A review of some physiological and toxicological responses of freshwater fish to acid stress

Abstract

SynopsisData relating to the specific effect of low pH on growth of freshwater fishes are ambiguous. Reproductive failure resulting from acid stress appears to be related to an upset in calcium metabolism and to faulty deposition of protein in developing oocytes. It appears that the 'no effect' level of pH depression for successful reproduction is around 6.5. Data on behaviorial responses of freshwater fish to acid stress and CO, are described. Most fish appear to be indifferent to pH within the range of approximately 10.5 to 5.5 and between 7.4 and 4.5 CO, appears to be the main directive factor. In cases of severe acid stress alteration of gill membranes and/or coagulation of gill mucus occurs and death due t o h~p o x i a may result from a lengthening of the water-blood diffusion distance. Several reports agree that acid stress causes an upset of electrolyte homeostasis in fiih but effects of low pH on osmotic permeability are largely lacking. Most hatcheryreared salmonids can tolerate pH 5.0 indefinitely but below this level the homeostatic electrolyte and osmotic regulatory mechanisms become inadequate. When fish are subjected to debilitating acid stress blood pH decreases possibly as the result of flux of H* ions across gill membranes into the blood. This could change transepithelial potential and allow a bloodto-water diffusion of Na+ ions down an electrochemical gradient. Lowered ambient pH may interfere with gill calcium levels increasing permeability to both H+ and Na+ ions or an acidemia may occur as the ~esult of a decrease in the excretion of metabolically produced H+ ions and CO,. When the capacity of the buffer mechanisms is exceeded the blood pH drops and the capacity of hemoglobin to transport oxygen is decreased.

Cellular responses of the skin and changes in plasma cortisol levels of trout ( Oncorhynchus mykiss ) exposed to acidified water

“…However, since mucus is initially synthesized as basic or neutral glycoproteins (Mittal et al 1980), an increased PAS-reactivity could also reflect a predominance of newly differentiated mu cous cells, and an increased turnover of this cell-type (Iger et al 1988;. The presence of a mucus layer attached to the epidermal sur face after fixation of the skin from acid-exposed trout may indicate that its composition has changed, although Fromm (1980) has suggested that this might be caused by coagulation in low pH conditions. The appearance of newly differentiated mucous cells close to the skin sur face in acid treated fish has not previously been reported for rainbow trout.…”

Tilapia are able to withstand long‐term exposure to low environmental pH, judged by their energy status, ionic balance and plasma cortisol

“…As a consequence of the acidification, fish populations are vanishing, especially in poorly buffered areas like Scandinavia, Canada and the north-eastern U.S.A. (Beamish, 1976;Leivestad & Muniz, 1976). The main causes for the disappear ance of fish populations are osmoregulatory stress, disturbances of the acid-base balance, increased gill diffusion distance resulting in hypoxia, reduced reproduc tive capacity, changes in predator-prey interactions, impaired growth, and an accumulation of aluminium and heavy metal ions in the acidified waters (Fromm, 1980). The resulting physiological and endocrinological changes observed in fish after exposure to water acidification can be studied systemati cally only in the laboratory.…”

Cellular responses of the skin of carp (Cyprinus carpio) exposed to acidified water

“…It caus es disturbance of water and ion balance, reduced growth and reproduction, and may eventually lead to disappear ance of fish populations (Fromm 1980;M cD onald 1983;Wendelaar Bonga and Dederen 1986). The disturbance of the water and ion balance is mainly caused by dam age inflicted upon the gills, and, as a consequence, many au thors have examined the histopathological effects of acid Correspondence to: S.E.…”