Evaluation of Potentially Nonlethal Sampling Methods for Monitoring Mercury Concentrations in Smallmouth Bass (Micropterus dolomieu)

We evaluated three potentially nonlethal alternatives to fillet sampling for the determination of mercury (Hg) concentrations in smallmouth bass (Micropterus dolomieu). Fish (n = 62, 226–464 mm total length) from six sites in southern Missouri were captured by electrofishing. Blood samples (1 mL) from each fish were obtained by caudal veinipuncture with a heparinized needle and syringe. Biopsy needle (10 mm × 14 gauge; three cuts per fish; 10–20 mg total dry weight) and biopsy punch (7 mm × 5 mm in diameter, one plug per fish, 30–50 mg dry weight) samples were obtained from the area beneath the dorsal fin. Fillet samples were obtained from the opposite side of the fish. All samples were freeze-dried and analyzed for total Hg by combustion amalgamation atomic absorption spectrophotometry. Mean relative standard deviations (RSDs) of triplicate samples were similar for all four methods (2.2–2.4%), but the range of RSDs was greater for blood (0.4–5.5%) than for the muscle methods (1.8–4.0%). Total Hg concentrations in muscle were 0.0200–0.8809 μg/g wet weight; concentrations in plug, needle, and fillet samples from each fish were nearly identical. Blood Hg concentrations were 0.0006–0.0812 μg/mL and were highly correlated with muscle concentrations; linear regressions between log-transformed blood and fillet Hg concentrations were linear and statistically significant (p < 0.01), and explained 91–93% of the total variation. Correlations between fillet Hg concentrations and fish size and age were weak; together they explained ≤37% of the total variation, and the relations differed among sites. Overall, any of the alternative methods could provide satisfactory estimates of fillet Hg in smallmouth bass; however, both blood and plug sampling with disposable instruments were easier to perform than needle sampling. The biopsy needle was the most difficult to use, especially on smaller fish, and its relative expense necessitates reuse and, consequently, thorough cleaning between fish to prevent cross-contamination.