Research Document - 1999/115

Developing an overall indicator for monitoring temporal variation in fish size at age and its application to cod (Gadus morhua) in the Northwest Atlantic, NAFO subdivision 3Ps.

By Y. Chen and L.G.S. Mello

Abstract

Size-at-age data are commonly available life-history information for a fishery. They are essential in formulating management policies for fish stocks. Because fish size-at-age values are likely to be influenced by many biotic and abiotic environmental variables, monitoring temporal variations in size-at-age can often provide fisheries managers with important information about status of fish stocks and help them identify the necessary changes in management policies. However, because of the multivariate nature of size-at-age data, it is often difficult to objectively evaluate temporal variations in size across all age groups. Commonly used single-age-based approaches ignore covariances among sizes of different age groups and may derive different results in evaluating temporal variations when different age groups are used. In this study, we propose a two-step approach to developing an indicator for monitoring temporal variation in fish size-at-age data. A robust approach, minimum volume ellipsoid analysis, is used to identify outliers in size-at-age data, and then a weighted principal component analysis is applied to the data with identified outliers down-weighted. An indicator is defined from the resultant principal components for monitoring temporal variations in size-at-age data. The proposed approach was applied to size-at-age data of cod (Gadus morhua) in the Northwest Atlantic, NAFO subdivision 3Ps. An overall size-at-age indicator was developed for the cod stock. Sizes for the pre-1980's year classes were found to be much higher than those for the post-1980 year classes.

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