The effect of ration on acclimation to environmental acidity in rainbow trout

Freshwater salmonids exposed to low environmental pH typically suffer a net loss of ions, primarily Na+ and Cl-, across the gills, resulting in reduce d plasma and tissue ion concentrations. However, in recent experiments in o ur laboratory, juvenile rainbow trout, Oncorhynchus mykiss, fed a ration of 1% body weight d(-1) or greater showed no ionoregulatory disturbance durin g chronic, sublethal acidification. This raised the possibility that these fish had acclimated to low pH in that they would be better able to withstan d further, more severe acidification than fish that had no prior experience of acid conditions: previous studies had concluded that such acclimation d oes not occur. This hypothesis was tested by measuring unidirectional ion f luxes during a 24 h acute acid challenge (pH 4.2) in juvenile rainbow trout that had previously been exposed to either ambient pH 6.2 (naive fish) or sublethal low pH 5.2 (acid pre-exposed fish) for 90 days, and fed a ration of either 1.0 or 0.25% d(-1) (wet basis). No mortalities were observed duri ng the acute acid challenge in the fish fed the higher ration and no differ ences between the two groups in the response of Na+ fluxes were observed. S odium influx in both groups was significantly inhibited throughout the chal lenge and Na+ net flux was significantly stimulated over the first 6 h. Pri or to the acute acid challenge, the fish fed the lower ration that had prev iously been exposed to pH 5.2 had significantly lower plasma ion concentrat ions than those fish previously exposed to pH 6.2. Both groups suffered mor talities; those of the naive fish (22% by 24 h) being markedly lower than t hose of the acid pre-exposed fish (68% by 24 h). However, there were no sig nificant differences in either Na+ or Cl- fluxes between the two groups of fish during the acid challenge: both showed significant inhibition of ion i nfluxes and significantly greater net ion losses, resulting in reduced plas ma ion concentrations. These results indicate that rainbow trout are unable to acclimate to environmental acidification irrespective of the availabili ty of dietary salts.