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

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salmon infected with pancreatic disease (PD) escaped from a farm in southern Norway in 2007 6 .
Several studies have mapped the dispersal of adult farmed salmon after escape and have shown that
they disperse over large geographical areas and that they migrate in significant numbers up rivers
during autumn where they spawn together with wild salmon (e.g. Furevik et al 1990; Whoriskey et al.
2006, Hansen 2006). Identification of escaped salmon is routinely performed by analysing growth
patterns in scales (Fiske et al. 2006) and attempts to use genetic markers to trace salmon escapees are
under development. Research to investigate the genetic effects of aquaculture on native species has
been broadly developed through the FP-6 EU Coordinating Action GENIMPACT 7 . GENIMPACT’s
objectives are to study the genetic impact of farming activities and its implications for aquaculture
management and the conservation of wild stocks. Among the project’s conclusions was that improved
technological measures to prevent escapes of fish from sea cages were paramount to reducing
negative genetic effects (Triantafyllidis et al. 2007).
In comparison to Atlantic salmon, knowledge of the extent and effect of escapes of Atlantic cod is
very limited. Methodologies for simple and cost effective identification of escaped individuals of
these species do not exist and detailed knowledge of the short- and long-term behaviours and dispersal
of fish following escapes of farmed juveniles is sparse. Uglem et al. (2008) documented the short- and
long-term dispersal of escaped adult cod in a single Norwegian fjord using telemetry. In general, the
escaped adult cod dispersed rapidly away from the farm in random directions. Recapture rates of these
escapees in the local commercial and recreational fisheries was surprisingly high (approximately
40%), indicating that recapture of high numbers of escapees may be feasible if recapture fisheries are
properly organised. Results from releases of hatchery-produced cod juveniles for stock enhancement
purposes have shown that released cod generally have a variable fidelity to the release site, depending
on the release location (Svåsand et al. 2000). At a scale of dispersal of within 50 km of the release
site, Svåsand et al. (2000) stressed that results obtained in one area cannot be generalized to other
areas. Moreover, fish released for enhancement purposes differ from escaped farm fish since the
former were produced and released in a manner intended to make them fit for the life in the wild.
For other cultured species, like sea bream, sea bass and meagre, knowledge regarding identification of
escaped fish and how escapes might affect ecosystems is virtually non-existent. Intentional releases of
cultured sea bream for stock enhancement have been reported from the southern Atlantic coast of
Spain, and in the Bay of Cadiz (Sanchez-Lamadrid 2004). Released fish moved less than 10 km from
the release point. Good growth rates and condition indices indicate that the released fish adapted to
life in the wild, and suggest that populations of wild fish could also be altered by released fish. For
example, there is correlative evidence of a substantial increase in wild populations of sea bream after
fish farming began in the Messolonghi lagoon, Greece (Dimitriou et al. 2007). For the Canary Islands,
Toledo et al (2008) highlighted that sea bass (Dicentrarchus labrax), a introduced species in these
islands, is commonly find in shallow coastal habitats. The escaped individuals use natural trophic
resources, and their abundance depends strongly on distance from fish farms. Escaped sea bass are
able to exploit natural resources by feeding mainly on wild fishes. Consequently, its diet overlaps with
that of top predator species in the Canary Islands, and establishment of this species in the archipelago
is assumed to lead to new and strong competition with the local species, affecting traditional fisheries.
Genetic interactions between wild and cultured fish have been deeply studied for salmon but very
little attention have paid to cod (but see Moe et al. 2005) and Mediterranean species like sea bass and
sea bream. Apparently, the interbreeding risk for sea bream is low, since there are no distinct genetic
populations in the biogeographical area extended between the Azores and the Eastern Mediterranean.
Nevertheless, a small reduction in genetic variability in cultivated sea bream has been detected
(Youngson et al. 2001). In the case of sea bass, however, it seems that three different groups of sea
bass populations exist, being present in the Atlantic, Western and Eastern Mediterranean (Cesaroni et
al. 1997). Even though not well documented, Spain may have used a mixture of sea bass stock from
the Atlantic and Western Mediterranean populations, while France, Italy, Greece and Eastern
6 www.kyst.no
7 http://genimpact.imr.no/
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