A bone to pick with age estimation using hard parts: A case study of swordfish, Xiphias gladius, in the southwest Pacific Ocean

Life history parameters of fish can vary spatially, and these differences have been used to identify separate stocks for management. However, inconsistency in the methods used to obtain the parameters is often overlooked as a possible explanation for the variability in parameters. Here, we sought to understand whether large differences in growth parameters obtained for broadbill swordfish (Xiphias gladius) in the southwest Pacific Ocean and central Pacific Ocean were due to methodological differences in estimating age, and to evaluate the most appropriate hard part for providing annual age estimates. Sectioned anal fin rays were obtained from an earlier study undertaken in the southwest Pacific (1995-2003) and the assumed annuli were re-counted for a subset, using criteria confirmed through an inter-laboratory calibration exercise. Sagittal otoliths were also obtained from the same study (in which otoliths had been collected but annual age had not been estimated) and the assumed annuli present in transversely sectioned otoliths were counted using new criteria for reading growth zones. Ray-based growth curves were different to the curves obtained in the original 1995-2003 study, confirming that methodological differences exist among studies that use fin rays in the Pacific. More importantly, a direct comparison of the new age estimates from fin rays and otoliths showed a clear bias; ray age was lower on average than otolith age for females > 7 years and males > 4 years. Considering that otoliths have been shown to be more reliable than alternative hard parts for estimating the age and longevity for many species, the results suggest that swordfish live longer and grow more slowly than previously estimated in the Pacific. A maximum age of 21 years was obtained from otoliths and 14 years from rays. We recommend that protocols are established to ensure that hard part reading methods are standardised among laboratories and that otolith-based age estimation is investigated and validated for all billfish stocks, and potentially other fish species.

Automated summary

This study investigates the differences in growth parameters of swordfish and assesses the most appropriate hard part for age estimation. The results reveal methodological differences in estimating age using fin rays in the Pacific, and a clear bias in age estimates between fin rays and otoliths. The study recommends standardizing hard part reading methods and investigating otolith-based age estimation for billfish stocks and other fish species.