,700 t of mussels over 4 years, the restored stock
only reached about one fifth its original level (Burnell, unpublished data). It is
conceivable that the original estuarine population, which was removed to deepen a
shipping channel, was physiologically adapted to the fluctuating salinity of the
estuary. Hydrographic modelling has demonstrated the retention of larvae close to
the mouth of the estuary, which supports the hypothesis of a self-recruiting
population.
4.2.1.2 Breeding Programmes for Species Other than Salmonids
For cultured strains of European marine or anadromous species other than Atlantic
salmon, breeding programmes (where they exist), are at a much earlier stage. This
might be taken to indicate that there will be less genetic difference between wild
populations and reared strains (since they have been less generations in captivity and
therefore less accidental or deliberate anthropogenic selection has been applied).
Certain cultured species such as cod Gadus morhua and Atlantic halibut Hippoglossus
hippoglossus show substantially better survival of progeny following mass spawning