Alien fish eradication improves water quality at Century City - case study
NCC planned and implemented an alien fish eradication programme to solve a looming water quality crisis at one of South Africa’s premier retail, office and residential developments.
Century City, first established in 2002, is a large mixed-use development covering 250 hectares of land along the N1 near the centre of Cape Town. It includes the Canal Walk shopping mall, Ratanga Junction theme park, a retirement village, office parks and several residential complexes.
At the heart of the development is the 16ha Intaka Island wetland, including one seasonal pan that is among the largest water fowl breeding sites in South Africa. About half the wetland has been artificially created to help filter the water in the development’s 6km of navigable canals. The closed system is fed from a nearby wastewater treatment works, as well as via groundwater infiltration and run-off from the surrounding developed areas.
Water plants and fish species such as Cape Kurper and Cape Galaxias were intended to provide natural water quality management. But alien fish species such as carp, banded tilapia and sharptooth catfish were soon introduced to the system, probably by fishermen, with severe consequences for water quality. Mud stirred up by the bottom-feeding carp stopped plant growth, and the carnivorous banded tilapia upset the balance of fish species.
The result was a dramatic deterioration in water quality throughout the system, including a noticeable unpleasant smell and rapid growth of toxic blue-green algae.
NCC and the Century City Property Owners’ Association, together with a consultant freshwater ecologist from CapeNature, established that the only effective and economically viable way to eliminate alien fish species was to use the piscicide Rotenone. This affects only gill-breathing animals and breaks down quickly.
NCC planned project implementation in several steps, drawing on its extensive logistics and resource management experience:
- We chose late summer for the eradication, to avoid killing off tadpoles and to take advantage of lower water levels to use less of the piscicide
- Indigenous fish were netted and kept in portapools for re-introduction
- Water samples were taken and an application plan worked out to ensure complete effectiveness
- We invited the SPCA to monitor the eradication and informed residents, anglers and other users of the canals through brochures, information boards and media publicity
- Obtained the special permits needed to dispose of dead fish at the Vissershok landfill site
- Recruited and trained clean-up teams to gather dead fish
- Sourced boats, water scoots and other equipment needed for the operation
The Rotenone application was carried out over two days, starting early in the morning to avoid disruption to residents and tenants. NCC drew on its firefighting experience to set up a field operations centre that co-ordinated application and cleanup teams, maintained radio communications and supplied refreshments to work teams. Cleanup teams remained on site for a week to ensure that all dead fish were removed before the saved species were re-introduced.
Following the eradication operation, NCC supported Century City in an ongoing campaign to educate anglers and prevent any re-introduction of problem fish species.