Towards a catchability constant for trawl surveys of Namibian hake

Fine-scale environmental effects on Cape hake survey catch rates in the northern Benguela, using data from a trawl-mounted instrument package

“…Almost no diel effect on catch rate was found in the present study. This is in contrast to observations by Huse et al (1998Huse et al ( , 2001, Johnsen & Iilende (2007) and Kainge et al (2015). But similar to Kainge et al (2017), who found the same results, the dataset contained very few stations which were sampled outside the daylight period.…”

Improving the precision of length frequency distribution estimates from trawl surveys by including spatial covariance—using Namibian Merluccius capensis as an example

“…For instance, photos show that hakes swim in front of the trawl and actively avoid the gear (Paper III). These findings, in combination with a study of the herding effect on hakes (unpublished data, Engås et al), indicate that the hakes are less passive in the catching process than previously believed (Huse et al 2001). Clearly, when the escapement is size and species dependent (Paper III), any change in the species ratio or length composition could have important consequences for the Namibian hake assessment as neither species nor size dependent catchability are included in the assessment model for Age > 2 (Paper III).…”

“…Clearly, when the escapement is size and species dependent (Paper III), any change in the species ratio or length composition could have important consequences for the Namibian hake assessment as neither species nor size dependent catchability are included in the assessment model for Age > 2 (Paper III). It is possible to generalize and estimate corrections factors based on in situ observations, and Huse et al (2001) estimated a catchability factor for the Namibian hakes based on an experimental in situ study in 1999. Similarly, Dickson (1993 a,b) established correction models for smaller cod as they were under-represented in the catches.…”

Factors affecting the diel variation in commercial CPUE of Namibian hake—Can new information improve standard survey estimates?

“…For improved assessments it is crucial to identify and quantify key factors that affect the precision and accuracy of the survey estimates. Several studies have identified marked diel variation in the catch rates of Namibian hake (Pillar and Barange, 1997;Huse et al, 1998Huse et al, , 2001Iilende et al, 2001), which may seriously bias the survey estimates if not corrected for. Due to its bias potential, much effort has been put into the study of diel variation in catch rates elsewhere, analysing data from both dedicated experiments (Michalsen et al, 1996;Aglen et al, 1999), and standard surveys (Fréon et al, 1993;Korsbrekke and Nakken, 1999;Hjellvik et al, 2001Hjellvik et al, , 2002Hjellvik et al, , 2004Adlerstein and Ehrich, 2003).…”