An analysis of the small scale surveys of anchovy abundance around Robben and Dassen islands from 2009 to 2013

Robinson, William M L; Butterworth, Doug S

An analysis of the small scale surveys of anchovy abundance around Robben and Dassen islands from 2009 to 2013

Working Paper

2014

Publisher

University of Cape Town

Department

Marine Resource Assessment and Management Group

Faculty

Faculty of Science

Abstract

The results from the small scale hydroacoustic surveys of the abundance of anchovy around Robben and Dassen islands over the 2009-2013 period are analysed under the assumption of a Gaussian form for the trends in density at each island over the course of the winter months. Based primarily on AICc, the model selected from amongst a number of variants has the same trend in abundance with year for the two islands, compatible with the assumption used by Robinson (2013) in his GLM analysis of the impact of closures to pelagic fishing around these islands on penguin recovery, though the data have limited power to distinguish deviations from that assumption. The abundance estimates from the island surveys, though compatible also with the May recruitment survey trends, show appreciably larger variance. This raises the question of whether these small scale surveys merit continuation, unless it is possible to increase their frequency considerably during the winter months each year to improve the overall precision of the integrals over local abundance which they can provide.