Environmental Engineering Reference
In-Depth Information
processes such as gynogenesis, whereby their eggs are activated by the sperm of
other species. The gibel carp is a highly invasive freshwater species with gynoge-
netic triploid populations throughout Europe (Vetemaa
et al
. 2005). Although
triploid species are believed to be able to reproduce only via parthenogenesis,
gynogenesis, or hybridogenesis, a bisexually reproducing triploid amphibian has
recently been discovered (Stöck
et al
. 2002). This highlights the need for extreme
caution in the production and spread of triploid animals. Immunocontraceptive
control and the genetic modifi cation of fi sh to produce single-sex progeny (daugh-
terless) have both been investigated in Australia for the control of common carp
and other nuisance vertebrate species, but both are still in experimental planning
or development (Thresher 2007). Immunocontraception relies on activating the
fi sh's immune system to block fertilization. The required immunocontraceptive
antigen could be delivered by a viral vector or by baits. Daughterless induction in
fi sh relies on the heritable deactivation of the aromatase enzyme responsible for
converting androgens into oestrogens, the result being that all offspring of mutants
are sexually reproducing males capable of spreading the mutation throughout the
population. Should the daughterless technology be successfully developed to pro-
vide a workable management option, considerable public resistance to the concept
of the large-scale release of genetically modifi ed organisms (GMOs) would need
to be overcome and although these organisms are intragenic rather than trans-
genic (that is their own genetic material is modifi ed without any addition of for-
eign DNA) they are likely to be subjected to the same stringent risk assessment
required for the release of GMOs (Russell and Sparrow 2008). Moreover, genetic
suppression of oestrogen production in females may potentially be reversed by the
presence of environmental xeno-oestrogens (Jobling
et al
. 1998).
13.8.2.4 Physical removal
Complete eradication of fi sh from water bodies by physical removal is generally
considered a hopeless task because the effective effort rises exponentially as the
population is fi shed down. However, removing every last individual may not be
necessary if one is able to hold the population at a suffi ciently low density to hope
for stochastic extinction to occur via the Allee effect (Courchamp
et al
. 1999c).
Any control programme aimed at eradication also needs to consider the risk of
illegal reintroduction which may render very expensive control measures ineffect-
ive. If a source population for reintroduction is locally available, or if the site has
easy human access, then eradication efforts may ultimately be a waste of time, espe-
cially if the target species has some perceived recreational or other benefi t. Physical
removal often aims to reduce the ecological impact of invasive species rather than
to achieve complete eradication but is probably the only effective measure in very
large water bodies due to the impracticality or cost of chemical renovation.
One simple strategy to eradicate fi sh in small ponds and lakes is complete dewa-
tering. h e water body can be drained or pumped dry. Care must be taken to ensure
that no wet refugia remain to harbour fi sh and the technique is best employed dur-
ing times when eggs that might survive prolonged emersion in damp vegetation
