Assessment of invasion extent of Asian swamp eels in ponds and backwater marshes adjacent to the Chattahoochee River: With consideration of management and control strategies

Taylor, Andrew; Long, James

doi:10.36967/nrr-2291487

Asian Swamp Eels (Monopterus albus; ASEs) were first documented in the contiguous U.S. in 1994 within several ponds on the Chattahoochee Nature Center (CNC), a private nature preserve in Roswell, Georgia. Subsequent surveys from 2000 to 2005 detected adult and juvenile eels within adjacent marsh habitats along the Chattahoochee River, within the boundaries of the Chattahoochee River National Recreation Area. However, previous studies were somewhat limited by low capture rates of eels within the marshes. Herein, we expand upon these previous studies to provide updated estimates of invasion extent (spatial distribution and adult abundance) and to assess potential management and control strategies for ASEs within the Chattahoochee River marshes. Leaf litter trapping of juvenile ASEs allowed for the estimation of detection and occupancy probabilities within the marshes immediately adjacent to the CNC, as well as surrounding marsh habitats. In 2015, a juvenile eel was detected 1.5 km downstream from the CNC ponds, and in 2016 juveniles were detected in a marsh situated across the mainstem Chattahoochee River. Occupancy probabilities increased with vegetation cover and amount of silt substrate, and maps of occupancy probability suggested eels could occupy several marsh areas where they had not been previously detected, including portions of the Willeo Creek marsh and an unnamed tributary to the North Marsh. Subsequent canoe-based backpack electrofishing captured six adult eels from the unnamed tributary to the North Marsh in 2017, confirming eels have expanded their range within the marshes and interconnected habitats. Because capture rates of adult eels from the marshes were low, we used next-generation sequencing of 60 juvenile eels to estimate an approximate minimum bounds on the number of reproducing adults in the marshes in 2015. The pedigree reconstruction of maximum likelihood indicated that 86 adults contributed to our sample of juveniles, and point estimates of the effective number of breeders ranged from 57 to 154. Canoe-based electrofishing routes produced five adult eels from the marshes adjacent to the CNC in May through July 2016, but electrofishing circuits failed to detect eels in the Middle, North, or South marshes in July 2017. We assessed the potential use of canoe-based electrofishing to deplete eels within Beaver and Kingfisher ponds on the CNC property, both of which were known to produce high catch rates of eels. Catch rates fell with subsequent depletion passes within a day (likely trap-shyness), but catch rates were approximately equal (~0.20 eels/minute) across four consecutive days of effort with no noticeable depletion trend in either pond (despite the removal of a total of 142 eels). Baited stationary traps and baited eel pots failed to capture eels within the marshes or CNC ponds. Finally, we integrate our results with existing literature to examine management and control options for ASEs within the Chattahoochee River marshes. Given the reproductive strategies of the species (e.g., protogynous sequential hermaphroditism and polygamy wherein females visit multiple males within a breeding season) and low catchability of adult eels from the marshes, removal and control efforts will be costly and eradication will be virtually impossible. Moving forward, the costs of management actions should be balanced with known and other, perhaps unforeseen, ecological risks of population persistence or expansion.