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Research Document - 2012/079

Age Determination without Tears: Statistical Estimation of Silver Hake (Merluccius bilinearis) Age Composition on the Basis of Otolith Weight and Fish Length

By S.E. Campana and M. Fowler

Abstract

It is well documented that slow-growing fish produce otoliths that are heavier than fast-growing fish of the same size. As a result, the analysis of combined otolith weight and fish length measurements is better able to distinguish among age groups than is either variable on its own. Here the accuracy of a statistical model for estimating the proportions at age in an unknown sample is tested by taking advantage of the relationship between otolith weight, fish length, and age in a calibration sample. The accuracy of statistically-estimated age proportions in research vessel samples of silver hake (Merluccius bilinearis) was reasonably good, and on the order of that expected of inter-reader age variability. If a suitably large subsample of silver hake from a given fishery and sex were aged, and then used as a reference for estimating the age composition of the remaining fish of the same groups, the results are very likely to be acceptable to the stock assessment. Based on results to date, it also appears that recently-aged years could be used to estimate the age composition of an unknown recent year with acceptable accuracy. On the other hand, continuous use of the statistical method for estimating age composition on a routine annual basis (across many years) would be inadvisable without periodic calibration.

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