Respiratory area of carp (Cyprinus carpio), redrawn from Oikawa and Itazawa (1985), illustrating its fast hyperallometric growth in teleost larvae (dG ≫ 1) and early fingerlings (dG > 1), and the slower, but still hyperallometric growth of its gills in juveniles and adults (dG = 0.794). The inset shows the gill area in juvenile and adults, divided by the corresponding body weight and plotted against that same body weight. The resulting scope for growth (which requires O2) declines with body weight, down to a level (at W∞1, or W∞2) where all the available O2 is used for maintenance. Higher temperatures, by increasing metabolic rate, will shift the asymptotic weight from W∞2 to W∞1, irrespective of assumptions or data on the shape of the maintenance metabolism (dotted lines). Graphic: Pauly and Cheung, 2017 / Global Change Biology

By Craig Welch
21 August 2017

(National Geographic) – Warming temperatures and loss of oxygen in the sea will shrink hundreds of fish species—from tunas and groupers to salmon, thresher sharks, haddock and cod—even more than previously thought, a new study concludes [responding to Models projecting the fate of fish populations under climate change need to be based on valid physiological mechanisms –Des].

Because warmer seas speed up their metabolisms, fish, squid and other water-breathing creatures will need to draw more oxyen from the ocean. At the same time, warming seas are already reducing the availability of oxygen in many parts of the sea.

A pair of University of British Columbia scientists argue that since the bodies of fish grow faster than their gills, these animals eventually will reach a point where they can't get enough oxygen to sustain normal growth.

"What we found was that the body size of fish decreases by 20 to 30 perent for every 1 degree Celsius increase in water temperature," says author William Cheung, director of science for the university's Nippon Foundation—Nereus Program.

These changes, the scientists say, will have a profound impact on many marine food webs, upending predator-prey relationships in ways that are hard to predict. (Read about how climate change is resulting in smaller mountain goats.)

"Lab experiments have shown that it's always the large species that will become stressed first," says lead author Daniel Pauly, a professor at the university's Institute for the Ocean and Fisheries, and principal investigator for the Sea Around Us. "Small species have an advantage, respiration-wise." [more]

Climate Change May Shrink the World’s Fish


ABSTRACT: One of the main expected responses of marine fishes to ocean warming is decrease in body size, as supported by evidence from empirical data and theoretical modeling. The theoretical underpinning for fish shrinking is that the oxygen supply to large fish size cannot be met by their gills, whose surface area cannot keep up with the oxygen demand by their three-dimensional bodies. However, Lefevre et al. (Global Change Biology, 2017, 23, 3449–3459) argue against such theory. Here, we re-assert, with the Gill-Oxygen Limitation Theory (GOLT), that gills, which must retain the properties of open surfaces because their growth, even while hyperallometric, cannot keep up with the demand of growing three-dimensional bodies. Also, we show that a wide range of biological features of fish and other water-breathing organisms can be understood when gill area limitation is used as an explanation. We also note that an alternative to GOLT, offering a more parsimonious explanation for these features of water-breathers has not been proposed. Available empirical evidence corroborates predictions of decrease in body sizes under ocean warming based on GOLT, with the magnitude of the predicted change increases when using more species-specific parameter values of metabolic scaling.

Sound physiological knowledge and principles in modeling shrinking of fishes under climate change


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.

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

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