Survivorship of captively propagated Carolina heelsplitter and a potential surrogate, eastern creekshell, in South Carolina streams

Abstract The Carolina heelsplitter Lasmigona [ Platynaias ] decorata is a federally endangered mussel endemic to North and South Carolina, USA. Captive propagation is aimed at bolstering wild populations via augmentation of juveniles, though factors affecting survival of captive‐reared Carolina heelsplitter have not been evaluated. We compared survivorship of 100 captive‐reared juvenile Carolina heelsplitters originating from two geographically isolated river basins ( n = 50 of Pee Dee River basin origin; n = 50 of Catawba River basin origin) over 1 year (Oct 2021–Oct 2022). We retained a subset ( n = 10 per origin) of mussels in a hatchery to serve as a control cohort and housed the remainder in concrete mussel silos deployed in one of four natural streams. We integrated a second species (eastern creekshell Villosa delumbis , n = 11 control and 20 in silos) in our study design to evaluate its potential use as a surrogate for assessing suitability of water quality for the Carolina heelsplitter at potential release sites, given the high rate of co‐occurrence between the two species in South Carolina. We used known fate models to estimate weekly survival rates and ranked models to assess support for effects of species, basin of origin, release site, and time. Only 50 of 151 mussels (14% of all Carolina heelsplitters; 70.5% of eastern creekshells) survived the study. Eastern creekshell survival in the hatchery (100%) was higher than in natural streams (63%). However, we detected no evidence that Carolina heelsplitter survivorship was higher in the hatchery or varied by stream. Mussel survivorship increased with size at release but declined over time (except for eastern creekshell in the hatchery), with the most severe declines observed for the Carolina heelsplitter. Further, origin was a better predictor of Carolina heelsplitter survival than species alone or release site, with Catawba‐origin juveniles having the lowest survivorship at most time points. Consistency in outcomes of Carolina heelsplitters in silos and the hatchery (control) provides strong evidence that their poor survivorship was due to an inherent attribute of our release cohort, rather than poor stream quality or cage effects. Further, the stark differences in species survivorship indicate the eastern creekshell may be a poor surrogate for investigating the resource requirements and release site suitability for Carolina heelsplitters of the size range we considered (25–38 mm). To improve propagation and release success for Carolina heelsplitters, we recommend investigating the species' basic requirements, the efficacy of rearing juveniles to a larger size prior to release, and whether extremely low rates of survival described here are consistent among first‐generation (F1) offspring harvested from a wider range of wild adults in each natal river basin.