ter lakes) have affected wild fish populations. Aquaculture's substantial demand for fish meal is driving a large wild cap­ture of small fishes (that are the base of food chains) (Naylor et al., 2000). In part, the over-fishing of some fish popula­tions is to support the aquaculture industry. Recognition of this has lead to research and efforts to replace fish protein and lipids in fish meal with vegetable sources and byprod­ucts from livestock processing (e.g., Glencross et al., 2003, Montero et al., 2003; Higgs et al, 2006).
     A second issue with caged cultures in natural waters is habitat degradation in the areas of the cages due to the large inputs of organic matter and nutrients (nitrogen and phosphorus) in the feed for the aquatic livestock. These in­puts can lead to reduced water quality, undesirable algal blooms and alteration in benthic communities in the near vicinity of the aquaculture operations (e.g., Gyllenhammar and Hakanson, 2005).
     Caged aquaculture inevitably loses some of the cultured fish, through small accidental escapes and through occa­sional large losses in storms, to the wild. The escapees may interfere with native populations (Canonico et al., 2005). While the number of escaped fish are small relative to native populations, the impacts of the escapees are probably mi­nor. However, in the case of Atlantic salmon (Salmo salar), escaped populations may be relatively large compared to

native populations. Although aquaculture salmon may be more aggressive and may outcompete native populations they are less reproductively viable and may cross-breed, with native populations leading to reduced viability of off­spring, which threatens the survival of the native gene pool (Naylor et al., 2005).
     A final concern of caged populations is that dense aqua­culture populations are incubators for diseases and parasites (e.g., Heuch et al., 2005), which can then spread to wild populations. Because fish diseases have led to major eco­nomic losses in aquaculture, there is increased use of vet­erinary drugs and vaccines in intensive production systems. The use of antibiotics in aquaculture can rapidly lead to the adaptation of disease microbes and loss of effectiveness of the antibiotic (Garcia and Massam, 2005). However, anti­biotics are not used either as prophylactic (before disease occurs) agents or as growth promoters in temperate wa­ter aquaculture production in Europe and North America (Alderman and Hastings, 1998). In recent years the use of antibiotics has fallen dramatically in the farmed salmon in­dustry in Norway from about 50 to less than one tonne annually (Figure 3-2). This is largely as a result of the suc­cessful development and use of vaccines against the prin­ciple fish pathogens (Alderman and Hastings, 1998).
     Closed-system aquaculture, such as in farm-based cat-