• ammonia excretion;
  • body size;
  • eutrophic;
  • gills;
  • ion transport;
  • sodium uptake kinetics

A number of different freshwater fish species (perch Perca fluviatilis, roach Rutilus rutilus and rudd Scardinius erythrophthalamus) from either eutrophic (Slapton Ley, a seasonally alkaline lake) or non-eutrophic waters were compared with respect to their sodium uptake kinetics and tolerance to acute (1 h) exposure to pH 9·5. Further comparisons were made with rainbow trout Oncorhynchus mykiss and brown trout Salmo trutta. The influence of fish size was also investigated in rainbow trout. Exposure to pH 9·5 was found to disrupt sodium balance and inhibit ammonia excretion in all species and sizes of fishes. The origin of fishes did not have a significant effect on the sodium uptake kinetics or the physiological responses to high pH water. The fishes from the eutrophic lake therefore did not appear to have any increased tolerance to acute exposure to alkaline water. In contrast to previous studies there was no inhibition of Na+ uptake during exposure to high pH. Indeed in some groups of fish Na+ uptake was actually stimulated, as was Na+ efflux. These differences are attributed to experimental water composition and interspecific differences in physiology. It was not always possible to size-match fishes of the different species, so rainbow trout were used to assess the effect of body mass (from 2 to 40 g), on Na+ uptake kinetics and Na+ or ammonia fluxes during alkaline water exposure in rainbow trout. Size had no significant effect on these measurements within this narrow range, which helps validate the comparison between species in this study.