Kai Wieland

Diel effects on bottom-trawl survey catch rates of shallow- and deep-water Cape hakes Merluccius capensis and M. paradoxus off Namibia, using solar zenith angle

Diel patterns in survey trawl catches for the Cape hakes Merluccius capensis and M. paradoxus 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 M. capensis and M. paradoxus, respectively. For M. capensis, the response to zenith angle increased sharply for values above 100°, which represents the time between sunset and sunrise. For M. paradoxus 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 M. capensis were more greatly affected than was the case for M. paradoxus, 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.

Using underwater television surveys to assess and advise on Nephrops stocks

General information State: Published Organisations: National Institute of Aquatic Resources, Section for Ecosystem based Marine Management, Section for Monitoring and Data Contributors: Bell, E., Clements, A., Dobby, H., Doyle, J., Feekings, J. P., Leocádio, A., Lordan, C., Weetman, A., Wieland, K., Leocádio, A. (ed.), Weetman, A. (ed.), Wieland, K. (ed.) Number of pages: 49 Publication date: 2018

Estimating escapement of fish and invertebrates in a Danish anchor seine

11 The codend is generally presumed to be the place where the main selectivity of fish occurs in towed 12 fishing gears, but other parts of the net have been found to contribute to the selectivity process of several 13 invertebrate species. This means that conventional selectivity or survival studies may ignore the 14 selectivity of net parts other than the codend for certain species. By attaching 12 small meshed collecting 15 bags to different parts of a Danish anchor seine net and conducting normal commercial fishing activities, 16 this study showed that there is a substantial escapement of fish and (especially) invertebrates from the 17 forward parts of the seine net. For seven species of demersal fish, most fish escaped through the lower 18 panel close to the codend. All invertebrate species were found in higher numbers in the collecting bags 19 than in the codend where many organisms escaped in the lower panel of the wings or the belly. Mean 20 levels of visible damage ranged from 1.00 to 3.25 for collected invertebrates and were similar for all gear 21