Developmental and Metabolic Constraints of the Scope for Activity in Young Rainbow Trout (Salmo Gairdneri)

The relationship between routine or ‘steady’ rate (Vo₂s.r.) and maximum rate of metabolism after forced bursts of activity (V^·o₂max) was studied in the fry and young (weight range 0.08-7 g) of Salmo gairdneri at 4 and 12°C.

The relationship between metabolic rate (y) and body weight (x) can be described by the function y = ax^b. The mass exponent b equals 0.96 and 0.93 for the routine rate, and 1.11 and 1.14 for the maximum rate of metabolism at 4 and 12°C respectively, the two sets of b-values differing significantly from each other.

The difference between the routine and the active rate of metabolism is called the relative scope for activity. Expressed as a percentage of the routine rate it increases from 85–166% in the sac fry, to approximately 400% in fish weighing 10g.

Within each of four size classes there is a linear relationship between V^·o₂s.r. and V^·o₂max, the slope of the relationship always being greater than one and increasing with the weight of the fish from 1.1 in the sac fry to 5.3 in the 3–7g size class. That is, in the larger size class each increase in the steady rate of oxygen consumption by one unit adds 5.3 units to the aerobic scope for activity.

It is speculated that routine and active rate of metabolism are related by a constant factor of expansion so that, up to a limiting value, an increase of the former will lead to an increase of the amount of oxygen available to each fish for additional locomotor activity.