Several shark species have declined steeply in the north-west Atlantic over the last 15 years, scientists say.

The populations of some sharks have fallen to less than a quarter of their former size.

With sharks high in the marine food chain, there is concern their fate may affect other creatures.

The scientists say marine reserves will not be enough to save the sharks, which need the sort of protection given to other large marine predators.

The scientists, from Dalhousie University, Nova Scotia, Canada, report their findings in the magazine Science.

They analysed catch data in the north-west Atlantic for six oceanic species from 1986 (and for a further three, including coastal sharks, from 1992) to 2000.

They say: "We estimate that all recorded shark species, with the exception of makos, have declined by more than 50% in the past 8 to 15 years."

Unsustainable losses

While fish like cod and haddock can reproduce fairly fast, possibly by up to 30-40% a year, sharks breed much more slowly.

Some take 15 years or longer to reach sexual maturity, and many have a long gestation period.

The spiny dogfish, one half of a British staple take-away food, fish and chips, under its other names of dogfish or rock salmon, has a 22-month gestation.

Many sharks have an annual replacement rate of only 3-4%, too low to make good the losses they are sustaining.

The Dalhousie team report a number of individual species' loss rates from 1986 to 2000:

• hammerheads: 89%
• white sharks: 79%
• tiger sharks: 65%
• coastal species (from 1992): 61%
• threshers: 80%
• blue sharks: 60%

The authors say they believe populations of thresher sharks "have collapsed" - in other words, they are close to commercial extinction.

They write: "Our results show that overfishing is threatening large coastal and oceanic sharks in the north-west Atlantic.

"The large and rapid declines we document are in addition to substantial historical reductions.

"Overexploitation of elasmobranchs (sharks, skates and rays) is known to have already nearly eliminated two skate species from much of their ranges.

Moving the problem

"The magnitude of the declines estimated here suggests that several sharks may also now be at risk of large-scale extirpation."

The authors used models to analyse the implications of large-scale marine reserves for shark conservation, and say the results show that reserves "can indirectly cause harm if fishing effort is merely displaced".

They say: "Conservation initiatives must explicitly consider impacts on the whole community of species.

"Emphasis on single-species conservation, without controlling effort, simply shifts pressure from one threatened species to another and may actually jeopardise biodiversity."

Dr Rachel Cavanagh is programme officer for the shark specialist group of the World Conservation Union (IUCN).

Warning sign

She told BBC News Online: "The outlook for sharks is bleak worldwide. And the north-west Atlantic and Australia are probably the best-studied and best-managed shark areas there are.

"If we're looking at massive declines there, it's going to be as bad or even worse in other areas."

Clive James, of the UK's Shark Trust, told BBC News Online: "These findings are very much in line with what we're hearing from elsewhere.

"Because of their slow breeding rates, sharks are biologically unable to withstand even a limited amount of exploitation, let alone what they're facing now.

"The problem is simple, straightforward over-exploitation. There's a real danger some species will become extinct if we do not act."