From: Peter Aldhous, New Scientist
Published May 22, 2009 07:34 AM

Shellfish reefs are 'most imperilled sea habitat'

An American costal shellfish reef. These are at risk, because their importance as ecosystem engineers has been overlooked until now (Image: Getty Images / Glowimages)

Globally, 85 per cent of reefs have been lost. Destructive fishing practices, disease and coastal development threaten many of the survivors. What sounds like an apocalyptic vision of the future for the world's tropical corals is in fact a chilling assessment of the current state of reefs built in cooler waters by oysters and other bivalve shellfish.

According to a report from The Nature Conservancy (TNC), released this week at the International Marine Conservation Congress in Washington DC, shellfish reefs are the world's most imperilled marine habitats - faring worse than coral reefs and mangrove forests.
TNC's team scoured the literature, surveyed scientists and analysed fisheries statistics to assess the health of reefs in 144 bays and estuaries in 44 "ecoregions" across the globe. In most bays, shellfish reefs are down to around 10 per cent of their historical abundance. In many former strongholds - such as in North America, Europe and Australia - they are all but extinct. Reasons for the decline vary, but include overfishing, introduction of exotic species, and disturbance from human activities.

In Europe, Pacific oysters introduced for aquaculture are now moving from southern latitudes into the North Sea, where they are outcompeting native mussels - with knock-on effects for other wildlife. In the Gulf of Mexico off the south-eastern US, meanwhile, the water demands of Atlanta and other cities mean river flows are down, making estuaries more salty and allowing invading marine predators to feast on native oysters.

The good news is that oyster reefs can bounce back, if managed with care. On the east coast of Florida, years of disturbance from boat wakes have created "dead margins" of dislodged oyster shells. Volunteers led by Linda Walters of the University of Central Florida in Orlando use an amphibious mechanical digger to remove the debris, then lay down mesh mats with empty shells tied on. These encourage larvae to settle, and after 18 months the mats host the same density of oysters as a pristine reef. The team has laid 8500 mats and will set 3500 more in the next few months.