Can anchovy age structure be estimated from length distribution data collected during surveys?

De Moor, Carryn L; Butterworth, Doug S; Coetzee, J C

Can anchovy age structure be estimated from length distribution data collected during surveys?

Journal Article

2013

Journal Title

African Journal of Marine Science

Link to Journal

http://www.tandfonline.com/toc/tams20/current

Publisher

National Inquiry Services Centre (NISC)

Publisher

University of Cape Town

Department

Marine Resource Assessment and Management Group

Faculty

Faculty of Science

Abstract

Historically, a time-series of proportions-at-age 1 from annual November hydro-acoustic surveys has been used to inform the assessment of, and in particular the choice of appropriate values for juvenile and adult natural mortality for, the South African anchovy Engraulis encrasicolus. However, information from direct ageing is limited and almost two decades old. A new method is developed to estimate the annual proportions of anchovy at age 1 directly from the population length distributions estimated from samples taken during the November surveys. This method involves modelling the annual length distributions of age 1 and age 2+ anchovy. The analysis provides a new time-series of proportions-at-age 1, together with associated standard errors, for input into assessments of the resource. The results also caution against the danger of scientists reading more information into data than is really there.