The cost of growth in young fish larvae, a review of new hypotheses

Abstract

Rapid growth is of paramount importance for young fish larvae, as mortality due to predation declines rapidly with increasing size of the larvae. Therefore, recent studies have focused on the rates of growth in larvae of freshwater and marine fish species. High rates of growth mean high rates of energy expenditure for synthesis of body components, and as energy may be limiting in the youngest life stages, the cost of growth has been a central issue. In juvenile and adult fish, the cost of growth represents approximately 40% of the energy content of the newly synthesised tissue, and with growth rates of up to about 25% d −1 the energy demands for growth represent a large fraction of the energy available. As growth is energetically costly, it has been an axiom that there is a positive relationship between the rate of growth and the rate of energy expenditure (above maintenance level). However, there is accumulating evidence from studies of larval Rutilus rutilus, Chondrostoma nasus, Coregonus wartmanni, Oncorhynchus tshawytscha, Esox lucius and Clupea harengus that the rates of metabolism and of growth are not correlated in the youngest stages growing at the highest rates, i.e., at very high growth rates, larval-weight specific metabolism does not increase in proportion to the rate of growth. Two recent hypotheses dealing with this apparent decoupling of the rates of growth and metabolism are discussed: (1) a possible decrease in the cost of synthesis of a unit of protein with increasing rates of growth or (2) a change in the energy allocation between growth and other energy demanding processes, so that energy is switched from maintenance functions to growth processes. Further, in order to channel as much energy as possible into growth, it has been hypothesised that protein degradation in larval fish may be low. A low rate of protein degradation means a high rate of retention of synthesised proteins and this would reduce the cost of growing. The few data on protein degradation in larval fish are conflicting but none support the hypothesis that it is low. Only the first of the two hypotheses concerning the decoupling of the rates of growth and of metabolism can be experimentally tested, namely the hypothesis dealing with a possible variability in the costs of protein synthesis. An appropriate experimental procedure for testing this hypothesis is forwarded.