Estimation of birthdates and catch-at-age using length frequency analysis (LFA), with application for skipjack tuna (Katsuwonus pelamis) caught in the Southwest Atlantic

Abstract

Converting length frequencies into age frequencies is an important component of a fisheries assessment. In this paper we use a length frequency analysis (LFA) to estimate birthdates after converting length data into catch-at-age, and use simulation studies to compare modelselection criteria and to examine the reliability of the resulting estimates. Deviance and an adaptation of the Akaike Information Criterion performed best. LFA results in useful estimates of birthdates and of catch-at-age if reliable length frequency data and estimates of growth parameters are available. The analysis is applied to skipjack tuna (Katsuwonus pelamis) caught in the Southwest Atlantic Ocean. Although spawning is reported to be seasonal in subtropical waters, the birthdates of the fish caught there were spread uniformly across the year. Young skipjack become vulnerable to fishing mainly in the first quarter of each year. Recruitment of strong year classes did not affect fishery yields equally in the Southwest Atlantic and the Caribbean Sea, so the assumption of a unit western stock for management purposes and the stock structure of skipjack in the Atlantic need further evaluation.