The mass of all major body constituents, with the exception of the
inorganic compounds, must, by definition, decrease with a decline in con-
dition, but it is the relative pattern of decline of each constituent that is
important. The lipid content (predominantly the triglyceride groups) declines
rapidly with decreasing condition (Figure 4) indicating a preferential mobil-
ization of triglyceride lipids during routine catabolism in fish of good con-
dition. When compared with protein mobilization, the initial preferential
utilization of storage fats as a catabolic fuel becomes even more evident
(Figure 5). Tilapia in good condition derive nearly 3 times as much metabolic
energy from fat reserves than from protein while fish in poor condition come
to rely almost exclusively on protein as a catabolic fuel when triglyceride
lipid has declined to a level which may be critical or almost non-existent
(Figure 4). Since lipid is a high energy compound containing 1.7 times more
energy per unit mass than protein, it follows that as the fish become more
and more reliant on a protein source for catabolism, so the rate of decline in
mass or condition accelerates. This may be one of the reasons why a decline
in condition so often appears to lag behind an expected decrease due either
to a predicted environmental stress or to some expected physiological stress
at the end of a period of good growth.
The preferential mobilization of either lipid or protein as the major
metabolic energy source has little effect on oxygen consumption since 13.72
J of energy is liberated from lipid for every mg of oxygen utilized while
only marginally less energy, 13.39 J, is liberated per mg of oxygen if protein
is the prime energy source. Thus, although fish that show rapidly declining
condition will be largely utilizing somatic protein as a catabolic fuel, oxygen
consumption basically remains a function of the mass of tissue and only in
extremely accurate work is the proportionate composition of the fish
important.
Fiure 4. Semiquantitative analysis of the various lipid fractions from Tilapia rendalli of
varying condition (a = triglyceride, b = phospholipid, kn = relative condition factor).