Otoliths and vertebrae as potential hard structures for ageing South African yellowfin tuna Thunnus albacares

Abstract

Sagittal otoliths and vertebrae from 46 yellowfin tuna Thunnus albacares (118-175 cm fork length) collected off the west coast of South Africa were assessed for potential usefulness in age estimation. Growth increments were found on both structures. Comparisons of growth increment counts showed that agreement in the counts of the number of growth increments was low (33-44%) but that most (90-94%) agreed to within one growth increment. Growth increment counts from whole otoliths, sectioned otoliths and vertebrae were not significantly different (f-test, p < 0.05). Among-reader precision was high for both whole otoliths (average percent error [APE] = 9.8%) and vertebrae (APE = 12.1%), but low for sectioned otoliths (APE = 34.9%). Age-bias plots indicated non-linear bias when sectioned otoliths were assessed in relation to other structures. Whole otoliths and whole vertebrae were considered equally precise, but the practicality of collecting vertebrae from the commercial fishery was better. Vertebrae are recommended as the most suitable structure for ageing yellowfin tuna.