Most studies of metabolic rates and energy requirements in aquatic animals have been conducted using indirect methods wherein heat production estimates were based on O2 consumed and CO2 produced. The purpose of the present study was to determine the feasibility of direct calorimetry as a method for measuring heat production of fish and to determine the effect of environmental temperature and body size on the metabolic rate of several species. An adiabatic bomb calorimeter was modified to hold living fish and to measure small increments of heat. Heat production of rainbow, brook and lake trout, and Atlantic salmon weighting 1 to 4 g each was measured at temperatures ranging from 3° to 18°. Heat production of rainbow trout having individual body weights from 0.85 to 57.0 g was measured at 15°. Direct calorimetry was found to be a satisfactory method for measuring heat production of fish. Tests were reproducible and could be completed in 6 to 8 hours. The heat production changed linearly with temperature over the range tested. Regression equations for heat production for fish weighing 1 to 4 g were: for rainbow trout, HP = 0.59 + 0.0525T; for Atlantic salmon, HP = 0.66 + 0.0339T; for brook trout, HP = 0.37 + 0.0770T; and for lake trout, HP = 0.41 + 0.0680T where HP is kcal/kg/hr and T is temperature. The effect of size on metabolic rate was linear from 1 to 4 g and proportional to W0.63 from 4 to 57 g.
