The effects of temperature and size on the growth, energy budget and waste outputs of barramundi (Lates calcarifer)

The effects of temperature and size on the growth, energy budget and waste output were examined in small (21 g) and large (142 g) juvenile barramundi (Lates calcarifer) grown for 26 days at one of six temperatures (23 °C, 26 °C, 29 °C, 32 °C, 35 °C and 38 °C). Fish were fed once a day and faeces was collected by stripping at the end of the growth trial to measure the apparent digestibility of dry matter, energy, nitrogen and phosphorous. Survival was not affected by temperature and was high (87–100%) in all treatments. The influence of size (W) on the daily growth rate (DGR, g fish− 1 d− 1) was modelled with the power function DGR = αWγ, where α and γ are constants fitted with regressions to incorporate the effect of temperature into the model. Following a parabolic response to temperature, DGR and feed intake were high in both size classes across a temperature range extending from 29 °C to 35 °C. Temperatures for maximum growth and feed intake lay within the thermal zone for optimal feed efficiency from 26 °C to 35 °C. Feed efficiency was higher in small fish than in large fish. Although size had no effect on nutrient digestibility, temperatures influenced the apparent digestibility of dry matter, energy and phosphorous. The partitioning of consumed energy between growth, heat loss, nitrogen excretion and faeces showed increased energy losses through nitrogen excretion at 38 °C. Size and temperature had significant effects on waste outputs in the form of solid, nitrogen and phosphorous waste. In both size classes lowest levels of waste output were seen from 29 °C to 35 °C with levels rising at the cooler temperatures. The energetics and waste output of juvenile barramundi raised at different temperatures are discussed with respect to temperature dependent nutrient digestibility, feed intake and protein turnover.