Abstract
The basis of the widely used Fraser‐Lee equation for back‐calculating lengths is discussed and a new algebraic method of calculating the constant is given. This algebraic method gave satisfactory results for the back‐calculation of European perch (Perca fluviatilis L.) lengths in Lake Mahinerangi, New Zealand. Errors in back‐calculation are much higher than regression techniques would suggest. Causes of the Rosa Lee phenomenon are discussed and methods are given for detecting it and compensating for it using a computer program, BACKCALC.