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

  • CL de Moor
  • DS Butterworth
  • JC Coetzee

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.

Keywords: anchovy, effective sample size, length distribution, proportion-at-age, proportion-at-length

African Journal of Marine Science 2013, 35(3): 335–342

Author Biographies

CL de Moor
Marine Resource Assessment and Management Group (MARAM), Department of Mathematics and Applied Mathematics, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa
DS Butterworth
Marine Resource Assessment and Management Group (MARAM), Department of Mathematics and Applied Mathematics, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa
JC Coetzee
Branch Fisheries Management, Department of Agriculture, Forestry and Fisheries, Private Bag X2, Rogge Bay 8012, South Africa
Published
2013-10-24
Section
Articles

Journal Identifiers


eISSN: 1814-2338
print ISSN: 1814-232X