Age estimation of roundnose grenadier (Coryphaenoides rupestris), effects of uncertainties on ages

Abstract

For the roundnose grenadier (Coryphaenoides rupestris), one of the larger macrourid species exploited at the upper and mid-slope depth in the North Atlantic, the reading scheme of the otoliths is quite easily agreed upon by most workers, and the readings are in acceptable agreement with age validation formerly obtained on the juveniles. In the North East Atlantic, the roundnose grenadier fishery operates over large areas and does not target concentrations of pure adult fish, unlike some other deep-water fisheries. The changes in the age compositions of the exploited populations are likely to appear in the catch and changes in the length distribution of the landings have appeared since the onset of fishing. In the present paper the growth of the roundnose grenadier from the west of British Isles is investigated from age readings of otoliths collected in 1996 and 1999. Age readings are made from 923 otoliths and compared to pre anal fin length (PAFL) measurements. The observed growth is adjusted to the Von Bertalanffy growth model (VBGM) and the difference in the estimated growth parameters according to the method used to fit the model is explored. The sexual dimorphism in growth is investigated and the implications of the uncertainties in growth and age estimations for the assessment and for ecological studies are discussed. The results show that age uncertainties cannot be disregarded and the usefulness of such parameters for assessment purposes is discussed. The relationships between length, age and otolith weight were analysed and the otolith weight appeared as a possible alternative solution to estimate the age of the roundnose grenadier.