Résumé: 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.

Résumé: The effect of salinity was studied in natural populations of the black-chinned tilapia (Sarotherodon melanotheron) from West Africa. This euryhaline species colonizes nearly all coastal environments from bays to lagoons characterized by salinities ranging from fresh water to hypersaline water over 100 aEuro degrees. Individuals were sampled during the dry season at several locations characterized by different levels of salinity (3-102 aEuro degrees). Their osmotic status and their gills were analyzed. The branchial mitochondria-rich cells (MRC), localized at the basis of the filaments and along the lamellae in fish taken from the saline stations, showed a wide plasticity with significant differences in their number and size. The most striking results were a significant larger area (a parts per thousand 3x) and a higher number (a parts per thousand 55x) of MRC at high salinity (102 aEuro degrees) compared to low salinity (3 aEuro degrees). The major ion transporters and channels were localized by immunocytochemistry and different expression patterns have been recorded between stations. Despite an increased Na+/K+-ATPase (NKA) alpha-subunit expression and NKA activity, pointing to an increased monovalent ion excretion, a severe osmotic imbalance was recorded in animals living in hypersaline environments.

Résumé: European sea bass Dicentrarchus Iabrax undertake seasonal migrations to estuaries and lagoons that are characterized by fluctuations in environmental conditions. Their ability to cope with these unstable habitats is undeniable, but it is still not clear how and to what extent salinity acclimation mechanisms are affected at temperatures higher than in the sea. In this study, juvenile sea bass were pre-acclimated to seawater (SW) at 18 degrees C (temperate) or 24 degrees C (warm) for 2 weeks and then transferred to fresh water (FW) or SW at the respective temperature. Transfer to FW for two weeks resulted in decreased blood osmolalities and plasma Cl- at both temperatures. In FW warm conditions, plasma Na+ was similar to 15% lower and Cl- was similar to 32% higher than in the temperate-water group. Branchial Na+/K+-ATPase (NKA) activity measured at the acclimation temperature (V-apparent) did not change according to the conditions. Branchial Na+/K+-ATPase activity measured at 37 degrees C (V-max) was lower in warm conditions and increased in FW compared to SW conditions whatever the considered temperature. Mitochondrion-rich cell (MRC) density increased in FW, notably due to the appearance of lamellar MRCs, but this increase was less pronounced in warm conditions where MRC's size was lower. In SW warm conditions, pavement cell apical microridges are less developed than in other conditions. Overall gill morphometrical parameters (filament thickness, lamellar length and width) differ between fish that have been pre acclimated to different temperatures. This study shows that a thermal change affects gill plasticity affecting whole-organism ion balance two weeks after salinity transfer.