Relative condition factorFigure 5. The energy production ratio of fats and proteins during change in relative con-
dition factor for Tilapia rendalli (after Caulton 1978b).
The problem of accurately predicting the tissue mass of a live fish prior to
respirometry is difficult because the relationship between tissue mass, water
content and condition is continually changing. The proportionate relationship
between tissue (lipid and/or protein) and water is seldom a simple inverse
linear function, as so often suggested, but changes with changing condition.
The ratio of water and tissue in T. rendalli, for example, is lowest in fish of
good condition (Figure 6), highest in medium condition fish and marginally
declines in fish of poor condition. This complicating factor will certainly
have a small but important effect on oxygen consumption when it is related
to the fresh mass of the fish and thus for precise measurement of metabolism,
oxygen uptake or the metabolic demand should be related rather to the dry
tissue mass or the energy content of the fish and not simply the fresh mass
unless the fish are all in a similar condition.
The information already discussed can be correlated and cross-referenced
to metabolism: an exercise that gives confidence to some of the results
discussed. For example, a morphometrically similar group of young T.
rendalli with an average mass of 42.63 g and having an average condition of
2.23 were maintained without food for a period of ten days at a constant
temperature of 18" C. The average mass loss over this period was measured as
2,020 mg per individual. From our knowledge of oxygen consumption and
energy utilization by T. rendalli (Figures 3 and 5), it is calculated that over