The Mzingazi gauging weir and its effects on the fish and macrocrustacean communities of Lake Mzingazi
Loading...
Date
2012
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Zululand
Abstract
Lake Mzingazi is a freshwater coastal lake in Richards Bay, KwaZulu-Natal. Because of
ongoing industrial and residential developments in the Richards Bay area, the need for
potable water increased and led to the construction of a weir across the outlet of Lake
Mzingazi. The first weir on this historically estuarine lake was built in 1955, after which it was
raised several times to satisfy the increasing demand for water. Due to potential salt-water
intrusion via groundwater from the saline Mzingazi River into the lake, another weir, namely
the Saltwater Barrier, was built 1.4 km downstream of Lake Mzingazi to limit saltwater
intrusion into the lake. The Mzingazi weir largely prevented the migration of fish and
crustaceans between the marine/estuarine environment and the freshwater environment. In
2005-2006, the DWA designed and constructed a new and improved gauging crump weir
across the lake outlet, which included a pool and weir fishway to allow the migration of fauna
in and out of the lake. The fishway was aimed at allowing euryhaline fish species to migrate
out to sea to spawn and back to the lake to complete their life cycle since Lake Mzingazi
historically served as an important nursery habitat for juveniles of a number of euryhaline
fish species. Monitoring of the fishway was required to assess the efficiency of the design,
and if necessary, to modify flow conditions to meet the requirements of migrating fauna. This
study aimed firstly at monitoring the operation and efficiency of the Mzingazi fishway and
secondly, at monitoring its effect on the fish community of Lake Mzingazi.
In order to assess the operation and efficiency of the Mzingazi fishway, the fish and
macroinvertebrate fauna were sampled at six locations in the Mzingazi system, i.e at the
Saltwater Barrier, in the upper Mzingazi River, at the entrance to the fishway, in the fishway
pools, at the fishway exit and in the lake immediately above the fishway. The fishway was
sampled over two days on a monthly basis over the period August 2007 to July 2008, using
funnel traps, dip nets and a small seine net. In order to monitor the effect of the fishway on
the lake fish community, seasonal sampling in the lake was conducted for a period of two
years, from August 2007 to June 2009, using seine and gill nets. A total of 3 288 fish,
representing 29 species, were recorded throughout the study. Of these 29 species, 20
species were able to locate and enter the fishway. The fishway was found to allow passage
for 17 fish species between the estuary and the lake. The dominant species that managed
successfully to migrate through the fishway were Myxus capensis, Glossogobius callidus
and Eleotris fusca, making up 77% of the total catch. The fish migrating through the fishway
included five marine species, five estuarine resident species, six freshwater species and one
unclassified Gobiid larvae sp.
Abstract
The Mzingazi gauging weir and its effects on the fish and macrocrustacean communities of Lake Mzingazi ii
A total of 8 188 macrocrustacea representing eleven species were also recorded in the
Mzingazi system, of which seven prawn and one crab species were able to locate and enter
the fishway. The fishway provided migratory passage for seven prawn species, these being
dominated by Macrobrachium equidens, Caridina nilotica and Caridina indistincta
respectively, together making up 97.6% to the total catch.
This study proved that the fishway was effective and efficient for upstream migration of the
target species. The design of the fishway was found to create suitable hydraulic conditions
for migration of juvenile and sub-adult fish (10 to < 100 mm SL) and for macrocrustacea (3 to
< 26 mm CL). Peak upstream migration of fish through the fishway occurred in August,
September, October and December, which coincides with the peak recruitment period of
most estuarine and marine spawning species. Peak migration and species abundance of
both fish and macrocrustaceans were found to be a natural effect of seasonal recruitment
and breeding, which was indirectly driven by seasonal temperature variations.
The fish community of Lake Mzingazi during the study comprised 16 species, which included
four marine species, five estuarine resident species and seven freshwater species. The
community was dominated by freshwater species such as Oreochromis mossambicus and
Tilapia rendalli and estuarine species such as Glossogobius callidus and Gilchristella
aestuaria. The operation of the Mzingazi fishway was found to be causing a gradual change
in the lake fish community by re-introducing euryhaline fish species into the freshwater
coastal lake.
The following important recommendations were made. Firstly, further long-term monitoring of
the fishway and the lake fish community is required to validate the absence of previously
recorded euryhaline species and to determine the long-term effect of the fishway on the fish
and macrocrustacean community. Secondly, the operation of the fishway is entirely
dependent on water levels in the lake and ongoing water abstraction for domestic and
industrial purposes often results in the water level in the lake being drawn below the overflow
level of 3.0 masml. For this reason, the Department of Water Affairs should limit abstraction
from the lake so that there is sufficient water allocated to allow the fishway to operate. The
water level should never be drawn below the minimum level of 3.0 mamsl, at which the
fishway can still operate.
Description
Submitted to the Faculty of Science and Agriculture in partial fulfilment of the requirement for the degree of Masters Science in the Department of Zoology University of Zululand, South Africa, 2012.
Keywords
Macrocrustacean communities, Freshwater lakes