In a comprehensive review of the effects of escaped Atlantic salmon on wild populations, Thorstad et al. (2008) concluded that while outcomes of escapee-wild fish interactions vary with environmental and genetic factors, they are frequently negative for wild salmon. As fish farm areas are typically located close to wild fish habitats, and escaped fish may disperse over large geographic areas (e.g. Furevik et al. 1990, Whoriskey et al. 2006, Hansen 2006), escaped salmon may mix with their wild con-specifics and enter rivers tens to hundreds of kilometers from the escape site during the spawning period. The average proportion of escaped salmon in Norwegian rivers monitored close to the spawning period varied between 11 and 35% during 1989-2008 (16% in 2008), with the highest proportions during the late 1980s and early 1990s (Anon 2009). Consequently, the potential exists for escapees to interact negatively with wild populations, through competition, transfer of diseases and pathogens, and interbreeding. Hindar & Diserud (2007) recommended that intrusion rates of escaped farmed salmon in rivers during spawning should not exceed 5% to avoid substantial and definite genetic changes of wild populations.
Transfer of diseases and pathogens
Escape incidents may heighten the potential for the transfer of diseases and parasites, which are considered to be amplified in aquaculture settings (e.g. Heuch & Mo 2001, Bjørn & Finstad 2002, Skilbrei & Wennevik 2006, Krkošek et al. 2007). Escapees from salmon aquaculture in Norway have been identified as reservoirs of sea lice in coastal waters (Heuch & Mo 2001). In addition, 60000 salmon infected with infectious salmon anaemia (ISA) and 115 000 salmon infected with pancreas disease (PD) escaped from farms in southern Norway in 2007, yet whether these precipitated infections in wild populations is unknown. The ability for escaped fish to transfer disease to wild fish depends on the extent of mixing between the two groups, which in turns varies with the life stage, timing and location of the escape (summarized by Thorstad et al. 2008). However, while escaped and wild fish mix, little direct evidence for disease transfer from escapees to wild salmon population has been documented, other than for the possible case of furunculosus, a fungal disease accidently introduced to Norway from Scotland in the 1990s with the transfer of stock and then believed to have been spread from farmed to wild populations by escapees (summarized in Naylor et al. 2005).
Successful spawning of escaped farmed salmon in rivers both within and outside their native range has been widely documented (see review by Weir & Grant 2006). The ability of escaped salmon to interbreed with wild salmon depends on their ability to ascend rivers, access spawning grounds and spawn successfully with wild partners. While the spawning success of farmed female salmon may be just 20-40% that of wild salmon and even lower for males (1-24%; Fleming et al. 1996, 2000), high proportions of escaped fish in many rivers can lead to a high proportion of farm×wild hybrids. Escaped female salmon may also interfere with wild salmon breeding through destroying the spawning redds of wild fish if they spawn later (Lura & Sægrov 1991, 1993).
Wild Atlantic salmon are structured into populations and meta-populations with little gene flow between them, and evidence for local adaptation in wild Atlantic salmon is compelling (reviewed by Garcia de Leaniz et al. 2007). Farmed salmon differ genetically from wild populations due to founder effects, domestication selection, selection for economic traits and genetic drift (reviewed by Ferguson et al. 2007). Hybridisation of farmed with wild salmon and later backcrossing of hybrids may change the level of genetic variability and the frequency and type of alleles present. Hence, hybridisation of farmed with wild salmon has the potential to genetically alter native populations, reduce local adaptation and negatively affect population viability and character (Ferguson et al. 2007). Several studies have shown that escaped farmed salmon breeding in the wild have changed the genetic composition of wild populations (e.g. Clifford et al. 1998, Skaala et al. 2004).
Large-scale field experiments undertaken in Norway and Ireland showed highly reduced survival and lifetime success of farm and hybrid salmon compared to wild salmon (McGinnity et al. 1997, 2003, Fleming et al. 2000). The relative estimated lifetime success ranged from lowest for the farm progeny to highest for the local wild progeny with intermediate performance for the hybrids. Farmed salmon progeny and farm×wild hybrids may directly interact and compete with wild juveniles for food, habitat and territories. Farm juveniles and hybrids are generally more aggressive and consume similar resources in freshwater habitats as wild fish (Einum & Fleming 1997). In addition, they grow faster than wild fish, which may give them a competitive advantage during certain life stages. Invasions of escaped farmed salmon have the potential to impact the productivity of wild salmon populations negatively through juvenile resource competition and competitive displacement. Fleming et al. (2000) determined that invasion of a small river in Norway by escapees resulted in an overall reduction in smolt production by 28% due to resource competition and competitive displacement. Local fisheries could therefore suffer reduced catches as wild fish stocks decline (Svåsand et al. 2007).
Competition for food
Escaped salmon consume much the same diet as wild salmon in oceanic waters (Jacobsen & Hansen 2001, Hislop & Webb 1992) and could potentially compete for food with wild stocks. Substantial competitive interactions in the ocean, however, appear unlikely to occur as ocean mortality of salmon appears to be density-independent (Jonsson & Jonsson 2004), although limited information exists to assess if this is also the case for coastal waters (Jonsson & Jonsson 2006).