Inter-individual differences in rates of routine energy loss and growth in early juvenile Atlantic cod (Gadus morhua)

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Young juvenile cod, Gadus morhua, within the same cohort exhibit marked growth variability, the physiological mechanisms of which are poorly understood. We evaluated whether differences in routine energy loss by juvenile Atlantic cod contributed to observed growth differences. The rates of respiration (O2 consumption, CO2 production) and ammonia (TAN) excretion were measured for 64 individual juveniles of the same cohort within a narrow size range (25 to 40 mm standard length, LS, ages 79 to 112 days-post-hatch) growing at different rates. The relationships between body size and rates of O2 consumption, CO2 production, and TAN excretion were determined via regression analysis. Residuals from some of these regressions were significantly related to growth rate such that faster-growing juveniles had relatively lower rates of routine energy loss. Fish condition factor, weight (LS)-3, also significantly decreased with increasing growth rate. Results of this study indicated differences in both the rates of routine energy loss and the patterns of growth allocation among individuals within the same cohort. Since these physiological attributes were positively correlated with growth rate, they may be indicative of survivors in field populations.