Results from sensitivity tests related to the models used to standardize the offshore commercial trawl CPUE data of M. capensis and M. paradoxus respectively

Glazer, Jean Patricia; Butterworth, Doug S

Results from sensitivity tests related to the models used to standardize the offshore commercial trawl CPUE data of M. capensis and M. paradoxus respectively

Working Paper

2009

Publisher

University of Cape Town

Department

Marine Resource Assessment and Management Group

Faculty

Faculty of Science

Abstract

General Linear Models (GLMs) are applied to the catch per unit effort (CPUE) data from each of the two hake stocks, Merluccius capensis and M. paradoxus, in order to derive standardized indices of abundance which are then input into the stock assessment models and the Operational Management Procedure (OMP). Since it is difficult to distinguish between the two hake species at the landing site catches are reported for both species combined. An algorithm that makes use of size and depth information is thus used to split the catches into the two species (Gaylard and Bergh, 2004). In recent years, for various reasons, a large proportion of the drag records do not have the catches reported by size rendering them unusable; hence an extensive exercise was undertaken to allow for the inclusion of these data in the analyses (Fairweather et al, 2009). The results reported here are for various sensitivities related to GLM3 of Fairweather et al (2009). This is the version of the data that corrects for the errors encountered in the historic extracts used previously, and also includes those data that were previously excluded given that size information was not reported. A procedure was applied to interpolate the size information from data for which there is size information within a given cell which, for each year, is defined by a depth range, latitude range (for the West Coast) or longitude range (for the South Coast), and quarter (Jan-Mar, Apr-Jun, July-Sept and Oct-Dec).