MONETTE, M.Y.*; MCCORMICK, S.D.; Univ. of Mass., Amherst; USGS, Conte Anadromous Fish Research Center: A comparison of responses of Atlantic salmon parr and smolts to acid/aluminum exposure; why are smolts more sensitive?

Acid rain coupled with resulting increased aluminum (Al) is a potential cause of Atlantic salmon decline in many North American rivers. Smolts appear to be the most sensitive of the salmon lifestages to acid/Al (AA), however the mechanisms underlying increased sensitivity are unknown. Our goals were: to confirm that smolts are more sensitive to AA than parr, and to determine the mechanisms underlying increased sensitivity. In the lab, parr and smolts were exposed to either control or AA water, and sampled after 2 and 6 days. In AA smolts, plasma chloride decreased 11% and plasma glucose increased 3-fold after 6 days, however, in parr plasma chloride and glucose were unaffected. Gill Na⁺, K⁺ ATPase activity (NKA) and hematocrit of both lifestages were unaffected by AA. Gill Al increased &gt 7-fold after 2 and 6 days of treatment in both lifestages. After 6 days, parr gill Al was 2-fold &gt than smolts. In the field, caged parr and smolts were placed in either a reference (RS) or an AA impacted stream (AAS) and sampled after 2 and 6 days. Plasma chloride decreased 8.3% in AAS parr and 27% in smolts after 2 days. After 6 days, plasma chloride in AAS parr continued to decline, but partially recovered in AAS smolts. Plasma glucose increased 2.7- and 2.2-fold in AAS parr and smolts, respectively, and remained elevated after 6 days. Gill NKA decreased 45% in both AAS parr and smolts after 6 days. Gill Al increased &gt 8-fold after 2 and 6 days of treatment in both lifestages. After 6 days, parr gill Al was 2-fold &gt than smolts. Our results confirm that smolt ionoregulatory ability is more sensitive to AA than that of parr, however neither decreased gill NKA nor elevated gill Al appear to be the mechanism of increased smolt sensitivity.