Diel effects on bottom-trawl survey catch rates of shallow- and deep-water Cape hakes <i>Merluccius capensis</i> and <i>M. paradoxus</i> off Namibia, using solar zenith angle
<div><p>Diel patterns in survey trawl catches for the Cape hakes <i>Merluccius capensis</i> and <i>M. paradoxus</i> off Namibia were studied in order to examine the effect of diel bias on catchability, and its implication for survey abundance estimation and the consistency of the survey time-series. Catch rates (numbers per haul) by species and length from summer demersal biomass surveys conducted during the period 2002–2013 were used, together with a computation of the corresponding light-level data from which the solar zenith angles were obtained. Generalised additive models were fitted to assess the relationship between the catches and a number of explanatory variables. Significant covariates were zenith angle, depth and geographical position. The final models explained 78% and 59% of the variability in catch rates of <i>M. capensis</i> and <i>M. paradoxus</i>, respectively. For <i>M. capensis</i>, the response to zenith angle increased sharply for values above 100°, which represents the time between sunset and sunrise. For <i>M. paradoxus</i> there was a moderate increase in the response to zenith angle during the night. In cases where some fishing took place at night in shallow water, the survey results for <i>M. capensis</i> were more greatly affected than was the case for <i>M. paradoxus</i>, which is related to the different depth preference of the two species. Fishing in depths shallower than 400 m outside daylight hours should therefore be avoided in order to reduce bias and ensure consistency in abundance estimates from surveys.</p></div
