FISH NUTRITION
473
any weight change." Earlier work on maintenance has brought out
the difficulty in accurate measurement of it because the fish adapt
to different levels of feeding (Brown, 1957). However, Brett et a,?.
showed that when stable growth rates have been obtained accurate
measurement of maintenance level by interpolation is possible.
Maintenance levels are of interest in eco-systems in assessing the
efficiency with which fish can subsist during those periods when food
supply is inadequate. However, the food requirements of fish which
are neither gaining nor losing weight are commonly equated with the
theoretical requirements for maintenance of a growing fish.
Thus, the equation, net efficiency of food conversion =
Growth
Food intake-maintenance ration
is said to express " the efficiency with which an animal utilizes for
growth that food consumed over and above the amount it would
require to just maintain its tissues '' (Warren and Davis, 1967). The
difficulty for the present authors is that the most complete data (that
of Brett and his colleagues) illustrate that the non-growing fish differs
in chemical composition from the growing fish ; so it appears that the
non-growing animal is not maintaining its tissues by comparison with
the growing animal. This is amply illustrated by Brett's data. At
16"C, for example, fish fed to excess had a body composition of 71.3%
water, 19.7% protein and 743% fat ; those receiving a ration of 1.87%
dry body weightlday had 76.7% water, 16% protein and 4 4 % fat.
Now the maintenance level of these fish at 16' on the diet in question
is about 1 % dry body weight/day. Thus feeding fish on a ration which
maintains body weight constant results in significant alterations to the
body composition of the fish. I n the growing animal (fully fed) tissues
are likely to be replete whereas they may not be in the non-growing
animal ; depleted cells may have t o be replenished with protein before
increase in cell size or numbers can occur. The relative deficit of protein
in the non-growing fish may sigmfy differential changes in enzyme
levels within tissues which may alter the emphasis on particular metabolic pathways. So the maintenance ration, as presently measurable,
represents not that of a growing fish, but that of a fish with (in the
case of sockeye salmon) enhanced water and lowered fat and protein
contents. It seems that the maintenance requirement of the one may
differ from that of the other and it is tenuous to equate the food
required to maintain a fish at constant weight with the portion of its
food which a growing fish uses for basal metabolism, tissue wear-andtear, and other forms of maintenance.
473
any weight change." Earlier work on maintenance has brought out
the difficulty in accurate measurement of it because the fish adapt
to different levels of feeding (Brown, 1957). However, Brett et a,?.
showed that when stable growth rates have been obtained accurate
measurement of maintenance level by interpolation is possible.
Maintenance levels are of interest in eco-systems in assessing the
efficiency with which fish can subsist during those periods when food
supply is inadequate. However, the food requirements of fish which
are neither gaining nor losing weight are commonly equated with the
theoretical requirements for maintenance of a growing fish.
Thus, the equation, net efficiency of food conversion =
Growth
Food intake-maintenance ration
is said to express " the efficiency with which an animal utilizes for
growth that food consumed over and above the amount it would
require to just maintain its tissues '' (Warren and Davis, 1967). The
difficulty for the present authors is that the most complete data (that
of Brett and his colleagues) illustrate that the non-growing fish differs
in chemical composition from the growing fish ; so it appears that the
non-growing animal is not maintaining its tissues by comparison with
the growing animal. This is amply illustrated by Brett's data. At
16"C, for example, fish fed to excess had a body composition of 71.3%
water, 19.7% protein and 743% fat ; those receiving a ration of 1.87%
dry body weightlday had 76.7% water, 16% protein and 4 4 % fat.
Now the maintenance level of these fish at 16' on the diet in question
is about 1 % dry body weight/day. Thus feeding fish on a ration which
maintains body weight constant results in significant alterations to the
body composition of the fish. I n the growing animal (fully fed) tissues
are likely to be replete whereas they may not be in the non-growing
animal ; depleted cells may have t o be replenished with protein before
increase in cell size or numbers can occur. The relative deficit of protein
in the non-growing fish may sigmfy differential changes in enzyme
levels within tissues which may alter the emphasis on particular metabolic pathways. So the maintenance ration, as presently measurable,
represents not that of a growing fish, but that of a fish with (in the
case of sockeye salmon) enhanced water and lowered fat and protein
contents. It seems that the maintenance requirement of the one may
differ from that of the other and it is tenuous to equate the food
required to maintain a fish at constant weight with the portion of its
food which a growing fish uses for basal metabolism, tissue wear-andtear, and other forms of maintenance.