Wetland & Fish Ecology - Enviro Dynamics Namibia

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4.7 IMPACTS OF CHANGES IN WATER QUALITY (TURBIDITY, SALINITY,
NUTRIENT CONCENTRATIONS) ON IISHANA HABITATS AND FISH AND
FISHERIES:
In all cases where oshana bottoms are disturbed by deepening existing channels,
serious negative effects on water quality can be expected over a short-term period,
such as one summer season, as loosened silt takes very long to settle out. Fish
species of the iishana are not sensitive to slight changes and deterioration of water
quality and experience serious increases in salinity as the deeper iishana and pans of
the Omadhiya wetland complex dry out. With ever increasing salinities as the salts
become concentrated by evaporation, the fish will eventually all die off. But this is
no excuse to hasten the process by careless construction practises or bad timing.
The increasing soil salinity with substrate depth found in these pans has already been
discussed.
4.7.1 IMPACTRELATED TO DIKE AND ITS CHANNEL:
The dike and channel will have a short-term, one season, and negative effect during
construction on water quality and turbidity which will affect fish life by decreasing
available food organisms, which will lead to overall loss of condition and so to lower
spawning success rates and overall productivity of the system. Over the longer term,
rehabilitation of the dike and channel will alleviate these negative impacts. Some
erosion and ongoing leaching of salts from disturbed sediments, e.g. sediment taken
from the oshana bottom and used to build walls/ flood protection banks can
however cause water quality deterioration over a longer term, i.e. 10 years. (Refer to
photographs in Figures 8 and 10 below).
Figure 4-4 Bull dozed wall within oshana on outskirts of Oshakati to protect shacks. K.S. Roberts
Draft Environmental Impact Assessment: Oshakati Flood Mitigation Project
Specialist Study: Wetland Ecology and Fish Ecology Imputs
July 2012