Site selection and carrying capacity in Mediterranean ... - FAO Sipam

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contribute to a diverse ecosystem, consequently, the resilience capacity will be limited; salmon
aquaculture has been shown to have an adverse effect on this habitat, as shown by studies at three
Scottish fish farms which showed that the farm activities provoked significant reductions in live maerl
cover and diversity, while they led to an increased abundance of species tolerant to organic
enrichment (Hall-Spencer et al. 2006). The displacement of fish farms to deeper offshore locations to
avoid adverse effects on Posidonia oceanica meadows, can have a negative impact on maerl
communities if their spatial distribution is not well know and no habitat mapping has been carried out
at sites indentified for aquaculture activities.
5.2.4. Escapes from reared stocks
As in agriculture, the aquaculture industry has been striving to improve the characteristics (e.g. food
conversion ration) of farmed fish in order to enhance production. Breeding programmes have been
undertaken to enhance strains that feed and grow better. This has lead to largely different genetic
profiles of cultured fish compared to their wild counterparts; consequently, escapes of such cultured
fish strains can lead to interbreeding and competitive interactions that may provoke detrimental
effects on wild fish stocks. McGinnity et al. (2003) experimentally documented that hybrids from
crosses between cultivated and wild salmon had a lower survival capacity and reproduction success
than wild salmon; moreover, these workers notioned that this could lead to extinction of vulnerable
wild populations. Escapes of fish from sea cages have been reported for almost all species presently
cultured across Europe, including Atlantic salmon, Atlantic cod, rainbow trout, Arctic chart, halibut,
sea bream, sea bass and meagre. The present level of escapes is regarded by many as a problem for
the future sustainability of sea cage aquaculture (Naylor et al. 2005, WWF 2005) as escapees can have
detrimental genetic and ecological effects on populations of wild conspecifics (see review by Thorstad
et al. 2008).
Three decades of Atlantic salmon mariculture have shown that it is impossible to develop culture
technologies and routines which guarantee that fish will not escape from farms (Naylor et al. 2005).
Therefore, site selection for aquaculture facilities should consider the potential impacts of escapes on
natural stocks. Under several circumstances, including extreme weather events and operational
accidents, fish escapes will occur. The Europe-wide industry trend of locating sea cage farms in more
exposed locations (Sunde et al. 2003) in response to regulatory mechanisms (e.g. Turkey), together
with the search for production advantages related to water quality, has increased exposure to extreme
weather conditions and thus stresses on farm installations. Although knowledge-based improvements
of technology and operational routines are the key to success for substantially reducing escapes of
farmed fish, it is still highly likely that escapes will occur. In this context, efficient, accurate and costeffective
tools for identifying escapees are central for assessments of the extent and consequences of
fish escapes. Moreover, knowledge of short- and long-term post-escape dispersal of fish is directly
applicable to assessing the prospects for recapturing escapees and for developing efficient recapture
technologies. Knowledge of dispersal patterns after escape are fundamental for predicting possible
negative ecosystem effects caused by escapees, including competition for resources with wild fish,
interbreeding between escaped and wild fish and transfer of diseases from farmed to wild fish (e.g.
Naylor et al. 2005; Weir and Grant 2005; Uglem et al. 2008). In addition, knowledge of the movement
patterns of fish after escape may provide information that will assist in the selection of farms sites that
might minimize the negative effects of escapees.
Knowledge regarding identification of escaped fish and ecological consequences caused by escapees
varies greatly among the fish species being farmed in Europe. For Atlantic salmon, it has been
demonstrated that escapees can cause negative environmental effects through interbreeding with wild
conspecifics (e.g. Fleming et al. 2000; McGinnity et al. 2003; Hindar et al. 2006) and possibly also
through propagation of parasites and diseases (e.g. Heuch & Moe 2001; Bjørn & Finstad 2002,
Krkošek et al. 2007). Concerns that escapees may act as vectors of pathogens to wild fish and to
farmed fish at adjacent farms have heightened recently with the escape of 60,000 salmon infected
with infectious salmon anaemia (ISA) from a farm in northern Norway. In another case, 11 5000
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