Thermal tolerance exceedances of an endangered unionid mussel in the Rio Grande Basin, with implications for river management

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Abstract
Freshwater mussels are globally imperilled, which is attributed to their sensitivity to changes in streamflow and temperature. The Black River in south‐eastern New Mexico, USA, harbours a stronghold population of the federally endangered unionid Popenaias popeii (Texas hornshell). Decreasing discharge and elevated water temperatures are considered the primary factors responsible for the species’ decline. However, the impact of these factors has not been quantified directly, which may hinder conservation efforts. The upper thermal tolerances (LT05 and LT50) of Black River P. popeii larvae (glochidia) and newly transformed juveniles were evaluated. Individuals were acclimated to 27°C and then immersed at five experimental temperatures (28, 30, 32, 34 and 36°C) for 24 h (glochidia) or 96 h (juveniles). The thermal tolerances of P. popeii and its presumed host fishes were then overlain onto in situ water temperature and discharge data to determine thermal exceedances in the past 15 years. Temperature was hindcast back to 2007 using machine learning (random forest model) and coupled with discharge. For glochidia, LT05 was exceeded frequently (41%) and LT50 was exceeded occasionally (13%) during spring and summer periods, whereas juvenile thresholds were never exceeded. Upper continuous duration above threshold (UCAT) analysis revealed periods of catastrophic high temperature, which were used to determine discharge bottlenecks. This study demonstrates how laboratory‐derived physiological thresholds can be used in conjunction with environmental data to evaluate the hydrological needs of aquatic organisms, which is useful in efforts to maintain flow regimes that protect native ecosystems.

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