Effects of body chemical components on the allometric scaling of the resting metabolic rate in four species of cyprinids.

Metabolic scaling is hypothesized to be affected by ontogenetic changes in the body chemical composition because several body components do not consume significant amounts of oxygen but contribute more to body mass (M) as a fish grows. We tested this hypothesis by assessing the mass scaling of the resting metabolic rate (RMR) and body compositions of lipid, protein, ash, and water in four species of cyprinids, Hypophthalmichthys molitrix (M range 9.5-218.3 g), Cyprinus carpio (M range 14.6-226.9 g), Carassius auratus (M range 4.5-323.2 g), and Ctenopharyngodon idellus (M range 5.8-274.2 g). The oxygen consumption of fasting fish was determined using a continuous flow respirometer at 25 °C and was used to assess the RMR. Then, the fish bodies were dried to a constant weight and used to determine the proximate compositions. No significant correlations were detected between the RMR and the masses of water, lipid, or ash after controlling for the wet mass in any species. The scaling exponents for the masses of lipid and/or ash were significantly >1; however, lipid and ash represented only a small proportion of the body mass of all four fish species. These results suggest that the increasing proportion of lipid and ash only explains a limited part of the metabolic scaling, and factors other than inert components may have a greater contribution. There was no significant difference between the metabolic scaling exponent for the wet mass and dry mass in all four species, suggesting that using either the wet or dry body mass has only a limited effect on the metabolic scaling exponent.