Validation of roughhead grenadier (Macrourus berglax) otolith reading

Abstract

Roughhead grenadier Macrourus berglax Lacépède 1802, is becoming an important commercial fish in the north-west Atlantic fisheries and reliable age information is needed for its assessment. Macrourus berglax has been aged by counting otolith growth zones assumed to represent anulli. Three validation methods were applied: back-calculation of length-at-age, length–frequency analysis, and analyses of the progression of the length mode of an exceptionally large year class. Results on size-at-age estimated by otolith growth zone counts, length–frequency analysis of pre-anal fin length and modal progression were internally consistent, but estimates by back-calculation were different. This latter method produces unrealistic growth curves for each sex due to the weak relationship between otolith size as measured and pre-anal fin length. Up to age 8–9 the size-at-age were the same for males and females; thereafter males were smaller than females. Male growth rate declined when pre-anal fin length reached 18 cm at around age 9, whereas the growth of females started to decline at about 30 cm at around age 16. The Von Bertalanffy growth model derived from otolith analysis was similar to that obtained by the Multifan length frequency analysis model, and gave realistic values of Linf, close to the maximum anal fin lengths found in catches. The results showed that Macrourus berglax has a prolonged life cycle and differences in growth between males and females.