Why plots of results for the simple penguin model of SWG/EAF/SEABIRDS/13APR/01 are misleading, and some broader consequential implications

Butterworth, Doug S; Plagányi, Éva E

Why plots of results for the simple penguin model of SWG/EAF/SEABIRDS/13APR/01 are misleading, and some broader consequential implications

Working Paper

2007

Publisher

University of Cape Town

Department

Marine Resource Assessment and Management Group

Faculty

Faculty of Science

Abstract

SWG/EAF/SEABIRDS/13APR/01 investigates the extent to which observed trends in the numbers of African Penguins breeding at Robben Island and in the Western Cape can be matched by a simple population model, and concludes that there is a reasonable match from 1992–2006 in numbers observed to be breeding and the corresponding model estimates when assuming parameter values for juvenile and annual adult survival rates of S = ,51.0 S = 85.0 j respectively, and an age at first breeding of 3. This result (based on a process error estimation model) conflicts with that of the modeling results presented by Plagányi and Butterworth (SWG/EAF/SEABIRDS/13APR/03) (which currently uses an observation error estimation model), and hence merits further investigation to determine whether this conclusion is justified. Here we first show briefly (by repeating and extending the analyses) that the results presented in SWG/EAF/SEABIRDS/13APR/01 fail to satisfy standard statistical criteria for acceptable fits of a population model to data. We thus reiterate our earlier concerns expressed that the abundance index data are not compatible with the parameter values as given in SWG/EAF/SEABIRDS/13APR/01, and hence that serious attention needs to be given to identify the source of the incompatibility between present demographic parameter estimates and abundance index series. The remainder of this document expands upon this and related issues, and makes suggestions for further areas of investigation.

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