Acid stress in respect to calcium and magnesium concentrations in the plasma of perch during maturation and spawning

Synopsis Plasma calcium and magnesium concentrations in perch,Perca fluviatilis, were monitored in an acid lake, Lake Iso Tenhetty (pH ∼5, Al concentrations 0.3–0.5 mg l^-1) compared with a nearby reference lake, Lake P?ij?nne, in Southern Finland. The effect of acid stress could be noticed as 1/3 lower fecundity of perch in the acid lake. Plasma Ca concentrations were temporarily lower in the middle of the winter in the fish of the acid lake, and in April–May samples the situation was opposite. Plasma Mg concentration was also lower in the perch from the acid lake in midwinter months, when compared with the double as high values (∼3 mmol l^-1) from the reference lake. Elevated level of plasma Mg have been reported for fish in cold acclimation especially in winter. Thus overwintering may be specially stressing for some specimens in acid waters. Also ova did contain clearly less magnesium in the midwinter sample from the acid lake perch compared with the reference lake sample. This difference, however, was no more to be found later in spring. Mg content in the ova of perch was in both cases high, which may be necessary to the breeding in comparable acid waters. According to the literature fish cannot increase their magnesium content directly from the water. Mortality for the reference lake embryos was lower than those of the acid lake when incubated in the acid lake. No physiological acclimation inside the fish could be demonstrated in this case.