Transgenic tilapia hybrids (Oreochromis mossambicus × O. hornorum) carrying a single copy of a homologous cDNA growth hormone exhibit higher growth rates than their wild-type conspecifics (Martinez et al. 1999). Swimming respirometry was employed to determine whether the increased growth rate was linked to elevated rates of maintenance metabolism in transgenics, and if this, in turn, influenced their scope to increase O₂ uptake and perform exercise or their ability to maintain O₂ uptake during progressive hypoxia.

Individual transgenics or wild-types, of similar mass and length (60.0 ± 10 g and 14.7 ± 0.9 cm vs. 75.8 ± 16 g and 15.7 ± 1.0 cm, respectively, n = 6) were exposed to stepwise increments in swimming speed (U), of 0.5 body lengths s^-1 (bl s^-1) every 30 min, in a Brett-type swimming respirometer at 25¡C. Oxygen uptake was measured at each speed until fatigue. Extrapolation of the exponential relationship between U and O₂ uptake back to a notional stationary baseline was taken as an estimate of maintenance metabolism. This was significantly higher in transgenic vs. wild-type fish (8.63 ± 0.86 vs. 5.39 ± 0.27 mmol O₂ kg^-1 h^-1, respectively). However, maximum O₂ uptake was also slightly higher in transgenics, such that aerobic scope was similar in both groups, and there was no difference in maximum sustainable U (5.2 ± 0.5 vs. 4.5 ± 0.7 bl s^-1 in transgenics vs. wild-types, respectively).

Following 2 h recovery from exercise, tilapia were exposed to progressive hypoxia (stepwise reductions in water O₂ partial pressures (P_O2) from 18.4 down to 1.3 kPa), and O₂ uptake measured at each P_O2. Despite higher routine rates of O₂ uptake in transgenics, the critical P_O2 at which O₂ uptake fell below the normoxic rate was similar in transgenics and wild-types (2.75 ± 0.30 vs. 4.16 ± 1.53 kPa, respectively).

The results indicate that stimulation of growth consequent to ectopic expression of growth hormone in transgenic tilapia (Martinez et al. 1999) is linked to increased rates of maintenance metabolism. The swimming and hypoxia experiments indicate, however, that the transgenics were able to compensate for this metabolic load to preserve maximum exercise performance and tolerance of reduced oxygen supply.

All data are reported as means ± S.E.M., means compared by t test, significance attributed at P < 0.05.

Martinez, R., Ramon, L., Acosta, J., Morales, A. & Estrada, M.P. (1999). Aquaculture 173, 271-283.