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were used to describe the demersal assemblages of the northern Benguela along the Namibian coast. Multivariate analyses indicated a clear separation between shelf and slope habitats, which were divided into three and two assemblages respectively. These divisions were distinct, with clear distributional boundaries separating them. Species determining the structure of these divisions and subdivisions and their relative abundances were identified. Depth, bottom temperature, bottom salinity and dissolved oxygen were all significantly correlated with the spatial distribution of the assemblages. Minor changes or shifts in the distribution of assemblages were observed over the study period, although patterns of species associations remained relatively constant.

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The Cape hakes Merluccius capensis and M. paradoxus are dominant predators over the Namibian shelf. They are found in a water column that includes myctophids and other mesopelagic fish, euphausiids and cephalopods. Together with their cohabitant potential prey, hake are known to undertake diurnal vertical migrations, aggregating near the bottom during daylight, but migrating off the bottom at night. An attempt to determine the underlying mechanisms of mis diurnal migration by means of underwater acoustics and trawling was made at a single location on the central Namibian shelf at a depth of 350 m during four consecutive days in April 1996. Large M. capensis, 50-75 cm total length, dominated just over the sea bed, whereas 30-40 cm M. paradoxus were most abundant 5-50 m off the bottom, suggesting that the smaller M. paradoxus had to remain higher in the water column to avoid being eaten by the larger M. capensis. Large hake of both species preyed preferentially on fish, whereas the smaller hake preferred euphausiids, although there was some evidence of euphausiid consumption by most hake. There was no distinct daily feeding rhythm in either species of hake, although there was some evidence of evening predation dominating. This may indicate a feeding strategy where vision is not important. 365The Benguela upwelling system is one of the most productive in the world (Waldron and Probyn 1992), providing a large biomass of plankton and planktivorous fish (Ware 1992) that in tum are preyed on by the Cape hakes Merluccius capensis and M. paradoxus, which are dominant and highly successful demersal predators in the system (pillar and Barange 1995, Roel and Macpherson 1988, Ware 1992. Bottom waters over the Namibian shelf in particular are frequently depleted of oxygen, placing notable constraints on physical activity. It is therefore of relevance to study the behavioural ecology of Cape hake to determine what strategies they apply to tackle this demanding situation successfully. Prey availability, prey selection, feeding rhythm and vertical positioning are key parameters when such strategies are to be elucidated.Hake are also among the most commercially important fish resources in the area (Bakun and Parrish 1981, Gordoa et at. 1995, Payne and Punt 1995, and it is therefore vital to apply the best forms of stock assessment to facilitate optimal management of the stocks. Among the methods used off Namibia is a combination of bottom trawl survey and hydroacoustic abundance measurements, in which the quantity of hydroacoustically identified Cape hake off the bottom is added to the bottom trawl catches (Anon. 1995). However, pelagic hake recordings in daylight tend to be obscured by traces of mesopelagic fish and plankton. On the other hand, bottom trawl catchability of hake is higher during the day than at night (Botha 1973, Payne 1989, Pillar and997). The overall objective of the present investigation was therefore to describe and if possible to explain the diurnal vertical dynamics of Cape hake and their cohabitants in ord...

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