Water quality dependent recovery from aluminum stress in Atlantic salmon smolt

In Norway, a variable pH target (pH 6.2-6.4 during most of the year, but 6. 4 during the smoltification period) is used to reduce the cost of liming sa lmon rivers. Here we test the adequacy of this liming strategy. Atlantic sa lmon presmolts exposed to sublethal acidic water (pH 5.9, <25<mu>g Ali.L-1) for more than 3 months showed impaired seawater tolerance, elevated gill-A l concentrations, severe gill tissue changes, elevated blood plasma glucose concentrations, but no effect on blood plasma chloride. It is usually assu med that smolt will recover from prior aluminum (Al) exposure if water qual ity is restored. Recovery rate is here used as an indirect measure of water quality improvements achieved after treating acid water (pH 5.8, 85 mug Al i.L-1) with lime to reach pH-target levels of 6.0 - 6.3. Fish were exposed in a channel-tank set-up for >210 hrs in water aged from 1 minute up to 2 h ours after treatment (in a flow through system). More Al was eliminated fro m the gills when the fish were exposed to pH 6.3 than to pH 5.8 or 6.0, and when water was aged after pH increase. Recovery, defined as return of norm al gill morphology, blood homeostasis and establishment of seawater toleran ce was achieved within 210 hrs in channels treated with lime to pH 6.3, whi le a similar recovery was not obvious at lower pH values. Liming to pH 6.3 detoxified Al better than pH 6.1.