Muskellunge (Esox masquinongy) are a large, predatory fish critical to recreational fisheries throughout the Great Lakes. In 2005, populations in the upper St. Lawrence River substantially declined, largely due to viral hemorrhagic septicemia virus (VHSv). To restore muskellunge, regional restocking initiatives are underway. Gauging the success of such efforts, and refining management plans to support this species, is reliant on accurate assessment of distribution and abundance. Traditionally, these assessments are obtained by direct count methods including various forms of live trapping, visual surveys, and angler catch reports. For an elusive, low-density species like muskellunge, these methods are problematic because they are resource intensive, disturb the target species and associated habitat, and can introduce data biases as fish exhibit avoidance behaviors and shifts in habitat preference in response to certain techniques. Therefore, we seek to supplement traditional approaches with environmental DNA (eDNA) methodologies to improve data quality and allocate resources more effectively. To derive quantitative population estimates from environmental sampling data, we determined eDNA shedding and decay rates for muskellunge embryos, larva, and juveniles through controlled aquaria experiments. To assess where eDNA methods can be most practically and effectively applied in the field, eDNA sampling was paired with traditional monitoring techniques.
Results/Conclusions
We developed a sensitive and specific TaqMan® qPCR assay for muskellunge which was used to measure eDNA in water samples. In controlled aquaria experiments, embryos did not produce quantifiable amounts of eDNA, even when stocked at high density (150 embryo / 58.5 L). Larval and juvenile muskellunge produced quantifiable amounts of eDNA at various levels of stocked biomass, allowing eDNA shedding and decay rates to be measured. For larval muskellunge, eDNA shedding rates ranged from 4.42x104 to 2.02x105 gene copies per hour per gram of fish. First-order decay rate constants for larval muskellunge ranged from 0.062 to 0.120 per hour. For juvenile muskellunge, eDNA shedding rates ranged from 1.60x105 to 2.61X105 gene copies per hour per gram of fish. First-order decay rate constants for juveniles ranged from 0.059 to 0.144 per hour. Environmental sampling was conducted during spring spawning surveys, and muskellunge eDNA was detected from 40 of 69 water samples collected in potential spawning bays. This 0.58 detection probability, compared to a 0.03 capture rate obtained via trap-netting, highlights the potential for molecular methods to supplement traditional approaches. Shedding and decay rates for muskellunge eDNA determined in this study will be used in interpretation of future eDNA sampling data.