MONETTE, M.Y.*; MCCORMICK, S.D.; University of Massachusetts, Amherst; Conte Anadromous Fish Research Center, USGS: Physiological and endocrine mechanisms of acid/aluminum impacts on Atlantic salmon smolt development

Many rivers and streams in eastern North America and northern Europe experience episodic pulses of acidity due to anthropogenic acidification. These pulses combined with low buffering capacity of soils, and increased inorganic monomeric aluminum (Al) concentrations of surface waters are toxic to salmonids and may be causing the decline of Atlantic salmon populations in these regions. We examined the effects of short-term, sub-lethal acid/Al exposure on the osmoregulatory ability of Atlantic salmon smolts in order to determine the mechanisms underlying acid/Al impacts. We assigned fish to replicate tanks of either control (pH 6.7-7.0, 5.4-30.0 &microg/l total Al) or treatment (pH 5.8-6.3, 78-152 &microg/l total Al) conditions and exposed fish for 48 and 120 h. Gill aluminum, hematocrit and plasma cortisol did not differ between control and treated fish at 48 h, but were significantly elevated in treated fish after 120 h. Acid/Al exposure caused a 29% reduction in gill Na⁺, K⁺ ATPase activity at 120 h, though this effect was not statistically significant. Acid/Al exposure had no effect on plasma chloride. This is consistent with previous findings that similar short-term, sub-lethal acid/Al exposure compromises seawater tolerance without negatively impacting freshwater ionoregulatory ability. The effect of acid/Al exposure on chloride cell proliferation will also be examined using immunocytochemistry for, Na⁺, K⁺ ATPase, BrdU incorporation, and proliferating cell nuclear antigen (PCNA).