A MULTI-LIFE STAGE COMPARISON OF EMERGING CONTAMINANT TOXICITY ON THE EARLY DEVELOPMENT OF THREE CANADIAN FISHES
In recent years, emerging contaminants (ECs) have gained notoriety due to their increasing presence in aquatic environments as well as the lack of data available regarding their toxicity to wildlife and humans. ECs of concern such as silver nanoparticles (Ag NPs) and fluoxetine (Prozac™; FLX) primarily enter the aquatic environment as mixtures through municipal wastewater effluents (MWWEs). MWWE, which is typically a combination of industrial, commercial, and household wastes, may be released into receiving waters with little to no treatment. Such practices are not uncommon, especially in rural Canadian municipalities. While a few Canadian studies utilized rainbow trout as the test organisms for toxicological effects of select ECs, the majority of data on the effects of these compounds in fish has been garnered using laboratory species non-native to northern climates, which may not be particularly relevant considering the potential role of life history, trophic level, physiology, and climate on the species-specific toxicity of chemicals. Consequently, inaccurate extrapolation from common laboratory species to species native to northern ecosystems and with commercial, recreational, and aboriginal importance (CRA species) is concerning, as it represents a significant uncertainty factor in ecological risk assessment. This study was completed as part of a larger collaborative effort that aimed to generate novel knowledge and approaches for the extrapolation and characterization of EC-exposed CRA fishes from classic in vivo studies to contemporary in vitro studies. This is an important and necessary step that aims to co-validate the results of each study in the overarching project, as well as begin to build a method that will be able to reliably predict in vivo results from in vitro assays. Overall, the intent is to reduce the number of live animals required for toxicity tests of ECs, as well as lessen the superfluous efforts that many in vivo exposures entail. For the present studies, exposures were performed as continuous, multi-life stage bioassays for 124, 196, and 73 days with rainbow trout (Oncorhynchus mykiss), lake trout (Salvelinus namaycush), and northern pike (Esox lucius), respectively. Subsamples were taken at hatch, swim-up, and at the time of approximate sexual differentiation to determine potential effects on development, growth, and survival. Data on life stage-specific mortalities were analyzed together and separately in order to examine overall survival dynamics as well as to pinpoint differences in sensitivities between early life stages (ELS). As a whole, the data obtained will contribute to the development of more appropriate environmental risk assessment strategies for North American fishes to ECs of concern. Primarily, the results show that there were clear species- and life stage-specific differences for both chemicals analyzed. For Ag NP-exposed fish, endpoints during the embryonic stage were the most frequently affected, at lower concentrations, compared to larval and fry stages. Moreover, while mortality within life stages was not generally affected, the cumulative survival across life stages was significantly lower for all treatment groups in rainbow trout and the lowest (0.10 nM), highest (30.0 nM), and middle (1.00, 3.00 nM) concentrations for northern pike. Unfortunately, cumulative survival in lake trout could not be analyzed during the last life-stage due to complications associated with the novelty of lake trout culturing in the facility. Furthermore, certain endpoints, especially degree-days to hatch and swim-up, were consistently affected across species and life stages and seemed to be the most reliable when indicating exposure to ECs. Alternately, for FLX-treated fish, the larval stage appeared to be the most sensitive, as there was clear endpoint- and species-specific outcomes. Fork length appeared to be the most sensitive endpoint for rainbow trout exposed to FLX, with the lowest observable effect concentrations (LOAECs) for embryonic and larval stages being 2.0 and 0.5 μg/L, respectively. Lake trout depicted a significant hormetic response in fork length during the larval stage with a significant decrease at 125 μg/L. Lake trout also showed an increase in degree-days to transition to swim-up at 0.5 μg/L. Northern pike only exhibited significant effects in degree-days to hatch and swim-up, with embryonic and larval responses occurring at 125 and 500 μg/L, respectively. Moreover, both rainbow trout and lake trout demonstrated significant mortalities at 125 and 500 μg/L. There was a significant decrease in hepatosomatic index (HSI) in lake trout fry exposed to 2.00 μg/L of FLX, and histological analysis revealed increased hepatocyte volume index (HVI) in northern pike fry at 125 μg/L. For both Ag NPs and FLX, rainbow trout, which are a common and standard laboratory species used in toxicity tests for Canadian water guidelines, was either the most or equally sensitive at environmentally relevant concentrations (i.e. the lowest concentrations implemented). The results of this thesis suggest that rainbow trout seem to be an adequate representative species for both northern pike and lake trout with regard to assessing the toxicity of FLX and Ag NPs.
Emerging contaminants, developmental toxicity, silver nanoparticles, fluoxetine, freshwater fish
Master of Science (M.Sc.)