The mechanisms and costs of physiological and toxicological acclimation to waterborne silver in juvenile rainbow trout (Oncorhynchus mykiss)

Juvenile rainbow trout were exposed to 0, 0.1, 1, 3, and 5 μg/l silver (Ag, as AgNO₃) for 23 days. Specific growth rate, cumulative food consumption, food-conversion efficiency, and critical swimming speed (U_crit) were significantly reduced during 5 μg/l Ag exposure, demonstrating a physiological cost of silver acclimation. Only the 5 μg/l Ag treatment had significant cumulative mortality (5.2%). Fish were most susceptible to silver on days 5 and 15. Exposure to 5 μg/l Ag significantly lowered plasma Na⁺ and Cl^- on days 5 and 10, but plasma ion concentration recovered thereafter. Unidirectional Na⁺ uptake and gill Na/K-ATPase activity were significantly inhibited by 3 and 5 μg/l Ag exposure. Na⁺ uptake was inhibited by 3 μg/l Ag at day 5 alone, whereas the effects at the highest Ag exposure persisted until day 15. Gill Na/K-ATPase was inhibited on day 5 in both the 3 and 5 μg/l Ag treatments but increased to approx. 1.5 times of control levels by day 23. Only the 3 and 5 μg/l Ag treatments produced toxicological acclimation (at least twofold elevations in 168-h LC₅₀ values in fish subsampled on days 15 and 23). We conclude that physiological acclimation results from compensatory changes in Na⁺ transport at the gills, and that these changes may eventually lead to toxicological acclimation.