Pragmatic perspective on aerobic scope: peaking, plummeting, pejus and apportioning

Abstract

A major challenge for fish biologists in the 21st century is to predict the biotic effects of global climate change. With marked changes in biogeographic distribution already in evidence for a variety of aquatic animals, mechanistic explanations for these shifts are being sought, ones that then can be used as a foundation for predictive models of future climatic scenarios. One mechanistic explanation for the thermal performance of fishes that has gained some traction is the oxygen and capacity-limited thermal tolerance (OCLTT) hypothesis, which suggests that an aquatic organism's capacity to supply oxygen to tissues becomes limited when body temperature reaches extremes. Central to this hypothesis is an optimum temperature for absolute aerobic scope (AAS, loosely defined as the capacity to deliver oxygen to tissues beyond a basic need). On either side of this peak for AAS are pejus temperatures that define when AAS falls off and thereby reduces an animal's absolute capacity for activity. This article provides a brief perspective on the potential uses and limitations of some of the key physiological indicators related to aerobic scope in fishes. The intent is that practitioners who attempt predictive ecological applications can better recognize limitations and make better use of the OCLTT hypothesis and its underlying physiology.

Acclimation potential of Arctic cod (Boreogadus saida Lepechin) from the rapidly warming Arctic Ocean

“…This was evident in B. saida with the significant reduction in f H,max (∼7 bpm) at 6.5°C acclimation when compared with 0.5°C acclimation. A reduction of f H is predicted with an increase in tolerance to warmer water (Farrell, 1997(Farrell, , 2016 and, at least in rainbow trout, appears to be caused by modification of the pacemaker action potential (Haverinen and Vornanen, 2007). A Q 10 effect has been used to describe acclimation potential (Du et al, 2010;Seebacher et al, 2015).…”

“…In our study, the cardio-respiratory transition temperatures (i.e. performance limits) were incorporated into a modified Fry temperature polygon to graphically represent B. saida windows of thermal tolerance (Farrell, 2016). Previously, the rate transition temperatures for ƒ H,max and AAS have been placed in a hierarchy within a Fry thermal polygon for goldfish (Ferreira et al, 2014) and rainbow trout (Chen et al, 2015).…”

“…We also hypothesized that breathing rates in 1°C acclimated fish would always return to resting after being chased faster than 6.5°C acclimated fish, which in fact turned out to be the opposite. We did, however, anticipate that the suite of physiological rate transition temperatures would show a predictable order, as seen previously in goldfish (Ferreira et al, 2014;Farrell, 2016 (Drost et al, 2014). In brief, B. saida were held at 0°C for up to 4 weeks at Cambridge Bay to ensure good health before being transported by air at 0°C to the Vancouver Aquarium, British Columbia, Canada.…”

“…the T max ) (Drost et al, 2014). Yet the temperature when heart rate first starts to fail to keep up with acute warming (see Farrell, 2016), the first Arrhenius breakpoint temperature (T AB ), was just 3.6°C and 4.7°C, respectively (Drost et al, 2014). These T AB results are similar to the temperature preferendum of B. saida, which is between 2.8 and 4.4°C (over a 0-8°C range) depending on the time of day (Schurmann and Christiansen, 1994).…”

Exploring key issues of aerobic scope interpretation in ectotherms: absolute versus factorial

“…There are also a range of extrinsic factors that have profound effects on metabolism and aerobic scope in ecotherms. In response to an increase in temperature, for example, SMR and MMR generally increase, with at least some species displaying a decrease in AAS past a species-specific optimum (Farrell 2016;Lefevre 2016). This pattern is often due to a decrease in MMR beyond this point, though not all species display this response and instead reach lethal temperatures before MMR begins to decline (Jutfelt et al 2018;Lefevre 2016;Nati et al 2016).…”