Relationship between aerobic scope and upper thermal limits of Pacific white shrimp (Litopenaeus vannamei) in low-salinity culture systems

Abstract

Aquaculture of the Pacific white shrimp Litopenaeus vannamei in low-salinity water is a viable industry and production strategy in the southeastern United States. A major challenge facing this industry is a phenomenon called late-term mortality which is thought to be driven by thermal stress at the end of the growing season when water temperatures can reach or exceed 36 °C in shrimp production ponds. To investigate the physiological mechanisms behind upper lethal limits in shrimp, we evaluated linkages between empirically measured thermal limits and absolute aerobic scope (AAS), or ability to provide energy above that needed for basic maintenance. In this study, we tested whether thermal tolerance decreases with increasing shrimp age/size and whether AAS is a useful concept for understanding the physiological basis of thermal tolerance in shrimp. We exposed two size classes (small: 2.07 ± 0.86 and large: 24.64 ± 2.55 g) of shrimp to increasing temperature at a rate of 1 °C/h from 28 to 42 °C. At each temperature, we used intermittent respirometry to estimate resting metabolic rate and we directly measured lethal thermal tolerance by evaluating critical thermal maximum (CTmax). Additionally, we used the electron transport system assay to estimate maximum metabolic rate (RMR) at temperatures from 9 to 45 °C. Small shrimp had a higher CTmax than large shrimp, with upper lethal limits of 40.6 and 39.0 °C, respectively. For both size-classes, AAS reached its minimum (AASmin) at temperatures near the peak RMR (RMRpeak) and within 2 °C of CTmax. Large shrimp exhibited a lower temperature at AASmin than that of the smaller shrimp. Reductions in AAS appear to be one of the underlying physiological drivers of thermal tolerance in L. vannamei and an indicator of increasing thermal stress. Changes in the temperature at which AAS reaches its minimum may be a useful predictor of shifts in thermal tolerance among shrimp size-classes.