her of individuals entering rivers as adults and some data on hatchery escapes are available. Gross (2002) estimated 3% escapement from similar facilities in British Columbia and elsewhere (British Columbia Environmental Assessment Office 1997). Based on the number of fish being raised in Maine waters, 3% escapement in Maine would translate into about 180,000 escapees per year from net pens. Continued improvements are being made that will reduce, but not eliminate, the number of escapees. An escape rate as low as 0.17%, which would be impressive, would still pro vide 10,000 escapees per year, 100 times the number of adults that returned to spawn in Maine’s eight DPS streams in 2000.
These escapees might not have much impact on healthy wild populations, because farm (pen-raised) salmon have shown competitive inferiority in the wild (Fleming et al. 1996, 2000). However, because of the low numbers of wild adults returning to spawn in recent years, farm salmon represent a large proportion (100% in some years) of the adults entering the rivers to spawn. The effect might be ameliorated to some extent by the precocious wild parr in the streams and by the low reproductive success of farm adults (Fleming et al. 1996). In experimental facilities, farm males had only 1–3% of the success of wild males, and farm females had only 20–40% of the success of wild females, with most matings involving wild males (Fleming et al. 1996). In addition, Fleming et al. (2000) showed that farm salmon introduced experimentally into a wild population had only 16% of the success of wild salmon in producing recruits. Thus, it is possible for wild populations to “resist” genetic infiltration by farm fish, but that potential drops as the number of wild fish becomes small, relative to the number of farm fish. Even a 10:1 adaptive advantage for wild salmon might not be sufficient to overcome a 100:1 numerical advantage for aquaculture escapees. It remains unclear to what degree farm salmon have infiltrated wild populations genetically, or conversely, how resistant wild salmon have been to genetic infiltration. Based on samples taken in 1994–1998, genetic infiltration of farm fish into wild Maine populations was minimal (King et al. 1999). However, if salmon farming in Maine expanded further, the numerical impact (among likely spawners) of aquaculture escapees would have the potential to become significant.