LONG-TERM EXPOSURE TO SMALL TEMPERATURE INCREASE AND SUBLETHAL AMMONIA IN HARD WATER ACCLIMATED RAINBOW-TROUT - DOES ACCLIMATION OCCUR

Juvenile rainbow trout (Oncorhynchus mykiss; initially 2-5 g) were exp osed for 90 days to either ambient water temperature (natural thermal regime) or to +2 degrees C superimposed above the ambient water temper ature (simulated global warming scenario), in the presence or absence of a nominal 70 mu M total ammonia (1290 mu g l(-1) ionized (NH4+), 10 mu g l(-1) unionized (NH3) ammonia). The exposures were conducted in moderately hard de-chlorinated water from Lake Ontario ([Ca2+]=0.96+/- 0.02 mM, [Na+]=0.55+/-0.01 mM, [Cl-]=0.737+/-0.004 mh?) on three occas ions: over summer (temperature range, 13.0-21.0 degrees C; pH=7.57+/-0 .26) and winter (temperature range, 3.5-7.0 degrees C; pH=7.46+/-0.02) without food limitation (satiation feeding), and during summer (tempe rature range, 13.0-18.5 degrees C; pH=7.38+/-0.09) with food limitatio n (1% daily, or restricted ration). Lethal temperature, lethal ammonia (1.8 mM total ammonia; approximately 31 700 mu g l(-1) NH4+, 900 mu g l(-1) NH3), and lethal temperature plus ammonia challenges were condu cted after each 90-day exposure to determine whether or not chronic pr e-exposure conferred any increased tolerance to elevated temperature o r ammonia. In addition, acute sublethal ammonia challenges (1.0 mM tot al ammonia; approximately 17 800 mu g l(-1) NH4+, 200 mu g l(-1) NH3), together with unidirectional Na+ flux measurements, were conducted af ter the two summer exposures to gain further insight into the effects of prior sublethal ammonia exposure on Na+ regulation, as influenced b y ration. The juvenile trout on unlimited ration and exposed to a warm ing scenario of +2 degrees C exhibited a slight, but significant eleva tion in lethal temperatures in both summer and winter, but the effect was not observed in fish fed a restricted ration. A challenge to letha l temperature and ammonia in combination reduced the lethal temperatur e anywhere from 3-7 degrees C for fish from all treatments; pre-exposu re to ammonia offered some protective effect. However, prior ammonia e xposure did not prolong survival times (LT(50)s) during lethal ammonia challenge, and there was no evidence of acclimation to elevated exter nal ammonia with respect to Na+ balance, These results suggest that ju venile trout are likely to adapt to a small temperature increase, such as could be associated with a global warming scenario, but their pote ntial for doing so may be restricted by sublethal ammonia and by nutri tional status. (C) 1998 Elsevier Science B.V.