In both countries, a proportion of producers have chronic or episodic metal toxicity problems. In Norway, problems areprimarily related to low pH and aluminium (Rosseland and Skogheim, 1986; Rosseland et al., 2005; Fivelstad et al., 2003;Stefansson et al., 2007), with smoltifying salmon being particularly vulnerable (Rosseland and Skogheim, 1984; Kroglund et al., 2007, 2008). Sublethal exposures during smoltification have also been shown to increase susceptibility to salmon lice infections, further lowering the recommended safe levels (Finstad et al., 2007). Approximately 15% of production sites exceed the recommended levels for labile aluminium (LAl) in tank water (<10 mg/L). The high pH of Chilean water indicates that low molecular forms of Al will be present as aluminate (Lydersen et al., 2002). Aluminate has been shown to cause osmoregulation problems in smolts at low Ca concentrations (Skogheim et al., 1987), but is reported to be less toxic than cationic Al-species occurring at low pH (Poleo and Hytterød, 2003). However, anionic aluminate has recently beenfound to accumulate on gills and cause physiological effects (Teien, unpublished data). This may warrant further investigations of Al effects in high pH waters. Producers in all regions of both Norway and Chile have documented elevated Fe levels, and about 25% of production sites in both countries have total Fe concentrations of >100 mg/L (Fig. 3F). Toxicity is largely determined by the oxidation rate from Fe(II) to Fe(III), which is strongly affected by temperature, pH and ionic strength (Teien et al., 2008). In addition, TOC reduces bioavailability by adsorption/binding, making effects more prominent in low TOC waters more typical for Chile (Fig. 3A). The above mentioned water quality issues can be treated using current techniques. For Al: liming, seawater addition, silica lye addition, or combinations of the two latter may be used. For Fe, oxidation by oxygen or ozone, provided sufficient subsequent retention time.
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