Age determination for juvenile fourfinger threadfin (Eleutheronema rhadinum) by using otolith microstructure and length data obtained from commercial fisheries off northwestern Taiwan

Abstract

The fourfinger threadfin (Eleutheronema rhadinum) is a commercially crucial species for coastal and inshore small-scale fisheries, but is rarely studied to understand its life history, including age and growth. The present study examined the daily growth increment in otoliths for the species during its early life stages by using otolith microstructure analysis, and corroborated the age estimates using alternative information and approaches. The daily growth increments were discovered to be formed and spaced regularly. The fish samples were successfully aged at between 174 and 652 days, with the growth rate fairly consistent among individuals. The mean length at age 1 year of E. rhadinum was estimated at 29.7 cm fork length using the fitted growth equation. Hatch dates, estimated using the daily increments and the date of capture, were mostly from April to August. We further corroborated the age estimates by using identifiable modes of younger fish in the monthly length-frequency samples. The lengths-at-age, predicted using the growth equation with best fit, were discovered to correspond favorably with the progress of the modes in the length-frequency distribution of sampled landed fish. A framework that incorporates information available from alternative approaches to determine and corroborate estimates of fish age can be developed, as demonstrated in this study.