Validation of aging and length back-calculation in rainbow trout, Salmo gairdneri Rich., from Dee Lagoon, Tasmania

Abstract

Fin-clipped S. gairdneri yearlings, derived from wild stock, were released into Dee Lagoon, central Tasmania, in 1978 and 1979. A further release of yearlings without fin-clips was made in 1980. Recaptures from 1979 to 1983 allowed validation of scale aging and back-calculation of length from collected scale samples. Back-calculation of size at release, Lr was possible due to formation of a distinct scale check associated with liberation. All back-calculated estimates of Lr and the lengths at the third and fourth winters, L3 and L4, agreed within 5% of actual values. Difficulty in identifying the location of the second winter check initially led to consistent underestimates of L2 but did not affect the accuracy of scale aging. Retention of only precise observations of the second check in the calculations increased the estimate of L2 to agreement with real values. The importance of validation of aging techniques is stressed.