291 Fish Physiology and Biochemistry 16: 291-302, 1997. © 1997 Kluwer Academic Publishers. Printed in the Netherlands. Fast growth, protein turnover and costs of protein metabolism in yolk-sac larvae of the African catfish ( Clarias gariepinus ) L.E.C. Conceição 1 , D.F. Houlihan 2 and J.A.J. Verreth 1 1 Department of Fish Culture and Fisheries, Wageningen Agricultural University, P.O.Box 338, 6700 AH Wageningen, The Netherlands; 2 Department of Zoology, University of Aberdeen, Tillydrone Ave., Aberdeen AB9 2TN, Scotland UK Accepted: October 31, 1996 Keywords: larval fish, protein synthesis, protein turnover, protein synthesis costs, energetics Abstract In fish larvae the costs of rapid growth may be accommodated by a decrease in the rate of protein turnover or by a reduction in the costs of protein synthesis. Protein growth, synthesis and degradation were measured in yolk-sac larvae of Clarias gariepinus and the costs of protein synthesis and protein growth were estimated. Growth rates were over 100% protein weight day –1 . Protein synthesis retention efficiency (retained protein per unit of synthesis) was estimated to be 69.6%, a value comparable to that of larger fish. The larvae used 43% of their oxygen consumption for protein synthesis. Nevertheless, protein synthesis costs were close to theoretical minima. Therefore, the high growth rates of catfish yolk- sac larvae seem to be possible through minimisation of the costs of protein synthesis. These low costs are associated with high rates of protein synthesis (138%protein weight day –1 ), and elevated RNA con- centrations (107 μg RNA mg –1 protein), which together suggest very high RNA efficiencies (12.9 g protein synthesized g –1 RNA day –1 ). 1991), allows the comparison of turnover rates of animals with widely different protein growth rates. Fish larvae may grow at rates of 20% protein weight day –1 or more (Wieser et al. 1988b; Hou- lihan et al. 1995a), but as fish larvae have a lim- ited capacity for oxygen uptake the metabolic scope for growth is limited (Wieser et al. 1988a). In comparison to larger fish, larvae require spe- cific energetic solutions to accommodate the costs of their high growth: Kiørboe et al. (1987) and Kiørboe (1989) suggested that fast growth was possible because larval fish may have a re- duced protein turnover in comparison to juvenile and adult fish. Houlihan et al. (1995a) challenged this view because larval nase Chondrostoma nasus (Houlihan et al. 1992) and larval herring Clupea harengus (Houlihan et al. 1995b) were found to have levels of protein turnover (as meas- ured by the protein synthesis retention effi- Introduction Growth is primarily protein deposition, i.e., the increase in protein mass in a given time interval, and growth is an energetically expensive process. Jørgensen (1988) estimated the costs of growth in fast growing juvenile fish to be around 87% of their total energy expenditure. The costs of growth are related to the costs of protein deposi- tion (Jobling 1985; Brown and Cameron 1991; Houlihan 1991) with protein growth being the net result of protein synthesis and protein degrada- tion. The costs of protein turnover include the costs of both protein synthesis and protein degra- dation, and turnover of proteins may account for 11 to 25% of the basal metabolic rate (Reeds et al. 1985; Hawkins et al. 1989; Waterlow and Millward 1989). Protein synthesis retention effi- ciency, defined as the quotient between the rates of protein growth and protein synthesis (Houlihan