Models projecting the fate of fish populations under climate change need to be based on valid physiological mechanisms

Abstract

Some recent modelling papers projecting smaller fish sizes and catches in a warmer future are based on erroneous assumptions regarding (i) the scaling of gills with body mass and (ii) the energetic cost of 'maintenance'. Assumption (i) posits that insurmountable geometric constraints prevent respiratory surface areas from growing as fast as body volume. It is argued that these constraints explain allometric scaling of energy metabolism, whereby larger fishes have relatively lower mass-specific metabolic rates. Assumption (ii) concludes that when fishes reach a certain size, basal oxygen demands will not be met, because of assumption (i). We here demonstrate unequivocally, by applying accepted physiological principles with reference to the existing literature, that these assumptions are not valid. Gills are folded surfaces, where the scaling of surface area to volume is not constrained by spherical geometry. The gill surface area can, in fact, increase linearly in proportion to gill volume and body mass. We cite the large body of evidence demonstrating that respiratory surface areas in fishes reflect metabolic needs, not vice versa, which explains the large interspecific variation in scaling of gill surface areas. Finally, we point out that future studies basing their predictions on models should incorporate factors for scaling of metabolic rate and for temperature effects on metabolism, which agree with measured values, and should account for interspecific variation in scaling and temperature effects. It is possible that some fishes will become smaller in the future, but to make reliable predictions the underlying mechanisms need to be identified and sought elsewhere than in geometric constraints on gill surface area. Furthermore, to ensure that useful information is conveyed to the public and policymakers about the possible effects of climate change, it is necessary to improve communication and congruity between fish physiologists and fisheries scientists.

Rediscovering and Reviving Old Observations and Explanations of Metabolic Scaling in Living Systems

“…Body shape and associated surface area may have evolved to match metabolic demand [133]. This reverse causation likely explains parallel scaling of gill surface area and metabolic (oxygen consumption) rate in amphipod crustaceans [136] and perhaps fishes, as well [137,138] (also see Section 3.3).…”

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

“…Hypoxia can also have a strong effect on aerobic scope by limiting oxygen supply and reducing MMR (Claireaux and Lagardère 1999). Given the importance of understanding how body size and environmental conditions interact to affect responses to climate change (Lefevre et al 2017;Lindmark et al 2018;Pauly and Cheung 2018), a consideration of how to best quantify changes in aerobic scope due to these factors is clearly warranted.…”

Temporal resolutions in species distribution models of highly mobile marine animals: Recommendations for ecologists and managers

“…In particular, the physiology and behaviour underlying the modelled species-habitat relationships may change under different climate regimes (Lefevre, McKenzie, & Nilsson, 2017;Myers, 1998), reducing the validity of the projections.…”