Mercury contamination in freshwater turtles of eastern Oklahoma: Evaluation of non-destructive sampling techniques

Recent studies in Oklahoma have found levels of mercury (Hg) contamination in fish that exceed safe consumption limits in several lakes. This study investigates the degree of Hg pollution in aquatic turtle species of Oklahoma that are used most commonly for human consumption. Turtles have been used as monitors of chemical contaminants in aquatic environments in both freshwater and marine habitats routinely. These studies are often complicated by the requirement to sacrifice long--lived and slowly reproducing species. A need for a nonlethal routine monitoring technique has been recognized due to a decline in turtle populations. Spiny softshell (Apalone spinifera), common snapping turtle (Chelydra serpentina), and red eared slider (Trachemys scripta) are the most commonly harvested turtles in Oklahoma and thus the indicator species chosen for analysis. Multiple tissue types (muscle, liver, claw, and scute) were collected from 72 turtles in eastern Oklahoma during the summers of 2010 and 2011 from 10 water bodies. Softshells had the highest Hg concentrations (0.04--0.72 mg kg-1), followed by snapping turtles (0.03--0.30 mg kg-1) and red eared sliders (0.01--0.20 mg kg -1). Based on the USEPA food consumption guidelines, seven of the ten sites had average Hg concentrations warranting consumption limits for at least one species. Average muscle Hg concentrations among sites were significantly different (p<0.01). No significant relationships were found between Hg burden and size, sex, or age. Liver/muscle ratios indicated current contamination. In addition, claw and scute were removed from each turtle to test the validity of using non--destructive (external) tissues as an alternative to lethal/destructive sampling of muscle and liver. Claw was the best overall predictor for muscle Hg burdens when comparing across species (R2=0.79) with similar slopes between hard and softshell turtles (slopes=0.087 and 0.099). Scute was not as reliable when all species were combined (R2=0.41). However, when turtles were separated between hard and softshelled species, relationships between Hg concentrations in scute correlated well with concentrations found in muscle (R2=0.84 and 0.83). Continuous monitoring programs are recommended to further protect human health and to track changes in contamination levels. These programs can be completed using the non--destructive tissue techniques and the corresponding linear regression models formulated here.