Response of oxidative stress transcripts in the brain of wild yellow perch (Perca flavescens) exposed to an environmental gradient of methylmercury

Methylmercury (MeHg) exposure and adverse health effects in fishes have been documented, but the molecular mechanisms involved in toxicity have not been fully characterized. The objectives of the current study were to (1) determine whether total Hg (THg) in the muscle was predictive of MeHg concentrations in the brain of wild female yellow perch (Perca flavescens) collected from four lakes in Kejimkujik National Park, a known biological mercury (Hg) hotspot in Nova Scotia, Canada and (2) to determine whether transcripts involved in the oxidative stress response were altered in abundance in fish collected across five lakes representing a MeHg gradient. In female yellow perch, MeHg in whole brain (0.38 to 2.00μg/g wet weight) was positively associated with THg in muscle (0.18 to 2.13μg/g wet weight) (R²=0.61, p<0.01), suggesting that muscle THg may be useful for predicting MeHg concentrations in the brain. Catalase (cat) mRNA levels were significantly lower in brains of perch collected from lakes with high Hg when compared to those individuals from lakes with relatively lower Hg (p=0.02). Other transcripts (cytochrome c oxidase, glutathione peroxidase, glutathione-s-transferase, heat shock protein 70, protein disulfide isomerase, and superoxide dismutase) did not show differential expression in the brain over the gradient. These findings suggest that MeHg may be inversely associated with catalase mRNA abundance in the central nervous system of wild fishes.

Reproductive health of yellow perch (Perca flavescens) from a biological mercury hotspot in Nova Scotia, Canada

Methylmercury (MeHg) exposure is known to adversely affect the reproductive health of laboratory fish, but its impacts on the sexual development of wild fishes are not well studied. Kejimkujik National Park and National Historic Site (KNPNHS) region of Nova Scotia, Canada, has been identified as a biological mercury (Hg) hotspot. To determine whether Hg was adversely affecting the reproductive health of wild yellow perch (Perca flavescens), sexually mature male and female perch were collected from 12 lakes within KNPNHS (mean muscle total Hg: 0.28-0.54 μg/g ww). Gonadosomatic index and germ cell development of male and female perch were measured, as well as the plasma 17 β-estradiol concentrations of females. These endpoints were compared between lakes, and were related to Hg concentrations measured in perch muscle and liver tissues. Our results indicate that the reproductive health of male and female perch was not adversely impacted by Hg, although a positive relationship existed between the proportions of primary spermatocytes in male testes and muscle total Hg concentrations. Perch were sampled at an early stage of recrudescence, and the tissue Hg concentrations in these perch were generally lower than those in laboratory studies reporting impacts on reproductive health, both of which may explain the absence of effects. Based on the measured endpoints, it appears that reproduction in perch was not affected at Hg concentrations known to affect fish eating wildlife.

Evidence of impaired health in yellow perch (Perca flavescens) from a biological mercury hotspot in northeastern North America

Few studies have investigated the effects of mercury (Hg) on wild fish from remote areas, even though these fish can have high total Hg concentrations. In Kejimkujik National Park and National Historic Site (KNPNHS), Nova Scotia, Canada, concentrations of total Hg in many yellow perch (Perca flavescens) currently exceed the estimated threshold level for adverse effects in fish (0.2 µg Hg g(-1) (wet wt), whole body). To determine whether Hg exposure is adversely affecting the general health of these fish, the authors collected male and female perch in the fall of 2009 and 2010 from 12 lakes within KNPNHS. The health endpoints condition, liver somatic index (LSI), and macrophage aggregates (MAs; indicators of oxidative stress and tissue damage) in the liver, kidney, and spleen were examined, and in female perch were compared between lakes and related to Hg concentrations measured in the muscle and liver tissue. No negative relationships between fish condition or LSI and Hg were found. However, within the liver, kidney, and spleen tissues of females, the relative area occupied by MAs was positively related to both muscle and liver Hg concentrations, indicating the health of these perch was adversely affected at the cellular level. These findings raise concerns for the health of these perch as well as for other wild fish populations known to have similarly elevated Hg concentrations.