Acute silver toxicity to Cnesterodon decemmaculatus (Pisces: Poeciliidae) in a river with extreme water-quality characteristics

A 96 h acute silver toxicity test was performed in order to determine silver toxicity (LC50) to a local fish species (Cnesterodon decemmaculatus) in a river with extrem water-quality characteristics (Pilcomayo River, South America)    and evaluate a cross-fish-species extrapolation of the Biotic Ligand Model. The dissolved silver concentrations tested were 0.095, 0.148, 0.175 and 0.285 mg Ag L−1. The 96 h Ag LC50 calculated for C. decemmaculatus was 0.14 mg L−1 (0.18 - 0.10)  and the value predicted by BLM for Pimephales promelas was 0.051 mg Ag L−1. Test water elevated hardness may have exerted some protective effect. High mean water pH may have exerted a major protective effect by reducing silver free ion form and causing silver precipitation. The mortality pattern observed in this toxicity test may lend some support to a relationship between gill silver accumulation and mortality. A cross-fish-species extrapolation of Ag BLM for  P. promelas was not valid in Pilcomayo River water and experimental conditions of this toxicity test.