The Effects of Starvation, Exercise, and Exercise with Pre-Training on Aerobic Fuel Use in Juvenile Rainbow Trout (Oncorhynchus Mykiss Walbaum)

Abstract

Metabolic fuel use in rainbow trout (𝘖𝘯𝘤𝘰𝘳𝘩𝘺𝘯𝘤𝘩𝘶𝘴 𝘮𝘺𝘬𝘪𝘴𝘴 W.) was investigated using closed system respirometry and proximate body analysis. During short term starvation (15 days, routine activity) the utilization of protein as a substrate, as determined by respirometry, increased from 14 to 24% of total fuel supply. However, even by the end of the experiment, the contribution of protein (24%) did not approach the classically reported values for fish of between 40 and 90%. Indeed, from respirometry data, during the first quarter of the experiment lipid contributed the majority of the fuel ( >60%) while carbohydrate contributed about 20%. Thereafter, lipid and carbohydrate became essentially equivalent in importance (about 37% each). However, from proximate body analysis, a more traditional fuel mixture was found (protein, 58%; lipid, 40%; carbohydrate, 2%) suggesting the possibility that the two procedures were measuring fundamentally different parameters. Instantaneous fuel use during sustainable swimming at different speeds was investigated by respirometry using three day test periods. While protein catabolism remained constant over time, and uniform between groups, its relative contribution tended to increase with time as total M₀₂ declined with sustained swimming. Protein catabolism was highest in nonswimming fish (30-45%) and lowest in the high speed swimmers (20%); lipid was the most abundant (41-55%) fuel used in all groups at all times. In the nonswimmers and lowspeed swimmers, lipid use tended to increase slightly over time whereas in the high speed swimmers, lipid use dropped from 54 to 44%. Carbohydrate use (up to 38%) was higher than predicted by earlier literature, but decreased greatly in both the nonswimmers and low speed swimmers over the three days, whereas in the high speed swimmers the contribution increased with time. The low speed swimmers from the last set of experiments were used as controls for the final set of experiments in which another group of fish were trained for two weeks at 1.0 L·s⁻¹ prior to testing using an otherwise similar regime. Even though there was no difference in gas exchange, the make-up of the fuel mixture was different for the two groups. Protein use was significantly lower, while lipid use was higher in the trained fish. In addition, relative protein use in the trained fish was constant over the three day period, a feature found only in the the high speed swimmers of the previous experiment. A critical evaluation of the respiratory quotient is given since its use by fish physiologists has been without complete conversion from that used by the mammal physiologists. In addition, the often quoted term "fuel use" is differentiated into 𝘪𝘯𝘴𝘵𝘢𝘯𝘵𝘢𝘯𝘦𝘰𝘶𝘴 𝘧𝘶𝘦𝘭 𝘶𝘴𝘦 and 𝘤𝘰𝘮𝘱𝘰𝘴𝘪𝘵𝘪𝘰𝘯𝘢𝘭 𝘧𝘶𝘦𝘭 𝘶𝘴𝘦 since the two describe fundamentally different principles, though this has not always been recognized in the literature.