Available at: https://digitalcommons.calpoly.edu/theses/3013
Date of Award
5-2025
Degree Name
MS in Biological Sciences
Department/Program
Biological Sciences
College
College of Science and Mathematics
Advisor
Sean Lema
Advisor Department
Biological Sciences
Advisor College
College of Science and Mathematics
Abstract
Climate change is leading to warmer atmospheric temperatures in many regions of the world, which in turn is raising the temperatures of aquatic habitats and simultaneously shifting the frequency and severity of extreme precipitation events. In fishes, metabolic demands are higher under warmer conditions, while dissolved oxygen levels typically decline in warmer water. This mismatch may limit the ability of fishes to cope with high water flow events like floods that can occur following extreme rainfall. A few species of pupfishes (genus Cyprinodon) in the Death Valley region of California and Nevada, USA, occupy stream or river habitats with high temperatures in summer. These pupfish populations are largely considered resilient to flash floods. However, if climate change results in extreme rainfall events becoming more severe and frequent during the warmer months of summer and early fall - when pupfish may also be experiencing even warmer temperatures from climate change - such high-flow events may have detrimental consequences on pupfish survivorship. A full recognition of what those potential effects might be requires understanding how elevated temperatures alter pupfish swimming performance and metabolic physiology. Here, we investigated how temperature affected the metabolic and swimming performance of adult Amargosa River Pupfish, Cyprinodon nevadensis amargosae. This species is endemic to the Amargosa River in the Death Valley region. Temperatures in this small river system can exceed 40˚C during summer. The river also frequently experiences flash flooding during rain events. Adult Amargosa River Pupfish were acclimated to temperatures of 16˚C, 27.5˚C, or 35˚C for a duration of 63 to 114 days, and then tested for differences in Critical Swimming Speed (Ucrit) and several measures of metabolic performance (standard metabolic rate, SMR; maximal metabolic rate, MMR; aerobic scope; critical swimming speed, Ucrit; and Cost of Transport at Ucrit, COT) using an intermittent swim-tunnel respirometer system. Following these swim trials, skeletal muscle tissue was collected from each fish. Muscle tissue was sectioned and stained for succinic dehydrogenase (SDH) to determine subsarcolemmal mitochondrial density, as well as muscle fiber morphological characteristics. Enzymatic assays were also conducted to determine lactate dehydrogenase (LDH), citrate synthase (CS), and pyruvate decarboxylase (PDC) activity levels in skeletal muscle. Critical swimming speed (Ucrit) increased in pupfish from 16˚C to 27.5˚C, but then declined at 35˚C. That decline in swimming performance was paralleled by a reduction in aerobic scope and increase in the metabolic cost of transport at 35˚C, implying that pupfish were unable to meet additional energetic demands aerobically at that 35˚C temperature. Those changes in aerobic metabolic capacity were associated with a lower oxidative capacity in both red (‘slow twitch’) and white (‘fast twitch’) skeletal muscle as indicated by mitochondrial density via SDH staining intensity, as well as reduced muscle citrate synthase activity in pupfish at 35˚C. Taken as a whole, these findings point to reduced aerobic swimming capacity in adult Amargosa River pupfish experiencing a prolonged period of high temperatures. That reduced aerobic swimming performance is likely to compromise the ability of pupfish to endure, and survive, high-flow regimes that occur during periods of high temperatures.