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, L_r was possible due to formation of a distinct scale check associated with liberation. All back-calculated estimates of L_r 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.