5 Non-Native Species in Aquaculture
169
where all variation resides below the family level (Youngson et al. 2001). Within
the population, genetic complexes will develop, often relating to the environment
in which the population has developed, constituting spatial, behavioural or temporal isolating mechanisms. Aquaculture practices of both inbreeding and selection
of individuals for specific traits magnifies the development of genetic complexes
in a population.
When aquaculture escapes breed with natural populations, hybridisation and
subsequent introgression can lead to a breakdown of the genetic complexes which
have developed, forcing a reduced fitness in the hybrid individuals (Skaala et al.
2006). This can lead to a decline in fitness and increased threat of extinction in the
now hybridised natural population (Mooney and Cleland 2001). Outbred largemouth bass Micropterus salmoides crossed from two distinct populations suffered
a reduction in fitness of approximately 14% relative to parental stocks (Goldberg
et al. 2005). F2 generation hybrids suffered higher mortality rates and increased
susceptibility to infectious disease. Collection and translocation between previously isolated stocks can have similar effects, which have been shown in stocks of
black-lipped pearl oyster, Pinctada margaritifera cumingii, in French Polynesia
(Arnaud-Haond et al. 2004). When large populations of invading species are
introduced, the threat to native species is unavoidable, however evidence suggests
that even when small populations of an invader are introduced (for example,
escaping aquaculture individuals) the native population is still threatened (Mooney
and Cleland 2001).
Hybridisation can be either inter- or intraspecific. Hybridisation between native
brown trout, Salmo trutta and Atlantic salmon, Salmo salar, in Europe is an
example of the former and female salmon escapees have been shown to hybridize
relatively freely with the brown trout (Youngson et al. 1993). Intraspecific hybridisation would involve escape of a different strain of the species into a native
population (Cross 2000). This is likely, due to the modification of aquaculture
species with traits chosen for performance. Spawning success is lower for
cultured salmon than for wild fish (Fleming et al. 1996, 2000), even when
released to the wild as smolts (Jonsson et al. 1990). In Spain, where rivers have
been highly stocked with non-native trout S. trutta, 25% of native populations
had evidence of introgression by genes of hatchery origin (Almodóvar et al.
2001). Evidence has also been found for the introgression of the Mediterranean
mussel, Mytilus galloprovincialis genes into native Australian populations
(Sanjuan et al. 1997).
5.3.6 Trans-Boundary Effects
Biological invasions, whether intentional or accidental, are by their very nature
not limited by geo-political boundaries. This is even more the case for marine
bioinvasions where oceanic currents and natural dispersal mechanisms can lead
to significant range expansions, following initial establishment, that transcend
169
where all variation resides below the family level (Youngson et al. 2001). Within
the population, genetic complexes will develop, often relating to the environment
in which the population has developed, constituting spatial, behavioural or temporal isolating mechanisms. Aquaculture practices of both inbreeding and selection
of individuals for specific traits magnifies the development of genetic complexes
in a population.
When aquaculture escapes breed with natural populations, hybridisation and
subsequent introgression can lead to a breakdown of the genetic complexes which
have developed, forcing a reduced fitness in the hybrid individuals (Skaala et al.
2006). This can lead to a decline in fitness and increased threat of extinction in the
now hybridised natural population (Mooney and Cleland 2001). Outbred largemouth bass Micropterus salmoides crossed from two distinct populations suffered
a reduction in fitness of approximately 14% relative to parental stocks (Goldberg
et al. 2005). F2 generation hybrids suffered higher mortality rates and increased
susceptibility to infectious disease. Collection and translocation between previously isolated stocks can have similar effects, which have been shown in stocks of
black-lipped pearl oyster, Pinctada margaritifera cumingii, in French Polynesia
(Arnaud-Haond et al. 2004). When large populations of invading species are
introduced, the threat to native species is unavoidable, however evidence suggests
that even when small populations of an invader are introduced (for example,
escaping aquaculture individuals) the native population is still threatened (Mooney
and Cleland 2001).
Hybridisation can be either inter- or intraspecific. Hybridisation between native
brown trout, Salmo trutta and Atlantic salmon, Salmo salar, in Europe is an
example of the former and female salmon escapees have been shown to hybridize
relatively freely with the brown trout (Youngson et al. 1993). Intraspecific hybridisation would involve escape of a different strain of the species into a native
population (Cross 2000). This is likely, due to the modification of aquaculture
species with traits chosen for performance. Spawning success is lower for
cultured salmon than for wild fish (Fleming et al. 1996, 2000), even when
released to the wild as smolts (Jonsson et al. 1990). In Spain, where rivers have
been highly stocked with non-native trout S. trutta, 25% of native populations
had evidence of introgression by genes of hatchery origin (Almodóvar et al.
2001). Evidence has also been found for the introgression of the Mediterranean
mussel, Mytilus galloprovincialis genes into native Australian populations
(Sanjuan et al. 1997).
5.3.6 Trans-Boundary Effects
Biological invasions, whether intentional or accidental, are by their very nature
not limited by geo-political boundaries. This is even more the case for marine
bioinvasions where oceanic currents and natural dispersal mechanisms can lead
to significant range expansions, following initial establishment, that transcend