REPORT NO: 816/1/03

March 2003


The overabundant proliferation of aquatic weeds in South African water conveyance systems cause a series of operational problems. Filamentous algae, such as Cladophora glomerata and pondweeds impede flow and reduce the capacity of irrigation canals to a significant extent. In worst case scenarios, irrigation scheme managers are faced with situations where they are unable to deliver water at the downstream ends of canal systems. This situation also contributes to water losses, crop losses and structural damage to concrete-lined canals.

This report is the product of a research project funded by the Water Research Commission to investigate the possible application of sterile (triploid) grass carp (Ctenopharyngodon idella) as biological control agent on aquatic weeds in concrete-lined irrigation canals. The aims of the project were firstly to investigate the suitability of a concrete-lined irrigation canal as grass carp habitat; secondly to test the efficacy of sterile grass carp as bio-control agent on filamentous algae; thirdly to evaluate the economic feasibility of this biological approach against the current chemical, physical and mechanical control methods; and fourthly to propose a management plan for the operational application of triploid grass carp as bio-control agent in irrigation canals.

To achieve the goals set for the project, the Ramah-3 Canal near the town of Crania, was selected to serve as experimental canal. This canal can be regarded as a typical South African concrete-lined canal, and is located in the Orange-Riet River Canal System, downstream of the Vanderkloof Dam.

As the majority of local irrigation canals are concrete-lined, skepticism existed amongst local scientists if the fish will be able survive in canals with a presumed high constant flow and low diversity of hydraulic biotopes. It was therefore a priority to establish the suitability of concrete-lined canals as habitat for this herbivorous fish specie.

The study found that flow velocities in the Ramah Canal System never exceeded 1 m/s, even under high flow conditions (full capacity and 110% plus conditions) of more than 5 m3/s. It was found that grass carp thrived at flow rates ranging from 0.48 to 0.80 meter/second, moving with ease upstream and downstream in a 16 km long experimental section of the Ramah-3 Canal.

The sterile grass carp controlled the algal biomass in the Ramah Canal to significant levels. Cladophora was efficiently controlled at stocking rates of 3 to 7 fish per km canal. Triploid grass carp retrieved from the canal system after a six-month experimental period were found to be in an excellent physiological condition and displayed a mean weight increase of more than 300%. An ideal stocking protocol will probably be 10 individuals of 20-30 cm in length per kilometer canal, with a 10-15% annual supplementation figure.

Civil structures, such as culverts, super-elevated canals and bridges will provide the fish with sufficient protection against possible predators. A few possible adaptations to the existing canal operation regimes should ensure that triploid grass carp could be managed as an effective biological control agent. This should be determined on a site-specific basis and could include additional civil structures such as sanctuary dams and small in-line fishways to ensure free migration throughout the target system.

An attempt was made to conduct a brief economic analysis, based on information on expenditures of local irrigation schemes on aquatic weed control in their canals. The outcome of this brief investigation was in line with overseas findings in that biological control with sterile grass carp will be more economical than the currently applied control methods of herbicidal and mechanical or physical control.

It can be concluded that the fish adapted to the artificial conditions experienced in a concrete- lined canal and perform their task as bio-control agent with ease. The authors are of the opinion that this bio-control technique will contribute to the current Integrated Aquatic Weed Management Programmes (IAWMPs) of the Department of Water Affairs and Forestry. Site- specific conditions for each irrigation scheme will however, dictate a different approach to the aquatic weed problem. It is therefore strongly recommended that suitable qualified specialists should develop all aquatic weed management programmes on a site-specific basis.