irrespective of the amount of ingested energy. As previously shown from
results of experiments to determine assimilation efficiency, the amount of
energy liberated per unit mass or energy of food ingested increases with
increasing temperature thus it may not be surprising that the processing costs
of food preparation increase proportionately. The increase in food processing
costs with the increasing temperature can, at warm temperatures, seriously
deplete the favorable gain in energy due to the greater assimilation efficiency
at those temperatures. For example, a fish that consumed 1 kJ of food at
18°C would have available 478 J of postassimilatory energy from which
a further 20 J of energy must be deducted for processing costs leaving
approximately 458 J of free energy available for metabolism and growth.
Likewise, a fish at 30°C would have 582 J, less a calorigenic cost of 109 J, or
473 J of energy available. The difference in assimilatory efficiency between
fish feeding at 18" C and those feeding at 30" C results in a gain of some 104
J, but when the cost of food processing is considered this difference is
reduced to an almost insignificant 15 J. This clearly demonstrates just how
energy demanding food processing is at higher temperatures. This example
also serves to illustrate that digestibility alone, as a comparative measure
used to forecast growth potential attributable to various diets, may not
always be a suitable measure upless the associated food processing costs are
also considered. This is especially important to the users of pelleted rations
and fish farmers should not be misled by manufacturers' claims of good
digestibility and consequently optimistic growth forecasts, as deduced from
digestibility trials alone.
Energy of food ingested (kJ)
Figure 9. The effect of temperature on the calorigenic cost of food processing shown by
Tilapia rendalli feeding on Ceratophyllurn dernersum (n = 39, after Caulton 1978a).