Effects of early-life exposure to contaminated sediments in fish

Early-life stages of fish are often more susceptible to environmental pollutants than juvenile and adult forms. In aquatic ecosystems, sediments can be an important source of exposure to contaminants in these early-life stage fish. The overall goal of this thesis was to investigate the effects of embryonic and larval exposure of fish to sediments from Lake Saint Louis, a fluvial lake affected by agricultural, industrial, and municipal inputs located in southwestern Quebec.In the first data chapter of this thesis (Ch.2), an in situ method was developed and validated by exposing early-life stages of walleye (Sander vitreus) to reference and contaminated sediments in Lake Saint-Louis, QC. Sites within the lake were characterized as ‘reference’ (1 site) or ‘contaminated’ (3 sites) based on analytical determination of levels of polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), dioxins, furans, and metals in lake sediments (data provided by Environment and Climate Change Canada (ECCC)). In order to assess the toxicity of these sediments to early-life stage fish, walleye eggs were placed into hatching boxes and deposited at the 4 sites in Lake Saint Louis in May of 2016 and 2017. A low percentage of the deposited eggs survived (0.9% - 6.6%), likely due to difficulties with the cage design. Live larvae were retrieved from 2 out of 4 sites over the two field seasons. We determined the number of individuals needed for future molecular work (e.g., DNA methylation, gene expression and thiobarbituric acid reactive substances (TBARS) assay which is used to detect oxidative stress). We were able to establish good quality DNA yields of 11-23 µg (5 pooled larvae), and good quality RNA yields of 7-9 µg (3 pooled larvae). A minimum of 30 mg of homogenized walleye larvae was required for TBARS analyses.In the second data chapter of this thesis (Ch. 3), a sediment contact assay was performed, based on the developed methods in Hollert et al. (2003), to assess the potential of contaminated sediments from Lake Saint Louis, QC to cause harm to early-life stages of fish using a model organism, zebrafish (Danio rerio). The sediment contact assay exposed zebrafish from ~3 to 120 hours post-fertilization (hpf) to sediments collected from a reference and two contaminated sites in Lake Saint-Louis, QC. At the end of the study, biochemical (DNA methylation and gene expression), and organismal-level (mortality, abnormalities, hatching rate) effects were evaluated. In comparison to the reference site, mortality was significantly higher in larvae exposed to sediment from one of the contaminated sites (43%), but not the other (21%). Additionally, the expression of 11 genes related to xenobiotic metabolism, embryonic development, oxidative stress or DNA repair were evaluated. Several genes from the aryl hydrocarbon receptor (AhR) pathway were significantly induced by exposure to the contaminated sediment; CYP1A (34, 42-fold induction), CYP1B1 (20, 22-fold induction), AhR2 (1.4, 1.4-fold induction). This study addresses effects…