A ban on fishing around a colony of threatened penguins in South Africa has brought swift benefits to the beleaguered birds, marine biologists reported on Wednesday.

The population of the African penguin (Spheniscus demersus) fell by 60 percent between 2001 and 2009, driven by a plunge in anchovies and sardines, with climate change and purse-seine trawling fingered as the main culprits.

Of the 26,000 surviving pairs, the biggest colony is on St. Croix Island in Algoa Bay, on the eastern coast of South Africa.

There, experts tagged adult birds and monitored them before and after a ban on purse-seine fishing that took effect in a 20-kilometre (12-mile) radius from January 2009.

Before the ban, 75 percent of the penguins had to venture beyond 20 kilometres to find food, they found.

Three months after trawling was stopped, 70 percent of the birds were feeding within the 20-km zone, tucking into fish that now became available.

Fifty kilometers away at Bird Island, there is also a large colony of African penguins, but fishing there is still permitted. The birds are still doing long-haul swims to find food, the investigators found.

The finding is important because the St. Croix birds have decreased their daily energy expenditure by 40 percent, "enabling them to invest energy in reproduction," said David Gremillet of France's National Centre for Scientific Research (CNRS).

It is too early to say whether the penguins will have more chicks and how many of the youngsters will survive until adulthood.

"It's something that has to be studied over the long term," Gremillet told AFP. The species is likely to be classified as "endangered" this month because of the sharp decline in the last decade.

The study, published by Britain's Royal Society in the journal Biology Letters, provides the first evidence about how quickly a threatened species can rebound when it is given a little help.

"A marine protected area closed to fisheries can have immediate benefits for an endangered marine top predator," say the authors.

Purse-seine fishing entails dropping a balloon-shaped net, or purse, to a certain depth and then raising it underneath shoals of fish that swim near the surface. The technique prevents the fish from swimming down to avoid capture.

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