By Desmond Bentley
For the first time anywhere, a species of freshwater fish classified as extinct in the wild has been successfully reintroduced to nature.
The Yarkon bleak, or Acanthobrama Telavivensis, used to be endemic to Israel’s coastal river system, but by the end of the last century the silver, 12- to 20-centimeter (5-8 inch) ray-finned fish had almost completely disappeared. By 1999, after several years of severe drought, only three small isolated populations survived, two of them in the Yarkon River that flows through Tel Aviv.
“The river’s ecosystem had been destroyed,” explains Yonathan Raz, chief ecologist of the Yarkon River Authority. “Now it is now being rehabilitated.”
According to Dr. Menachem Goren of Tel Aviv University’s Department of Zoology, 16 percent of native Israeli freshwater fish have become extinct and 19% are critically endangered.
The reason for their demise, says Raz, is simple: “Fish are a million times more susceptible to pollution than we are,” he says.
A team headed by Goren took it upon themselves to save the Yarkon bleak from extinction. Just days before the shrinking puddles completely dried up, they gathered up about 150 surviving fish.
“We originally intended to keep them in my laboratory for a year or two until the natural conditions improve and we could return them to the wild,” recalls Goren.
“The original ‘refugee fish’ were in very poor condition, and the 120 that survived were carefully bred in separate breeding stocks. We managed to produce a large number of them in laboratory conditions before reintroducing them to their natural habitat,” he explains.
Trial and error was part of the process, says Goren. “It was a multiyear effort. At first we knew little about the species – for example their sex ratio, best substrate and temperature for spawning, and what diet to provide them. We learned from our mistakes as we went along. Key to our success was the realization that we have to interfere with nature by constructing the breeding ponds.”
By 2004, over 14,000 fish swam and spawned in the university’s ichthyological laboratory fish tanks.
Engineering solution
The first attempt to reintroduce the Yarkon bleak to its natural habitat failed. During the winter of 2002-3, about 5,000 adult fish were released into the river as schoolchildren and journalists looked on, but despite a well-orchestrated public relations and educational campaign, the attempt was a failure: although the fish survived, they did not reproduce.
In 2006 the Yarkon bleak joined the International Union for Conservation of Nature’s list of threatened species and was officially classified as extinct in the wild. There were none to be found – except in the university fish tanks.
Goren concluded that a lack of suitable spawning areas and shelter for juvenile fish was the reason for the initial failure. “We needed an engineering solution – to interfere with nature in a positive way,” he says.
This took the form of a 400-square-meter experimental pond near the source of the Yarkon River, which was lined with gravel and freshwater plants before being stocked with the fish. Within months, thousands of juvenile Yarkon bleak swarmed the pond.
The Yarkon bleakAnother dozen or so sites were prepared, mainly in natural river basins where the fish had previously lived. During the winter of 2006-7, Goren’s team reintroduced some 9,000 laboratory-born bleak into the Yarkon and other rivers in central and northern Israel. Subsequent surveys showed the fish multiplying at most of the release sites. Three or four generations later, the population had increased significantly.
“This project has been successful because the authorities have coordinated their approach,” says Goren, referring to the Yarkon River Authority, the Environmental Protection Ministry, the Israel Nature and Parks Authority and Tel Aviv University. “I also have to thank my students for their many voluntary hours.”
River of hope
Nobody knows the Yarkon River better than Raz, who has been at the Yarkon River Authority since 1992.
The problem began, he explains, when Israel’s Water Commission decided in the 1950s to pipe water from the country’s natural springs to the Negev desert and constructed the National Water Carrier from the wet north to the arid south.
“The spring water that used to supply the Yarkon was diverted for use as drinking water in the center of the country. All this severely depleted the river’s potential water supply – the Yarkon was left with no water.”
What water remained became steadily more contaminated. “On top of the disappearance of source water, rapid development in the catchment released effluents into the river in varying degrees,” says Raz.
Attitudes and regulation have changed over the years. Now a permanent discharge of fresh water into the upper part of the Yarkon River is guaranteed by law. Raz says the system has recuperated to the extent that the fish have sensed the mid-section of the river is now suitable for them, though the situation remains tenuous.
“The restoration is succeeding, but it just takes one day of lack of oxygen for an entire fish population to perish. They are very sensitive to pollution and an indicator of the system’s health – our canary in the coalmine.”
Raz’s conclusion is that a zero-pollution policy is the way to rejuvenate dying rivers. “It’s not so much that you need to restore – but you do need prevention. Nature is amazing, and the ecosystem can restore itself.”