Thermal Tolerance Exceedances of an Endangered Unionid in the Rio Grande Basin with Implications for River Management

Abstract

Freshwater mussels are globally imperiled due to the human alteration of riverine systems. In semi-arid locations, they are jeopardized by declining streamflow and elevated temperatures. I measured the lethal thermal tolerances (LT05/50) of larvae (glochidia) and newly transformed juveniles from two populations of the federally endangered unionid Popenaias popeii (Texas hornshell) in the Rio Grande drainage. Mussels were acclimated to 27°C then exposed to a range of water temperatures from 28–38°C for 12-h and 24-h (glochidia) or 96-h (juveniles). I also compiled upper thermal tolerance information for P. popeii’s three presumptive host fishes (Carpiodes carpio [river carpsucker], Cyprinella lutrensis [red shiner], and Moxostoma congestum [gray redhorse]). I overlaid thermal tolerances from P. popeii and its hosts onto in situ water temperature data to identify thermal exceedances, which I linked back to discharge. For the Black River, I hindcasted water temperature for the past 15 years to temporally expand my dataset. I also performed a uniform continuous above-threshold (UCAT) analysis for the Black to determine the severity of exceedances. I then compared thermal tolerances and exceedances between the Devils and Black and overlaid tolerance thresholds onto water temperature data from two other rivers containing P. popeii populations (Rio Grande near Laredo and Pecos River near Pandale) to determine exceedances. Analysis showed that thermal tolerances were generally similar across populations, although Ruthies had higher glochidial LT05/50 compared to Grass Patch and Black individuals. Juvenile tolerances could not be directly compared. Fish thermal tolerances were equivalent to or higher than that of juveniles. Glochidial thermal tolerances (24-h) were rarely exceeded in the Devils, but exceeded frequently in the Black, including several catastrophic events. Thermal exceedances were also frequent in the Rio Grande and Pecos. Most exceedances occurred during the hot summer months when discharge was low. It appears that the three P. popeii populations in heavily modified rivers with intensive water extraction are thermally stressed, while the population in a relatively pristine river is not. The results of my study highlight the importance of protecting instream flows for mussel population persistence.