The cause of death of 15 crocodiles in the Nandoni Dam is attributed by the Department of Water Affairs to suffocation in fish nets in a provisional report. The laboratory tests results are not yet available.

“Our water quality professionals visited the Nandoni Dam on October 18. At that time, 15 crocodiles had died. According to the report we received, most crocodiles died of suffocation after being trapped in fish nets. The crocodiles were trapped in the fish nets as they went for the fish in the nets,” reads the media response of the department received on Tuesday. The prompt response was issued by Mr Ndivhuwo Thenga, Principal Communication Officer on behalf of Mr Alson Matukane, provincial chief director of Water Affairs.

Earlier on, Dr Paul Fouché of the Department of Zoology of the University of Venda said that crocodiles easily became strangled in fish nets and simply drowned because they were reptiles with lungs, requiring oxygen.

“The drowning of crocodiles as a result of being caught in nets is a probability, especially because the crocodiles seemed young and maybe not more than a meter in length,” Fouché said. Last week, Fouché observed about 20 to 30 operators who were netting fish. They concentrated their netting on the inlets of the dam, where the water is shallower, about 8 to 10 meters deep. At the far end of the dam, only one or two operators were observed. Fish netting is an illegal activity that is prohibited in South African dams.

Professor Ben van der Waal, also from Univen’s Zoology Department, is another expert who is very concerned about the large-scale illegal fish netting in Nandoni.

“They use nets of plastic material. The nets are used at night under the surface of the water and are difficult to see. They catch young tilapia and other fish species, mostly immature fishes. They do major harm to the fish population. Maroek is used to lure the fish to the nets. Throw nets are also used. With these, all sizes of fish are caught and they do a lot of harm, especially in the breeding season,” Van der Waal said.

The fish problem is described by the department of Water Affairs as serious.

“There are a lot of people who rely on fishing to earn a living. Fish netting may lead to depletion of fish species in the dam if it is not controlled properly,” Thenga said.

Van der Waal said that despite clampdowns by nature conservation officials in the past, where nets and canoes were confiscated, illegal fish netting continues.

Another concern surrounding the Nandoni is the possible inflow of sewerage into the dam. The department said they could not as yet confirm or deny that there could be sewerage contamination in the Nandoni.

“Carcasses and water samples have been sent to the laboratory for tests. We are still waiting for tests results from the laboratory. Some people found on the dam site on the day did not agree with the speculation that there is contamination,” stated Mr Thenga. It is said that the dead fish found might just be fish which died in an illegal net and were thrown away by the fisherman.

The department, however, did not rule out toxicity as the cause of the death of the fish. They explained that fish are usually employed as sensitive indicators of toxic pollution “but the situation is complex, because various environmental factors can considerably affect the toxicity”. Toxicity is the degree to which a substance is able to damage an exposed organism. As far as fish are concerned, the two most important environmental factors are dissolved oxygen (DO) and temperature. The lower the level of DO in the water, the more the activity of the fish may be impaired, so that their sensitivity to poisonous material is often increased. The metabolic rate of fish is closely linked with temperature, so that a rise of 10 degrees will increase the oxygen requirement by two or three times. Unfortunately, a raised temperature simultaneously reduces the available oxygen supply.