The rain may not be lashing down, but at 50 knots it feels like needles shooting onto your face.

Thankfully my glasses take the brunt of the abuse, while in front of me at the controls Tam Sneddon – one of the Environment Agency's water bailiffs – takes the bulk of it.

A midweek night and the Environment Agency is making its usual patrols of Plymouth Sound, its rivers and inlets, on the hunt for criminals.

Tam and his colleague Peter Gordon are joined by Pc Ryan Canning of Plymouth police. A patrol officer, he has the additional role of wildlife crime officer, with a specialism in migratory fish.

And fish, I learn on a rainy night in Plymouth, is big business for criminals.

The Nimrod, a 6.5m long rigid inflatable boat, patrols Plymouth's waters looking for poachers. While the phrase may evoke romantic notions of crafty but dashing folk, the reality is more akin to your bog-standard car-thief or burglar who cares nothing for ecology, fish stocks or personal property.

A bylaw means a line drawn across the Sound determines the point beyond which it is illegal to fish with a net or catch bass from a boat. Not the 99p bamboo net your children use, but nets dozens or even hundreds of metres long, stretched across channels to catch the increasingly expensive bass, sea trout, grey mullet or even salmon which Plymouth's rivers are quietly feted for.

Pc Canning explained that a few years ago it would be £2 per pound for salmon or trout. But that's gone up, to £10 or £14 per pound. A net across an inlet, about 100 metres, if put out during spawning season, could make a thousand or so in a night.

"The fish is then sold the next day," he said. "Salmon and sea trout can't be landed legally anywhere in Devon and Cornwall, even by a registered boat. They're the ones sold at the back doors.

"It's the bass and mullet that get netted in the estuaries that get landed 'legally' because the registered fisherman never admit to having caught them in a river.

"The fish merchants don't know that they haven't been caught outside the line across the Sound. Bass, mullet, salmon and sea trout all frequent the same estuaries so when the poachers shoot a net, they can't really discriminate – all types of fish get meshed up. The bass and mullet go to the fish quay or restaurants 'legally' and the salmon and sea trout go via the back door.

"Fish like bass can take up to five years to mature fully, so taking out a load like that could wipe out that year's fish."

While pausing somewhere along the Lynher River I'm told tales of notorious local poachers who regularly make a thousand pounds a week, or the really impressive ones who can make that in a night. They play a major part in the dwindling stocks of fish, but also have a hand in other criminal activity on – and off – the water.

Over time, police and the water bailiffs noticed how those who were intent on semi-industrial sized poaching were also turning their hand to thievery. Outboard motors, navigation equipment, anything left on a moored vessel would be lifted. Sometimes entire boats. When caught by police, the chances were they were also known to the Environment Agency for their poaching.

The cross-over has seen a united front between the EA and Devon and Cornwall police, with regular patrols by the two organisations, mixing and supporting each others' regulatory powers.

Pc Canning said: "The original idea of joint patrols was down to me, Tam, Pete and Martin Williams, head of fisheries enforcement at the EA. We knew we were targeting the same people so it made sense to work together, which we've done for around two years."

The wildlife side of their work drove the collaboration, but it's now spawned a much larger scheme, called Operation Bligh, which has involved police boats and divers from the Tactical Aid Group and EA boats up and down the coastline.

Pc Canning said: "We have powers of stop and search and targeting certain kinds of criminality, while the EA have powers which can see them confiscate boats, trailers and even the cars that pull them.

"This kind of criminality isn't isolated. The people who poach fish this way at night, invariably we find also don't pay their car insurance, are untaxed, will steal from other boats given the chance. Their criminality is a way of life and they carry it into every part of their lives."

The Nimrod – based at Mayflower Marina – has twin 90hp motors which give it the kind of "poke" that leaves you holding on very tight when it hits another vessel's bow wave.

Tam and Peter are at the controls as we roar across the Sound, nipping to each of the likely points where poachers operate. Intelligence gathered by land-based officers suggest one poacher has "gone to work" but could be at any number of locations in either Devon or Cornwall, let alone Plymouth.

Ribs like Nimrod help the EA and police penetrate inshore waters as well as vast natural harbours like the Sound, although other boats are on hand to cover deeper water, and land-based officers check a host of quiet slipways.

Pc Canning said: "There is the need to protect fish stocks and the valuable nurseries which Plymouth's rivers and estuaries possess, as well as target the criminals and variety of crimes they commit.

"We want to raise awareness, to the boating community, to fishermen and anglers, to those who live and work by the water's edge or regularly use the waters.

"What the poachers do is dishonest, but it's also impactive on the wildlife and the public.

"When we've picked up poachers, we find they're involved in an whole range of criminal offences."

At intervals Tam regales us with the variety of poaching techniques, including the art of "whiffing" with a bamboo pole in order to catch bass from a boat – again classed as illegal in the estuaries around Plymouth.

Other poachers who target salmon and sea trout in the freshwater rivers around Plymouth are less delicate, and he explains some are happy to use poisons, bleach and even cement powder to slaughter their catch, damaging all manner of aquatic life. Tam ominously noted: "One poacher we nabbed had a diary. In it he'd boasted of his large catches, writing 'if we carry on like this we will wipe out all the salmon in this river'.

"Hopefully, if we make people more aware of this illegal fishing, they will be more likely to call us if they see anything suspicious."