VN June 2020

Vetnews | June 2020 11 Article Raise the flag... <<< 10 “When I came back from overseas my first stop was at Pretoria University’s Faculty of Veterinary Science,” she said. “They had been approached by tilapia farmers to do some research on diseases, but the staff didn’t have time, so they grabbed hold of me.” She emphasised the proactive role played by the Tilapia Farmers Association of South Africa (TFASA), which spearheaded the project. “They understood that disease was a massive factor that could potentially impact the productivity of their farms, and they were concerned they might be harbouring something and wanted some surveillance done,” she said. The pathogen project identified key problems affecting fish health and ultimately stunting the success of farmers’ ventures. Taylor conducted a research survey of 19 tilapia aquaculture systems in the country’s Gauteng and Limpopo provinces. These Nile tilapia ( Oreochromis niloticus ) farms ranged in size from small 10,000-liter systems of emerging farmers, to 250,000-liter systems at more established enterprises. Setting up a small outdoor lab at each site, Taylor weighed, measured and examined fish gills and skin under microscope. The farmers also completed an extensive questionnaire to assess their farming practices. This included among others a water analysis – such as temperature, dissolved oxygen, carbon dioxide, ammonia, nitrates, pH, hardness and alkalinity – as well as questions about stocking densities and feed. In South Africa, Nile tilapia are classified as an exotic: “The government stipulated they have to be farmed in recirculating aquaculture systems to prevent them fromgetting into our national waterways,” said Taylor. “They are highly invasive and cross-breed with our indigenous tilapia.” But while tilapia are resilient fish, closed systems present their own challenges and Taylor’s macro and microscopic findings showed evidence of significant stress and subclinical disease. The farming practices affecting fish health included poor water quality, inadequate filtration, high stocking densities, uneconomical fish sex ratios, low quality or quantity of feed, poor disease management and lack of biosecurity. “The water quality was shocking,”saidTaylor.“That was probably the worst finding, and its potential impact on fish health is dramatic.” However, most fishwere not showing visible effects like skin lesions, so farmers were not aware of the problems that were building beneath the surface. Taylor explained that once fish immune systems become stressed, parasites proliferate, causing fish skin and gills to become irritated and susceptible to infection. “With fish there’s this kind of beautiful balance,” she said. “They are very tolerant, but if the environment starts going a bit wonky, they stress. However, it’s a subclinical finding, and so the farmers don’t see it. All of a sudden it’s crossed a threshold point and now you get a number of fish dying.” Mainly, however, the fish had extremely poor growth rates, spending up to 12 or 18, rather than six months, in the systems. “Farmers’ biggest costs are feed and heating water,” said Taylor. “If you’re doubling your feeding time and electrical costs during your grow-out period that has a massive effect on the final productivity of your system.” Dr. Gillian Taylor During a rural farm visit to stock tilapia fingerlings, Dr. Taylor speaks with TFASA members and farmers. >>> 12