Applying the direct duplicate method to simulated IDCR/SOWER survey data

Branch, Trevor A; Butterworth, Doug S

Applying the direct duplicate method to simulated IDCR/SOWER survey data

Working Paper

2005

Publisher

University of Cape Town

Department

Marine Resource Assessment and Management Group

Faculty

Faculty of Science

Abstract

The direct duplicate method (Palka, 1994) was applied to simulated IDCR/SOWER survey data. Estimates of whale density were generally negative biased, but less so than estimates obtained using the standard method. The mean bias across scenarios was -11% (range -31% to 8%) for the “2004” scenarios and -5% (range -19% to 10%) for the “2005” scenarios. Negative bias was more pronounced when a density gradient was present, when the detection function used to generate the simulated sightings excluded school size but included weather as a covariate, when errors in recorded school size were introduced, when weather and density were correlated, and when surveys were conducted in IO mode only. This method shows promise although further development is desirable to reduce the associated bias further, perhaps by including weather and school size as covariates.

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