Food and Growth of Fishes. II. Effects of Food and Temperature on the Relation Between Metabolism and Body Weight

In an earlier paper we described the growth of fishes, ΔW/Δt, in relation to the experimentally measurable variables, body weight (W), food intake (R), and total metabolism (T). Here we review experimental evidence of the nature of the relation between T and W, and its dependence on R and temperature. Making use of the basic energy equation, pR = T + ΔW/Δt, where p is the term for correction from injested to utilizable energy, we calculate T as the difference between the energy equivalents of R and ΔW/Δt, for comparison with results of oxygen consumption studies. Application to a number of published experimental results suggests that with constant food availability, this index of total metabolism, T, derived from feeding experiments, shows the same rate of change with body weight, W, as has been found by oxygen consumption studies under standard conditions. That is, the two sources of data provide estimates of a common γ in the relation[Formula: see text]where α and γ are the fitted parameters for the curve.When fish are fed on a "maintenance" diet, the value of α calculated from the food-growth difference (the growth change is rarely nil in a given experimental observation period), appears to correspond with that characterizing the "routine" metabolic level in oxygen consumption studies. Higher α levels result from higher levels of food availability, and at ad libitum feeding α appears to approach the levels known in oxygen consumption studies as "active" metabolic levels. Temperature effects in the experiments were estimated from multiple regression analyses and showed an elevation of α with increasing temperature. The long-term effect of temperature on α was comparable with that predicted by the Krogh correction at ad libitum feeding, but was significantly lower when food was limited, as at "maintenance" feeding.From a survey of effects of different designs of feeding experiments on these metabolic parameters, it appeared that apparently aberrant values of the weight exponent γ may instead be mistaken interpretations of changes in the level of metabolism α. That is, within the limiting conditions of standard or active metabolism, changes in temperature during experiments or manipulations of the availability of food by the experimenter, sometimes unintentionally, elicit adaptative responses in the level of metabolism, α. These show up in the results as effects on γ when the changes in conditions are gradual, hence confounded with body-size changes during growth.The ability to make distinctions between effects of various factors on these two metabolic parameters appears to depend upon a distinction between experiments conducted with a view to learning what fish do under particular circumstances, and experiments designed to explore what fish are capable of doing. The former type reveal a remarkable conservatism in the basic relation between metabolism and body size, γ. Results from the latter reflect possibilities of metabolic adaptation to environmental circumstances. The apparent predictability of the response of the total metabolism to various conditions of food energy supply and dissipation suggests that the remainder of the energy system, represented by the growth, may be similarly predictable. If this is true outside the laboratory, measurements of the metabolic parameters, α and γ, already familiar in physiological and behavioural research, could be directly used as indices of the (relative) positions of various sizes and species of fish in natural production systems.