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This week saw formal scientific publication of a report that produces one of the starkest conclusions I've seen about humanity's relationship with the oceans.

Globally, 85% of oyster beds have basically disappeared.

The paper, in (though not apparently on its website yet), formalises results from a study conducted a few years ago co-ordinated by , the US-based environment organisation.

The scientists behind the report () say this makes oyster beds "the most severely impacted marine habitat on the planet" - though researchers looking at the big ocean-going predators such as sharks, tuna and marlin might claim they're ahead in the race for this most undesirable of trophies.

In one sense it's not surprising. Oysters congregate in bays and estuaries - the easiest parts of the sea for humans to exploit.

From this, you might deduce that over-exploitation of oyster-beds (and indeed mussel-beds and other shellfish zones) isn't a new phenomenon; and you'd be right.

[pdf link], constructing artificial beds along inhabited parts of the Italian coast.

Excavations in south-western France yielded piles of more than one trillion oyster shells; while in the late 1800s, the UK's oyster industry supported 120,000 workers - a far cry from today.

What happened next is a story all too familiar to anyone who's looked at the history of fisheries for more than a few seconds: we industrialised, mechanising the process of excavation.

In the New World, settlement in oyster-rich areas such as increased demand for the shellfish many times over.

Oysters need a hard surface - in nature it is usually made from shells of other oysters

So fishermen increased the supply, until many of these grounds became shadows of their former glory.

Oysters need something hard to cling onto; a sea-floor of shifting sediment is no good to them.

What this means is fishing out an oyster reef basically means it won't come back.

Young oysters attach onto the shells of old ones, which are nice and hard.

When there are no shells left, there's nothing to cling onto, and even if there are any young around, they cannot survive.

Charles Clover, in that remarkable book , makes the case that parts of the North Sea owe their modern-day turbidity to the removal of beds that a century ago, were producing 100 times more oysters than today.

Oysters filter the water, clearing nutrients suspended in it; and the hardness of the bed means there's far less sediment stirred up by wave action.

"Nineteenth-century maps show oyster beds 200km (100 miles) in length on the Dutch and German side, but the last of these were fished out before the Second World War.

"Since then, there have been no oysters left to form a hard substrate across the bottom."

The implication is that if previous generations had looked after the resource better, present-day Britons (and Dutch and Germans) would not only have a much larger supply of oysters, we'd also have clearer waters for swimmers and divers to enjoy.

Without the luxury of being able to turn the clock back, two questions arise.

One is what can be done now to restore exhausted oyster beds.

The other is where the history of oyster overfishing should point us in terms of establishing regimes that protect and nurture valuable marine resources, so that our generation uses them sustainably and leaves some for the next.

Restoration projects in the Gulf of Mexico have successfully rebuilt some oyster reefs

The Nature Conservancy has pioneered the replenishment of defunct reefs and , many in the Gulf of Mexico - although there, restoration has been compromised by defences deployed against the Deepwater Horizon oil leak.

Fresh water was allowed to flow in far greater quantities than usual into the sea in an attempt to push oil away from the shoreline.

But fresh water kills marine oysters; and TNC says millions have indeed been killed along the coast.

Nevertheless, it appears that where there's money and will, replenishment can be made to work.

On the longer-term question, we've recently had , and in just over a year we'll have the second Rio Earth Summit - both events concerned largely with the sustainability of biological resources.

In terms of ocean conservation, Nagoya wasn't a huge success, with nations pledging to slap protection orders on just 10% of the marine world - although other components of the agreement there should also help conservation, such as the move from "harmful" subsidies towards an economic regime that penalises destruction and encourages sustainable use.

With , there's concern in some quarters that marine issues might be marginalised, given the attention now being focussed on the urban environment, forests, climate change, agriculture, food security, and such like.

There's no logical reason why that to happen - after all, climate change and food security are as relevant to the seas as they are to the land.

But the concern is there; and in an attempt to bring some attention to the issue, the Pew Environment Group recently launched [pdf link] that went before delegates to the first preparatory conference in the process leading up to Rio+20.

Its top line:

 "With 70% of the Earth covered by the ocean, and given the importance of the ocean as the life support system of Planet Earth, now is the time for [the UN Commission on Sustainable Development] to pay due attention to the needs of the ocean, and to the hundreds of millions of people who depend on healthy ocean ecosystems for their very survival."

One of the approaches to marine management that is working well in some places, and that environment organisations support, is giving control to local communities, allowing them to manage their resource in association with scientific advice.

Logically, oysters should be a prime candidate for this kind of approach. They nestle in inshore waters where regulations can be easily enforced, and they're relatively high-value commodities, meaning that communities who manage the fishery properly are virtually guaranteed a long-term, stable source of revenue.

One of the problems with assessing the state of our environment is that it's easy to assume what we're used to is "natural".

That's why the kind of historical study TNC has just published is so valuable - to show us what we might never have suspected we were missing, and what we might rebuild given the resources, the knowledge and the will.

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