Aerobic Maximum for Growth in the Larvae and Juveniles of a Cyprinid Fish, Rutilus rutilus (L.): Implications for Energy Budgeting in Small Poikilotherms

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Abstract
In this paper we address the problem of how young fish meet the metabolic costs connected with fast growth (up to 30% day-1 in juvenile roach). From the literature it appears that, due to the high costs of maintenance and activity, the energy budgets of larvae and juveniles growing at these rates should be very tight indeed, but no quantitative information is available on this. We conducted experiment at 15 and 20.degree.C using a respirometer equipped with an automatic infusion system allowing the intermittent presentation of Artemia nauplii to larvae and juveniles of roach, Rutilus rutilus (L.). The rates of food consumption, oxygen consumption and growth could then be determined simultaneously on fish ranging in weight from 15 to 700 mg. Feeding was always ad libitum, but by using fish of different size and by varying the length of the fasting period preceding the experiment, growth rates ranging from -2.4 to +16% day-1 at 15.degree.C, and from -0.8 to +19% day-1 at 20.degree.C were achieved inside the respirometer in experiments lasting from 4 to 5 days. Upon presentation of food the rate of oxygen consumption of the fish increased within 1 h to a maximum, made up of food-induced thermogenesis (FIT), feeding activity (FA) and standard metabolism (SMR). The metabolic cost of FA in .mu.mol O2 g-1 h-1 amounts to 1.9 .+-. 0.8 at 15.degree.C and 2.5 .+-. 1.3 at 20.degree.C, or less than 10% of the total metabolic rate during feeding. It takes 7-8 h for the rate of metabolism to return to the pre-feeding level. The measurement of food consumption allowed calculation of the coefficients of assimilation and of feeding-induced thermogenesis (equivalent to the ''SDA-coefficient''). Both coefficients are in the range reported for larger fish. The metabolic cost of growth can be determined by plotting the caloric equivalent of FIT + FA (the metabolic scope for growth) against the caloric value of somatic production during the experiment. Proportionality between the two variables holds only up to a rate of production of about 10 J g-1 h-1 (equivalent to a growth rate of about 6% day-1) at 15.degree.C and 13 J g-1 h-1 (= 8% day-1) at 20.degree.C. At higher growth rates there is no further increase in the rate of oxygen consumption. Thus an aerobic maximum for growth is defined, the energy equivalents of which are 5.3 and 7.7 J g-1 h-1 at 15 and 20.degree.C respectively. In the range of proportionality the cost of growth is almost independent of temperature, varying between 0.36 and 0.43 JJ-1; i.e. the synthesis of 1 J of body substance requires the expenditure of 0.36-0.43 J of metabolic energy. The metabolic energy for growth rates exceeding 8% day-1 can only be obtained in two ways: either by suppressing other energy-consuming functions (usually included in the term ''maintenance'') or by increasing the efficiency of the conversion of food energy into body proteins. The parameters of the energy budget of small roach feeding in total darkness are not different from those feeding in the light.

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