Growth of swimming muscles and its metabolic cost in larvae of whitefish at different temperatures

The temperature relationship of routine metabolic rate (Rr) of non-feeding, non-growing Coregonus lavaretus larvae between 2 and 15) C is characterized by Q10-values ranging from 1·8–2·45. The rate of growth, based on weight determinations, of first-feeding larvae amounted to 3·5, 7·6 and 9·4 % day"1 at 5, 10 and 12) C respectively, from which Q10-values between 4·0 and 4·8 can be calculated. The rate of increase of muscle mass between 5 and 10) C, based on the determination of the cross-sectional area of inner muscle fibres, resulted in a Q10-value of 4·5. Water temperature influenced the pattern of growth of the inner muscle fibres. At hatching, after 360 day degrees, total muscle mass of larvae reared at 4 and 8) C was independent of temperature, but at 4) C the rate of mass increase owed more to hyperplasia (increase in fibre number) than to hypertrophy (increase in fibre mass), whereas at 8) C the opposite was the case. The calculation of power budgets (including the metabolic cost of growth) of first-feeding larvae yielded net conversion eYciencies (K2) increasing with temper-ature from 46·3 % at 5) C to 54·7 % at 12) C. Comparing our data with literature data two general conclusions can be drawn. (1) In first-feeding larvae the net, but not the gross, conversion eYciency of food energy increases with temperature. This is due to net energy input being characterized by a much higher Q10-value than energy expenditures. (2) In embryos of freshwater fish so far investigated hyperplasia plays a greater role in the increase of fibre mass than hypertrophy at the lower temperature, whereas in embryos of marine fish hyperplasia prevails at the higher temperature. It is suggested that this discrepancy correlates with the high concentration of free amino acids in the eggs of marine species which provide an additional, easily available, source of metabolic energy absent in freshwater species