Scope for growth is optimized across a limited temperature range in an imperiled freshwater mussel

Abstract Predicting effects of rising temperatures on the health and physiology of aquatic organisms is receiving increasing attention, particularly in arid regions with limited freshwater habitat. One approach to estimating energetic health of aquatic ectotherms is scope for growth (SFG)—the net energy available for reproduction and growth after meeting basic maintenance costs. We examined the relationships between temperature, clearance rates, respiration rates, and other physiological parameters to estimate SFG of Popenaias popeii —a federally endangered mussel native to the semi-arid Rio Grande basin of the southwestern USA and Mexico. Scope for growth followed a three-phase pattern with values remaining stable but negative from 16 to 24°C, increasing and becoming positive from 24 to 28°C, and decreasing back down to negative values from 28 to 32°C. In New Mexico’s Black River, a last stronghold for P. popeii , seasonal thermal regimes show that maximum investment in growth and reproduction likely occurs from May to October, when positive SFG values are most common. Within this critical period, mid-summer increases from 28 to 32°C may temporarily dampen or eliminate growth and reproduction in P. popeii as SFG decreases from peak to negative values. Management of flow to minimize thermal stress during mid-summer appears critical to maximizing growth and reproduction of remaining populations.