In modelling effects of global warming, invalid assumptions lead to unrealistic projections

Abstract

In their recent Opinion, Pauly and Cheung () provide new projections of future maximum fish weight (W ). Based on criticism by Lefevre et al. (2017) they changed the scaling exponent for anabolism, d . Here we find that changing both d and the scaling exponent for catabolism, b, leads to the projection that fish may even become 98% smaller with a 1°C increase in temperature. This unrealistic outcome indicates that the current W is unlikely to be explained by the Gill-Oxygen Limitation Theory (GOLT) and, therefore, GOLT cannot be used as a mechanistic basis for model projections about fish size in a warmer world.

Is oxygen limitation in warming waters a valid mechanism to explain decreased body sizes in aquatic ectotherms?

“…Recently, the importance of oxygen supply as a determinant of body size has been questioned. Lefevre, McKenzie, and Nilsson (; ) challenged the claim that oxygen supply could limit growth and body size under most conditions, at least for gill breathing ectotherms, such as fish. Indeed, the current view among physiologists is that oxygen uptake can be easily modulated by organisms and therefore reflects oxygen demand rather than the other way around.…”

“…The mechanisms involved in short‐term acclimation are likely to differ from those that develop over longer evolutionary time‐scales. Broad‐scale interspecific comparisons therefore may not be relevant for understanding species‐specific responses to climate change over the next few decades (e.g., see debate in Lefevre et al, and Pauly & Cheung, ); Highlighting a range of alternative mechanisms that could help resolve the apparently conflicting evidence for oxygen supply as a limiting factor on body size (Figure ). Body size is an emergent property of multiple intrinsic physiological (development rate, metabolic rate, intake rate, allocation to reproduction) and ecological (food availability, predation risk) processes, and oxygen supply is only one of them.…”

“…The mechanisms involved in short-term acclimation are likely to differ from those that develop over longer evolutionary time-scales. Broad-scale interspecific comparisons therefore may not be relevant for understanding species-specific responses to climate change over the next few decades (e.g., see debate in Lefevre et al, 2018 andCheung, 2018); 3. Highlighting a range of alternative mechanisms that could help resolve the apparently conflicting evidence for oxygen supply as a limiting factor on body size (Figure 1).…”

On confusing cause and effect in the oxygen limitation of fish

“…This letter deals with the critique by Lefevre et al. (, ) of the use of Gill‐Oxygen Limitation Theory (GOLT) to explain observed and predict further decreases of the maximum body size of fish under warming.…”

“…Second, Lefevre et al. () wrongly assume that the catabolic term in the general von Bertalanffy growth function (VBGF) requires oxygen and that “[…] all evidence indicates that maintenance oxygen demand decreases with mass ”. However, the catabolic (second) term of the general VBGF, d W /d t = H · W d − k · W b , is oxygen‐independent as the spontaneous, temperature‐dependant denaturation (i.e., loss of the quaternary structure) of the body's protein molecules does not require oxygen (Cox & Nelson, ).…”