Comparison of sardine November proportion-at-age from different length frequencies and age readers

Abstract

de Moor and Butterworth (2009) recommended that a new method to weight the trawls when calculating the sardine length frequency observed during the November surveys be applied. This new method involved weighting the individual trawls equally for all trawls of sample sizes greater than 40 and down weighting those of smaller sample size. The method used previously weighted the individual trawls by the acoustic weighting associated by that trawl as a proportion of the total acoustic weighting of all trawls. This investigation was initially prompted in 2007 when considering the unusual lack of evidence for strong cohorts in length frequency data and the proportions-at-age which showed a higher proportion of older ages than implied by current estimates of natural mortality. This latter concern remained when the proportions-at-age were recently recalculated using ALKs provided by Deon Durholtz. The proportions-at-age are now compared using ALKs provided by Cynthia Mtengwane and the revised November length frequencies for sardine (Coetzee and Merkle 2011).