Energy partitioning was studied in the larvae and early juveniles of two species of freshwater fish with different life styles, Coregonus lavaretus and Esox lucius, by measuring simultaneously the rates of feeding, oxygen consumption and growth in long-term experiments. Comparison between the two species is based on measurements at 15 °C, but a group of C. lavaretus was also monitored at 10 °C.
The intensity of routine oxygen consumption was mass-independent in the smallest larvae, but above about 10 mg wet body mass (m) it scaled with an allometry of m−0.103 in C. lavaretus and m−0.154 in E. lucius. Routine, minimum and maximum rates of metabolism were higher in C. lavaretus than in E. lucius, whereas the mass-specific rate induced by feeding showed a quite different and more complex relationship to species.
The mass-specific rate of growth scaled with m−0.133 (r=0.54) in C. lavaretus but with m−0.24 (r=0.90) in E. lucius.
Comparison of five experimental groups, differing from each other with respect to species, type of prey (Artemia nauplii orRutilus rutilus larvae), experimental temperature and body size, suggested a general pattern of energy partitioning, with 62±3.7% of metabolizable food energy being allocated to growth, and 38±3.7% to metabolic expenditures.
The relationship between metabolic rate and growth rate, expressed as the ‘yield ratio’, mg dry body mass μmoll−1O2, the inverse of the net cost of growth, proved to be nearly invariant in all experimental groups, namely 0.043 mg μmol−1. However, two experiments with the smallest pike larvae led to higher growth rates per metabolic expenditure, suggesting that in E. lucius oxygen consumption may reach an aerobic maximum for growth at a routine rate of about 200–220 μmol−1O2g−1 dry body mass h−1.
Gross costs of growth include mass-dependent maintenance expenditures. Expressed in mass-independent units, E. lucius was more efficient in converting food energy into growth than was C. lavaretus; its superiority in this respect ranged from 7 to 17 % depending on body mass and diet.
The results are discussed with regard to the question of whether in small poikilothermic metazoans the allocation of metabolic energy follows the rules of addition or compensation. Furthermore, growth performance of the fish larvae is compared with that of microorganisms on the basis of the ‘yield ATP’ concept.