Reestablishing a Host‐Affiliate Relationship: Migratory Fish Reintroduction Increases Native Mussel Recruitment

Abstract

Co-extirpation among host-affiliate species is thought to be a leading cause of biodiversity loss worldwide. Freshwater mussels (Unionida) are at risk globally and face many threats to survival, including limited access to viable host fish required to complete their life history. We examine the relationship between the common eastern elliptio mussel (Elliptio complanata) and its migratory host fish the American eel (Anguilla rostrata), whose distribution in the Chesapeake Bay watershed is limited, in part, by dams. We examined population demographics of E. complanata across locations in the Chesapeake Bay watershed, primarily in the Susquehanna River in the absence of American eels, and conducted experimental restocking of eels to assess potential impacts on mussel recruitment. Compared to surveys completed~20 yr prior, E. complanata could be experiencing declines at several historically abundant sites. These sites also had extremely limited evidence of recruitment. Restoration of host fish improved recruitment, but results were not equivalent between stocking sites, indicating that host reintroduction alone may not be fully effective in reestablishing mussel populations. One site where eels were introduced (Pine Creek, Tioga County, Pennsylvania, USA) experienced an increase from 0 juveniles found during quantitative surveys prior to eel stocking to 151 (21% of individuals collected during quantitative surveys) E. complanata juveniles found four years after stocking. A second site (Buffalo Creek, Union County, Pennsylvania) experienced a more moderate increase from 2 to 7 juveniles found during 2010 and 2014 quantitative surveys, respectively. Continued examination of other potential interacting factors affecting recruitment, including water quality or habitat conditions, is necessary to target favorable sites for successful restoration.

Spatial analysis of American Eel (Anguilla rostrata), fish passage and land use in Chesapeake Bay tributaries

“…An additional benefit to improving access to upstream habitats for eels comes in the distribution of Eastern Elliptio mussels (Elliptio complanata). Larvae of freshwater mussels (Bivalvia: Unionidae) are host-dependent and attach to fish hosts until they become free-living juveniles [58]. The mussel, uses American Eel as its primary fish host, but both species are in decline [59].…”

“…The mussel, uses American Eel as its primary fish host, but both species are in decline [59]. In the Chesapeake Bay watershed, E. complanata recruitment is limited and this appears to be caused by host species distribution, since the mussels are much more abundant downstream of dams on the mainstem of the Susquehanna River than upstream [58]. Restoring American Eel to a stream improves E. complanata recruitment but not consistently, since water quality (especially nitrogen and sedimentation) and habitat also play a role [58], which emphasizes the need to improve riparian buffers as well.…”

“…In the Chesapeake Bay watershed, E. complanata recruitment is limited and this appears to be caused by host species distribution, since the mussels are much more abundant downstream of dams on the mainstem of the Susquehanna River than upstream [58]. Restoring American Eel to a stream improves E. complanata recruitment but not consistently, since water quality (especially nitrogen and sedimentation) and habitat also play a role [58], which emphasizes the need to improve riparian buffers as well. Eels can be restocked to areas but this is not a panacea since stocked individuals have different growth rates and sex ratios compared to naturally recruiting eels in the same water body [60] and can carry parasites from one watershed to another [61].…”

Assessment of Reintroduction of American Eel into Buffalo Creek (Susquehanna River, Pennsylvania)

“…However, Eastern Elliptio is less abundant in the Susquehanna River than in nearby watersheds (Blakeslee et al 2018), presumably in part due to the lack of eels. From 2008 to 2019, a long-term study evaluated eel as a host fish to Eastern Elliptio in the Susquehanna River watershed (Lellis et al 2013;Galbraith et al 2018). One objective of that study was to determine if increasing eel abundance would increase recruitment of Eastern Elliptio.…”

“…Before 2010, post-relocation assessment was relatively difficult due to the width and depth of stocking locations in the Susquehanna River. Beginning in 2010, eels were released into Buffalo Creek for the freshwater mussel study, with the assumption that they would disperse throughout the watershed, reaching a target density of 529 individuals per stream kilometer, similar to Susquehanna River tributaries below Conowingo Dam (Galbraith et al 2018). Relocation of eels into wadable reaches of Buffalo Creek provided an opportunity to examine movement, sexual differentiation, growth, and population density of reintroduced eels.…”