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.
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