Potential use of otolith weight for the determination of age-structure of Baltic cod ( Gadus morhua) and plaice ( Pleuronectes platessa)

Abstract

Stock assessment models used in fisheries management require an accurate estimation of the age-structure of the fish populations. This is often derived by ageing a number of individuals (counting rings in sectioned otoliths) and estimating the relative percentage per age class. In this paper, we investigated if a strong relationship between otolith weight and fish age could exist in two important commercial species: plaice ( Pleuronectes platessa) and Baltic cod ( Gadus morhua). A highly significant relationship was shown between otolith weight and fish age in both species and otolith weight was independent of fish length. The latter has important implications because it means that otolith weight is a much stronger predictor than fish length and it can be used to estimate fish age. The relative age-structures estimated using otolith weight were not significantly different from those estimated by traditional annuli counting in both species. For each new sample of fish, the linear regression between otolith weight and fish age needs to be confirmed in order to use the same parameters for the estimation of fish age using the otolith weight. However, the necessary recalibration was easily performed using a 100-individuals random sample. Therefore, we suggest that otolith weight could represent a valuable criteria for an age determination technique that is objective, economic and easy to perform compared to traditional methods that have been defined “as much an art as a science”.