Modifying effects of life stage, ionic strength and post-exposure mortality on lethality of H + and Al to lake trout and brook trout

Abstract

Lethality of H + and Al to egg and fry stages of lake trout ( Salvelinus namaycush) and brook trout ( S. fontinalis) was determined in a series of laboratory toxicity tests in soft water. Survival of egg and fry stages of lake trout was reduced at pH < 4.8 following exposures of 7–58 days in water containing 4.0 mg/l Ca. Brook trout were more tolerant and survival of eggs and fry was reduced at pH 4.2 and 4.4. Al lethality required an a priori H + stress (pH 4.2–4.8), was consistent with a mechanism of joint action with H + to produce ionoregulatory failure, and was not observed at pH 5.1–6.0. Lethality of H +, but not Al, was increased two-fold in low ionic-strength water (Ca = 1.6 mg/l) and addition of NaCl improved fry survival at limiting levels of Ca (0.6 mg/l). Mortality was not confined to the exposure period and often continued after transfer to control water following short-term exposure of eggs and fry. Short-term, single-episode exposures to H + and Al suggest that individual snowmelt and rainstorm events should have little impact on salmonid recruitment. Mortality of sac-fry during repeated episodic exposures, superimposed on mortality of cleavage eggs due to chronic acidification is suggested as a possible mechanism of recruitment failure in acidified waters. Survival of lake trout and brook trout populations cannot be ensured in low ionic-strength waters (Ca < 1.6 mg/l) acidified to pH < 5.1, based on their responses to H + and Al.