Corroborating the ages of walleye pollock (Theragra chalcogramma)

Abstract

Fish ageing researchers have long recognised the importance of validating age-reading methodologies. The strongest age validations require the acquisition of ageing structures from fish of known-ages, or specimens whose ages are appropriate for bomb carbon validation. Often such specimens are extremely difficult or impossible to acquire so researchers have sought alternatives to validation. The alternative to age validation is age corroboration. Corroboration of a fish ageing method occurs when fish ages are found to be consistent with some ancillary information when comparisons are made in an unbiased manner. The question pursued in this study is how desirable are such comparisons from a scientific viewpoint. Information is presented that corroborates otolith ages for walleye pollock (Theragra chalcogramma), one of the largest groundfish fisheries in the world. Walleye pollock ages were corroborated using marginal increment analysis, ages following the strong 1978 year class in the eastern Bering Sea, and a comparison of ages read from otoliths with ages read from vertebrae. A new statistical method is suggested for comparing otolith and vertebra age readings. The walleye pollock example demonstrated that corroborating evidence can improve confidence in fish ages and ageing techniques.