130
T.F. Cross et al.
A similar situation is apparent with aquaculture of the Manila clam, Ruditapes
philippinarum. Approximately 2.75 million tonnes are cultured worldwide each
year, with 90% of the production in China. In several southern European sites
(where it has been introduced) it appears to be flourishing at the expense of local
clam species. This has been documented in the Italian lagoons (Mantovani et al.
2006), where it out competes the native Tapes decussate, and similarly, in Portugal
where it has been blamed for the demise of this species (Gosling 2003). The
problem with the spread of this exotic clam is that it may initially go unnoticed due
to its benthic habitat and its superficial similarity to other clam species, and presently
its range is expected to extend northwards.
4.3 Introduction to the Wild
Cultured strains can be introduced inadvertently to the wild, as in farm escapes, or
deliberately, from stocking or ranching exercises. While the mode of introduction
is different, the genetic consequences for wild conspecifics or other species with
which the reared animals interact are similar.
4.3.1 Cultured Strains Inadvertently Introduced to the Wild
(Farm Escapes–see Ferguson et al. 2007)
Farmed marine or anadromous species can escape from onshore tanks or sea cages
(pens) due to equipment failure or human error. Escapes from cages are more likely
(though direct escapes from tanks may also occur) and usually occur due to storm
damage, commonly in winter or from predator attack (e.g., seals). It is important to
note that escapes can occur at any life stage, and the age and season at which
animals escape can greatly influence their subsequent behaviour and survival (e.g.,
for salmon, Hansen (2006) and references therein). There is usually increased
mortality (over natural levels for that stage in the life-cycle) directly after escape,
as cultured animals must adapt to capturing wild food and avoiding predators. Also,
the ultimate impact of an escape incident will be strongly influenced by the life
stage involved. It is clear from consideration of the regime of natural mortality, that
a million immature juveniles will have a much lesser effect than the same number
of animals nearing maturity (assuming similar levels of increased post-escape
mortality at the different life stages).
Recapture of individuals subsequent to escape using various fishing methods is
difficult because little is known about post-escape behaviour. In the case of Atlantic
salmon, seine netting following a simulated escape (deliberate release) from
Norwegian sea cages was largely unsuccessful, because it appeared that escaped
fish aggregated deeper than the level fished (as shown by the use of sonar tags)
T.F. Cross et al.
A similar situation is apparent with aquaculture of the Manila clam, Ruditapes
philippinarum. Approximately 2.75 million tonnes are cultured worldwide each
year, with 90% of the production in China. In several southern European sites
(where it has been introduced) it appears to be flourishing at the expense of local
clam species. This has been documented in the Italian lagoons (Mantovani et al.
2006), where it out competes the native Tapes decussate, and similarly, in Portugal
where it has been blamed for the demise of this species (Gosling 2003). The
problem with the spread of this exotic clam is that it may initially go unnoticed due
to its benthic habitat and its superficial similarity to other clam species, and presently
its range is expected to extend northwards.
4.3 Introduction to the Wild
Cultured strains can be introduced inadvertently to the wild, as in farm escapes, or
deliberately, from stocking or ranching exercises. While the mode of introduction
is different, the genetic consequences for wild conspecifics or other species with
which the reared animals interact are similar.
4.3.1 Cultured Strains Inadvertently Introduced to the Wild
(Farm Escapes–see Ferguson et al. 2007)
Farmed marine or anadromous species can escape from onshore tanks or sea cages
(pens) due to equipment failure or human error. Escapes from cages are more likely
(though direct escapes from tanks may also occur) and usually occur due to storm
damage, commonly in winter or from predator attack (e.g., seals). It is important to
note that escapes can occur at any life stage, and the age and season at which
animals escape can greatly influence their subsequent behaviour and survival (e.g.,
for salmon, Hansen (2006) and references therein). There is usually increased
mortality (over natural levels for that stage in the life-cycle) directly after escape,
as cultured animals must adapt to capturing wild food and avoiding predators. Also,
the ultimate impact of an escape incident will be strongly influenced by the life
stage involved. It is clear from consideration of the regime of natural mortality, that
a million immature juveniles will have a much lesser effect than the same number
of animals nearing maturity (assuming similar levels of increased post-escape
mortality at the different life stages).
Recapture of individuals subsequent to escape using various fishing methods is
difficult because little is known about post-escape behaviour. In the case of Atlantic
salmon, seine netting following a simulated escape (deliberate release) from
Norwegian sea cages was largely unsuccessful, because it appeared that escaped
fish aggregated deeper than the level fished (as shown by the use of sonar tags)