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Fisheries - Enviro Dynamics Namibia

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F I S H E R I E S , M A M M A L S A N D S E A B I R D S S P E C I A L I S T S T U D Y<br />

The target catch species is meso-pelagic (i.e. found at depths between 200 – 1000 m above the<br />

sea floor) and shoals migrate vertically upwards through the water column between dusk and<br />

dawn. Mid-water trawlers exploit this behaviour (diurnal vertical migration) by adjusting the<br />

depth at which the net is towed (this typically varies from 400 m to just below the water surface).<br />

The net itself does not come into contact with the seafloor (unlike demersal trawl gear) and<br />

towing speed is greater than that of<br />

demersal trawlers (between 4.8 and<br />

6.8 knots) – Figure 11 refers.<br />

Once the gear is deployed the vessel<br />

is hampered in its ability to<br />

manoeuvre as the gear may extend<br />

up to 1 km astern of the vessel<br />

(depending on the depth being<br />

fished).<br />

Trawl warps are heavy, ranging from<br />

32 mm to 38 mm in diameter. Net<br />

openings range from 40 m to 80 m<br />

in height and up to 120 m in width<br />

(Figure 11). Weights in front of, and<br />

Final Report<br />

<strong>Namibia</strong>n Marine Phosphate (Pty) Ltd.<br />

Figure 11. Typical gear configuration used during midwater<br />

trawling operations<br />

along the ground-rope assist in maintaining the vertical opening of the trawl. To reduce the<br />

resistance of the gear and achieve a large opening, the front part of the trawl net is usually made<br />

from very large rhombic or hexagonal meshes. The use of nearly parallel ropes instead of meshes<br />

in the front part is also a common design. On modern, large mid-water trawls, approximately<br />

three quarters of the length of the trawl is made with mesh sizes above 400 mm.<br />

In 2006, 12 rights-holders and 12 vessels were registered within the mid-water trawl fishery. The<br />

fleet operates exclusively out of the port of Walvis Bay and fishing grounds extend north of 25ºS<br />

to the border with Angola and effort is highest in the north (Figure 12). Juvenile Cape horse<br />

mackerel move into deeper water when mature and are fished mostly between the 200 m and<br />

500 m isobaths towards the shelf break.<br />

Mid-water trawl fisheries are not usually considered to have significant impacts on benthic<br />

biodiversity (Atkinson and Sink 2008). Nonetheless, as they tow their nets at a relatively high<br />

speed they regularly entangle sea birds, sharks, dolphin and seals (Nel 2004).<br />

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