Diurnal variation models of bottom trawl survey catches parameterised by using commercial CPUE data

Introduction: Diurnal variation is observed in almost all marine surveys and can cause biased abundance and length frequency estimates. Symmetric logistic models developed by Hjellvik et al. (2001) were utilized to investigate diurnal variation in catch rates of hake (Merluccius spp.) of trawlers for the period 1997-2002 and of hake surveys (1990-2003) in Namibian waters. The large numbers of commercial tows have a very good geographical, seasonal and temporal resolution that enabled us to study the influence of various explanatory variables on diurnal fluctuation model parameters. These results were thereby used to facilitate the modelling of diurnal variation in Namibian hake trawl survey catches