Escapement of Cape hakes under the fishing line of the Namibian demersal sampling trawl

Abstract

Swept area surveys in Namibian waters provide input data to the stock assessment model. To evaluate the model's predictions, it is important to understand the catch efficiency of sampling trawls. The objective of this study was to establish whether Cape hakes Merluccius capensis and M. paradoxus escape under the fishing line and to identify any species or length dependence of escape rates. Experiments were carried out in Namibian waters during two cruises in October 2002 and October 2003. A collection bag was mounted under the trawl to catch fish escaping below the fishing line. Environmental data and photographs of fish in front of the trawl were also taken. Escapement of hake varied by species, length, depth and year. It was generally below 5% for M. capensis but escapement in 2002 averaged 10–20% for M. paradoxus, and in 2003 it was over 50% in the shallow area (300m), decreasing to 10% in the deep area (570m). Oxygen level variation seemed to explain the marked between-year difference in escapement. Escapement of M. capensis decreased marginally with increasing fish length, whereas escapement of M. paradoxus showed a marked increase with increasing fish length in shallow waters and no length dependence in deep waters. Species differences in behaviour and vision may partly explain the observations.