Once, the poacher was a man with big pockets in his raincoat sneaking on to an aristocrat's land to steal game for his family pot. Now he is likely to be part of a gang from town, in it for hard cash, rampaging through the countryside with guns, crossbows or snares.

Police in rural areas across Britain are reporting a dramatic increase in poaching, as the rise in food prices and the reality of recession increases the temptation to deal in stolen venison, salmon, or rarer meat and fish.

Organised and sometimes armed gangs of poachers are accused of behaving dangerously, intimidating residents, causing damage to crops or to gates and fences. Squads have also been out in the countryside "lamping", poachers using lights to transfix animals.

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There have even been reports of drive-by poachers, aiming guns through the open windows of moving vehicles to pick off deer or other game. Others go about their work more discreetly, knowing that in some parts of the countryside, if they are careful, their activities can pass unnoticed for weeks.

Animals from the smallest shellfish to stags are in danger. Last month, a survey team who visited a river in a remote part of Scotland, were shocked to find that poachers had stolen mussels, with a potential value of nearly £20,000, from the river bed. They were prised from the bed of the South Esk river, near Brechin, Angus. Freshwater mussels have been protected by law since 1998. To kill, injure or disturb the habitat of a single mussel is punishable by a fine of £10,000, implying that the South Esk poachers, if caught, could face a fine of £1.3m.

A single pearl can fetch £150, and a necklace can be worth £15,000, but the poachers may have difficulty making that sort of money, because jewellers are banned by law from buying loose pearls.

"This was not an opportunistic half-hour in the river," Peter Cosgrove, the scientist in charge of the survey team that discovered the crime, said. "We worked hard to find these mussel beds and that suggests the pearl fishers must have made a similar effort. They would have had to have systematically been in the river for many days. In all my years doing this type of survey, this might be the largest kill I've seen."

Last week, rural landowners and businesses in Scotland launched a new campaign to get the public to report instances of poaching or illegal hare or deer coursing. Scotland's National Wildlife Crime Unit has records of 335 incidents of poaching in 18 months, with the numbers now running at more than 20 a month. During August, the number of recorded incidents was 22, almost double the previous year's figure.

"Poaching is particularly common on the urban fringes," a spokesman for the Scottish Rural Property and Business Association said. "It's idiots going out into the country with guns, or crossbows. And there are snares being set by idiots, really for the hell of it. Salmon poaching is on the increase. This is not a case of stealing one for the pot: it's on an industrial scale. There are people trawling fish traps in the river beds that will hold a dozen or two dozen fish."

In Hampshire, poachers have struck 14 times in the past month, and police have met local residents and warned them about taking the law into their own hands. Incidents are now three times as common as they were a year ago. One gamekeeper chased poachers from a farm near Corhampton at speeds of up to 80mph, until they rammed his vehicle to ensure their escape. The number of incidents reported in the area last month was three times the figure for the same period in 2007. Charlie Flindt, of Manor Farm, near Alresford, in Hampshire, said poachers were stealing deer, pheasants, partridges, and chickens. Two Shetland ponies have also been stolen, though unlike the stolen game, they may not be destined for the cooking pot.

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"I was born in that house across the yard, and for the first time in 47 years I'm keeping a baseball bat beside the back door," he said recently. "I'm having to do nightly sweeps of the fields. We are angry, and we are very scared, and someone is going to get hurt at this rate."

In Wales, the Welsh Assembly launched an environment action plan last month to combat wildlife crime, a category that covers poaching and other offences such as the killing of birds of prey, or badger-baiting, which used to be prevalent in South Wales although the culprits seem to have shifted their activities to west Carmarthenshire or Gloucestershire.

One of the more unusual prosecutions brought by wildlife officers in Wales involved the theft of 200,000 wild bluebell bulbs in North Wales, for which two men were arrested and fined £7,000.

Yorkshire police have also warned landowners to be on the look-out after complaints that poaching is on the increase. This month, police stopped a van in Wetherby High Street. In it they found four men, all from Bradford, dressed in camouflage, with three lurcher dogs, lighting equipment, and a large number of freshly caught rabbits and hares. The men are to appear before magistrates.

In Henley-on-Thames, in south Oxfordshire, the local MP, John Howell, has asked police to meet farmers and gamekeepers furious about the increase in poaching, after several dead hares and vehicles tracks were found on two farms. The owner of the farms, Michael Colston, said recently: "What I really fear is that with 10 attacks in just three weeks, some people are beginning to talk about taking the law into their own hands. Something could very well happen. I am very much against this, of course, but these gamekeepers, farmers and residents are terrified."

And it is not just rural areas that are being targeted. On Hampstead Heath, in north London, a woman out walking her dog last week had a scare when the animal emerged from a dip in the pond with blood pouring from his mouth, after an illegal three-prong hook left by poachers had caught in its tongue.

Dr Colin Shedden, the Scottish director of the British Association for Shooting and Conservation said: "Poaching has become an increasingly common problem over the past 10 years. Such offences are not usually committed by local people, but more likely by serious criminals travelling some distance.

"There are many aspects to these crimes, and poaching can involve different species, especially those with high resale value such as salmon and venison from deer."

The president of the Country Land and Business Association, Henry Aubrey-Fletcher, said deer-poaching posed a particular danger. "Many poachers are inexperienced shots and there are risks of the animal being only wounded and not killed outright," he said. "If they are shooting at night and on ground they do not know there is also a raised risk of accidentally shooting someone or something else by mistake."

Douglas McAdam, the chief executive of the Scottish Rural Property and Business Association, said: "Poaching and illegal coursing ranges from wide-scale criminal activity undertaken on a commercial scale, to one-off incidents often involving animal cruelty on the urban fringe. Such offences interfere with the activities of the land manager and are often coupled with other criminal action and rural crime such as theft or vandalism."