Field validation of age readings from the statoliths of chokka squid (Loligo vulgaris reynaudiid'Orbigny, 1845) and an assessment of associated errors

Abstract

Lipinski, M. R., Durholtz, M. D., and Underhill, L. G. Field validation of age readings from the statoliths of chokka squid (Loligo vulgaris reynaudii d’Orbigny, 1845) and an assessment of associated errors. ‐ ICES Journal of Marine Science, 55: 240‐257. This paper describes the first successful field tag-recapture experiment, aimed at validating daily increments in the statoliths of chokka squid (Loligo vulgaris reynaudii). Seven hundred and forty-two squid were tagged and injected with oxytetracycline (OTC) and 63 were subsequently recovered after 4‐20 days spent free in the sea. Forty-seven statoliths were successfully prepared for increment reading, eight of which satisfied stringent requirements for validation: good visibility of increments, sharp definition of the OTC band and a clear, undamaged margin. These eight statoliths yielded results indicating that the increments in chokka males in the size range 290‐370 mm mantle length (ML) were deposited daily and that the same was true for a 173 mm ML female. The results of an analysis of the errors involved in the increment recognition stage (counting) do show, however, that proper statolith preparation, increment identification and subsequent reading are crucial for obtaining accurate age estimates. The error in counting increments in a poorly prepared statolith may be three times higher than the error for a well prepared statolith in the technique used, which is felt to be one of the best presently available. This technique relies on the grinding of both sides of the statolith in the frontal plane (contrary to the most frequently used transverse sections). Precise position of the grinding plane is determined and maintained during the procedure. ? 1998 International Council for the Exploration of the Sea