A summary of the General Linear Modelling approach applied to standardize the CPUE data for the offshore trawl fishery for Merluccius capensis and M. paradoxus off the coast of South Africa

Glazer, Jean Patricia; Butterworth, Doug S

A summary of the General Linear Modelling approach applied to standardize the CPUE data for the offshore trawl fishery for Merluccius capensis and M. paradoxus off the coast of South Africa

Working Paper

2006

Publisher

University of Cape Town

Department

Marine Resource Assessment and Management Group

Faculty

Faculty of Science

Abstract

The models applied to standardize the CPUE data of Merluccius capensis and M. paradoxus caught offshore off the coast of South Africa are summarised here. This is not straightforward because CPUE indices are required at the species level, but the offshore trawl commercial catch data are recorded only for both species combined. Consequently algorithms developed by Gaylard and Bergh (2004), which make use of species proportions by size at depth, as estimated from research surveys, have been applied to split the hake catches by species at a coast level (west and south) before combining the data from both coasts to perform coast-combined species-specific analyses. Note that this approach can be used from 1978 onwards only, as prior to that the depth of drags was not recorded. The data used in the analyses are obtained from the Marine and Coastal Management (MCM) demersal database. A fair amount of pre-processing of the data is necessary to ready them for GLM analysis purposes. This includes accumulating the drag-by-drag data on a daily basis per vessel and applying rules to identify and exclude potential “outliers” from the analyses. The daily accumulation is necessary because in certain cases operational constraints prevent the recording of catches per trawl. In such cases effort is recorded per trawl, whereas the catch for the day is logged against the effort of only one of the trawls (usually the last) of the day. Alternatively, the skipper may average the daily catch across the drags of the day.