Long-term exposure to small temperature increase and sublethal ammonia in hardwater acclimated rainbow trout: does acclimation occur?

Juvenile rainbow trout (Oncorhynchus mykiss; initially 2–5 g) were exposed for 90 days to either ambient water temperature (natural thermal regime) or to +2 °C superimposed above the ambient water temperature (simulated global warming scenario), in the presence or absence of a nominal 70 μM total ammonia (1290 μg l−1 ionized (NH4+), 10 μg 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 mM) on three occasions: over summer (temperature range, 13.0–21.0 °C; pH = 7.57 ± 0.26) and winter (temperature range, 3.5–7.0 °C; pH = 7.46 ± 0.02) without food limitation (satiation feeding), and during summer (temperature range, 13.0–18.5 °C; pH = 7.38 ± 0.09) with food limitation (1% daily, or restricted ration). Lethal temperature, lethal ammonia (1.8 mM total ammonia; approximately 31 700 μg l−1 NH1+, 900 μg l−1 NH3), and lethal temperature plus ammonia challenges were conducted after each 90-day exposure to determine whether or not chronic pre-exposure conferred any increased tolerance to elevated temperature or ammonia. In addition, acute sublethal ammonia challenges (1.0 mM total ammonia; approximately 17 800 μg l−1 NH4+, 200 μg l−1 NH3), together with unidirectional Na+ flux measurements, were conducted after the two summer exposures to gain further insight into the effects of prior sublethal ammonia exposure on Na+ regulation, as influenced by ration. The juvenile trout on unlimited ration and exposed to a warming scenario of +2 °C exhibited a slight, but significant elevation in lethal temperatures in both summer and winter, but the effect was not observed in fish fed a restricted ration. A challenge to lethal temperature and ammonia in combination reduced the lethal temperature anywhere from 3–7 °C for fish from all treatments; pre-exposure to ammonia offered some protective effect. However, prior ammonia exposure did not prolong survival times (LT50s) during lethal ammonia challenge, and there was no evidence of acclimation to elevated external ammonia with respect to Na+ balance. These results suggest that juvenile trout are likely to adapt to a small temperature increase, such as could be associated with a global warming scenario, but their potential for doing so may be restricted by sublethal ammonia and by nutritional status.