The cost of growth in young fish larvae, a review of new hypotheses.
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, the cost of growth has been a central issue. In juvenile and adult fish, the cost of growth represents circa 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. Recent hypotheses dealing with this apparent decoupling of the rates of growth and metabolism are discussed, i.e. a possible variability in the cost of protein synthesis or a change in the energy allocation between growth and other energy demanding processes. Further, in order to channel as much energy as possible into growth it has been hypothesised that the protein turnover in larval fish may be low. The few data on protein turnover in larval fish are conflicting but none seem to support the hypothesis that turnover is low.
The RNA concentration has often been used as a biochemical correlate of growth in fish. However, recent studies on larvae and juvenile fish have questioned the validity of this approach during the early life stages, and these results are discussed.
(Marine Biological Laboratory, University of Copenhagen, Strandpromenaden 5, DK-3000 Helsingør, Denmark)
