The Functional Relationship between Body Size and Growth Rate in Fish

The relationship between specific growth rate (G w ) and body size (usually weight, Wg) in ectotherms is often described by a power function : G w = a / W b (usually fitted by linear regression of In G w on ln W). A constant value of the weight exponent b is a useful feature because it facilitates the adjustment of size variations so that comparisons can be made between samples in which weights are dissimilar. Opinions differ, however, on the constancy of b. 2. A new growth model for Brown Trout, Salmo trutta, (Elliott, Hurley & Fryer 1995) is used to show that spurious values of b can be obtained when different estimates of W are used in the power function. The error in estimating both b and a increases with the length of the period over which growth is measured and is greatest when growth rates are highest. The analyses show how it can be concluded erroneously that b is not constant. A new equation for data analysis is proposed to overcome this problem when a suitable growth model is not available.