The toxicity and bioavailability of nickel and molybdenum to standard toxicity-test fish species and fish species found in northern Canadian lakes
Pyle, Gregory George
Nickel (Ni) and molybdenum (Mo) are two metals that are commonly associated with northern Saskatchewan uranium deposits. Consequently, concentrations of Ni and Mo are elevated above background levels in uranium-mine effluent receiving waters. The objectives of this research were: (1) to determine if standard toxicity-test fish species, like fathead minnows or rainbow trout, were predictive of toxicity to fish species that inhabit lakes near northern Canadian uranium mining operations, like northern pike or white suckers; (2) to determine if toxicity results derived in the laboratory related to toxicity observed in the field; (3) to determine the relative toxicity of Ni and Mo; (4) to determine how water quality parameters, such as hardness, pH, and total suspended solids (TSS) affects Ni toxicity; and, (5) to determine if exposure to Ni or Mo induced metallothionein in fish. Field studies indicated that Mo concentrations in receiving waters were strongly associated with larval fathead minnow mortality. However, laboratory tests provided contradictory results. Laboratory toxicity tests on field-collected receiving waters gave different results than field tests. Laboratory results were interpreted by considering confounding variables, such as hardness and pH. Waters generally high in Ni, circumneutral or slightly acidic, and with low hardness, caused fathead minnow eggs to hatch earlier than controls. This is a significant result because time-to-hatch is an ecologically important endpoint often not considered in more conventional toxicity-characterization programmes. In laboratory tests involving Ni- and Mo-spiked laboratory dilution water, Ni was much more toxic than Mo. The most sensitive endpoint for Ni toxicity was time required for fathead minnow eggs to hatch. Tolerance to Ni varied by species in the following order: juvenile rainbow trout > alevin rainbow trout > white suckers > northern pike > fathead minnows. Waterborne Mo was not toxic to any life stage of any fish species tested. Indirect evidence suggested that dietary Mo may be toxic to fish, although further work is required. Water hardness, pH, and TSS reduced Ni toxicity to larval fathead minnows. Metallothionein was induced in juvenile rainbow trout gills, but not livers, in Ni-exposed fish, but not Mo-exposed fish.