Developmental changes in oxygen consumption and hypoxia tolerance in the heat and hypoxia-adapted tabasco line of the Nile tilapia Oreochromis niloticus, with a survey of the metabolic literature for the genus Oreochromis.

The genus Oreochromis is among the most popular of the tilapiine cichlid tribe for aquaculture. However, their temperature and hypoxia tolerance, if tested at all, is usually tested at temperatures of 20-25 degrees C, rather than at the considerably higher temperatures of 30-35 degrees C typical of tropical aquaculture. We hypothesized that both larvae and adults of the heat and hypoxia-adapted Tabasco-line of the Nile tilapia Oreochromis niloticus would be relatively hypoxia-tolerant. Oxygen consumption rate (MO2), Q(10) and aquatic surface respiration (ASR) was measured using closed respirometry at 2 (c. 0.2 g), 30 (c. 2-5 g), 105 c. (10-15 g) and 240 (c. 250 g) days of development, at 25 degrees C, 30 degrees C and 35 degrees C. MO2at 30 degrees C was inversely related to body mass: c. 90 mu M O-2 g(-1)/h in larvae down to c. 1 mu M O-2 g(-1)/h in young adults. Q(10) for MO2 was typical for fish over the range 25-35 degrees C of 1.5-2.0. ASR was exhibited by 50% of the fish at pO(2) of 15-50 mmHg in a temperature-dependent fashion. However, the largest adults showed notable ASR only when pO(2) fell to below 10 mmHg. Remarkably, p(crit) for MO2 was 12-17 mmHg at 25-30 degrees C and still only 20-25 mmHg across development at 35 degrees C. These values are among the lowest measured for teleost fishes. Noteworthy is that all fish maintain equilibrium, ventilated their gills and showed routine locomotor action for 10-20 min after MO2 ceased at near anoxia and when then returned to oxygenated waters, all fish survived, further indicating a remarkable hypoxic tolerance. Remarkably, data assembled for MO2 from >30 studies showed a > x2000 difference, which we attribute to calculation or conversion errors. Nonetheless, p(crit) was very low for all Oreochromis sp. and lowest in the heat and hypoxia-adapted Tabasco line.