Effects of an acute silver challenge on survival, silver distribution and ionoregulation within developing rainbow trout eggs (Oncorhynchus mykiss)

Rainbow trout eggs were acutely challenged with silver (as AgNO₃) at different stages of development from fertilization through to hatch in moderately hard water (120 mg CaCO₃ l^-1, 0.70 mM (25 mg l^-1) Cl^-, 1.3 mg l^-1 DOC, 12.3 ± 0.1°C) at measured total silver concentrations of 0.11 ± 0.004, 1.55 ± 0.15, and 14.15 ± 1.52 μg l^-1. Four separate acute challenges were conducted, each consisting of 5 days exposure to the respective silver concentration, followed by 4 days recovery after transfer to silver-free water (series 1, 1-10 days post-fertilization; series 2, 8-17 days post-fertilization; series 3, 16-25 days post-fertilization; series 4, 23-32 days post-fertilization). Mortality was not significantly different from control during exposure to 0.11, 1.55, and 14.15 μg l^-1 total silver in series 2, 3 and 4 (mortality for series 1 data could not be calculated for technical reasons). In the four days of recovery following silver exposure, however, there was significant mortality at 14.15 μg l^-1 total silver reaching 100, 31 and 72% in series 2, 3 and 4, respectively, indicating eggs are more sensitive in the period of 8-17 and 23-32 days post-fertilization at this temperature. Mortality following silver exposure was associated with ionoregulatory impairment in series 3 and 4, where up to 60% of whole egg [Na⁺] and [Cl^-] was lost relative to controls at 14.15 μg l^-1 total silver. Significant but smaller reductions in egg [Na⁺] and/or [Cl^-] were also observed at 0.11 and 1.55 μg l^-1 total silver. The greatest accumulation of silver in whole eggs and chorions occurred in series 4, reaching concentrations of 0.53 μg g^-1 (eggs) and 15.5 μg g^-1 (chorions) in the 14.15 μg l^-1 treatment. The accumulation of silver in the whole eggs and chorions of the 0.11 μg l^-1 treatment was not different from controls throughout embryonic development. Of the total silver content, only a small proportion of silver was found in the embryos (1-17%), indicating that the chorion is a protective barrier during acute silver exposure. (C) 2000 Elsevier Science B.V.