Using Salinity Tolerance to Inform Conservation of an Endangered Unionid Mussel, <scp><i>Popenaias popeii</i></scp> (Texas Hornshell), in a Salinized River

ABSTRACT Freshwater mussels are globally threatened due to changes in the hydrological cycle and water quality. The Rio Grande basin in North America contains the endangered unionid Popenaias popeii (Texas hornshell). Secondary salinisation has likely contributed to the decline of this species, leaving only a handful of populations remaining. Conservation groups are interested in restoring P. popeii in reaches where it formerly occurred and augmenting existing populations. Assessing habitat suitability before reintroducing live individuals is vital to ensuring these actions are successful. In this study, we evaluated water quality constraints on P. popeii by determining its salinity tolerance. Recent studies have shown salinity may be responsible for P. popeii ' s decline in the Pecos River, a major tributary to the Rio Grande in Texas. We determined lethal salinity tolerances (LC 05 and LC 50 ) of P. popeii from the Black River, New Mexico, a tributary of the Pecos River, by exposing individuals to a range of salinity concentrations (1, 2, 3, 4, 5, and 6 PSU). Larvae (glochidia) were exposed for 12‐h and 24‐h and newly transformed juveniles for 96‐h. We then overlaid P. popeii 's salinity tolerance thresholds onto in situ salinity data from sites across the Rio Grande Basin for the past six decades to determine exceedances. Reaches where the mussel still occurs had minimal exceedances, while reaches from which it has been extirpated had frequent exceedances. Proposed restoration for P. popeii in the Pecos River is likely to be unsuccessful unless salinisation is addressed, even in reaches with relatively higher spring flows.