HOW RELIABLE ARE GROWTH BACK-CALCULATIONS BASED ON OTOLITHS

Growth back-calculations from otoliths assume that the relationship between fish and otolith length is linear through time. However, recent studies have demonstrated that the fish/otolith size ratio varies systematically with somatic growth rate, resulting in relatively large otoliths in slow-growing fish. Such a growth effect will result in a fitted fish-otolith regression with differs significantly from that of the mean of the individual fish-otolith slopes. Fraser-Lee growth back-calculations made from such a regression consistently underestimate previous lengths at age. Back-calculation bias was eliminated through use of an algorithm defining individual fish-otolith trajectories and a biologically determined, rather than a statistically estimated, intercept. Adaptations of the biological intercept back-calculations procedure accurately predicted previous lengths in the presence of both stochastic error and time-varying growth rates. When used to reevaluate some published back-calculations, the biological intercept procedure resulted in more accurate values than those previously estimated, and reduced or eliminated the presence of Lee's phenomenon. -from Author