Aquaculture for all

Cracking the Demise of Oysters

Health Biosecurity Welfare +10 more

Oysters were once a dominant feature in temperate estuaries all around the world, but centuries of intensive fishing has decimated their populations. Now, renewed awareness of their importance has gained limelight, and aquaculture may play an essential role in their survival, writes Adam Anson, TheFishSite.

Photo: StockXchng

It is hard to imagine the size of some of the great oyster reefs that once thrived on our planet. They lived in an abundance unparalleled by today's measures, stretching miles down coasts and weighing millions of tonnes. But, due to centuries of intensive fisheries extraction and coastal degradation, we may never see these majestic reefs again.

Following the great demise of the oyster reefs comes a complete lack of public awareness that only further exasperates the problem and makes finding the solution that much more difficult. Unfortunately, time is of the essence as many of this world's our oyster reefs are already near or past the point of functional extinction.

Good food and dynamic industries are not the only things offered up by oysters. They make up an essential part of many aquatic ecosystems, filtering suspended particles from the water column and removing excess nutrients from coastal bays through denitrification. This creates habitat suitable for fish and seagrasses to thrive and reduces the likelihood of harmful algal blooms. Oysters also serve as natural coastal buffers, provide nursery habitat for fish that are valuable to coastal economies and increase water clarity. Yet for all the life they eminate, they remain mysteriously prone to sudden and unexpected ailments.

“A major problem with bivalve populations is determining whether they’re healthy, sick or in-between,” said Dr. Helen Gurney-Smith, Research Scientist at Vancouver Island University’s Centre for Shellfish Research (CSR). This is one of the specific challenges to shellfish aquaculture. Marine bivalves such as mussels, oysters, clams and scallops may appear healthy one day, but be dead a few weeks later.”

A new report, conducted by the Nature Conservancy and written by scientists across five continents, has -- for the first time in history -- comprehensively evaluated the global state of shellfish. Estimates were taken for oyster reefs in 144 estuaries and 40 ecoregions around the world. Their conclusions did not bode well for oysters. According to the report, they are one of, if not the most, severely impacted marine habitats on the planet.

A Global Assessment

Based on these data, the researchers concluded that oyster reefs have declined by more than 90 per cent from historic levels, in 70 per cent of bays and 63 per cent of marine ecoregions, whilst they are functionally extinct (more than 99 per cent loss) in 37 per cent of estuaries and 28 per cent of ecoregions.

"Although oyster reefs are beginning to receive some conservation attention, they remain an obscure ecosystem component and still are vanishing at sometimes alarming rates," said the report. Globally, an estimated 85 per cent of oyster reefs have been lost and with them some of the most prolific oyster industries.

"Just as important, perhaps, is the need to develop opportunities to support, pay or trade for these services from shellfish reefs."
Nature Conservancy

"Reefs are functionally extinct in many areas, particularly in North America, Australia and Europe," says the report. "Most of the world’s remaining wild capture of oysters comes from only five regions on the east coast of North America, and in most of these regions, oyster reefs are in poor condition or worse."

The driving forces behind the decline of oyster reefs include destructive fishing practices, coastal over-development, and associated effects of upstream activities such as altered river flows, dams, poorly managed agriculture and poor water quality. "Many of these threats have been around for decades and even centuries, but today there are two main issues that impede oyster recovery efforts," says the report.

The first is a widespread lack of awareness to the importance of oysters and that shellfish habitats are in trouble and the second is the common perception that as native shellfish decline, non-native shellfish can act as an ecologically suitable replacement. "Unfortunately, previous introductions of non-native oysters and other shellfish into new areas have spread disease and have had other negative impacts on the surrounding environment," says the report..

An accompanying factsheet to the report recommends a major shift in management to conserve and restore native oyster reefs, using realistic and cost-effective solutions. The key recommendations of the report are: improve protection for reefs of native shellfish; restore and recover reefs back to functioning ecosystems that provide multiple services to humans; manage fisheries sustainability for ecosystems and livelihoods; stop the intentional introduction and spread of non-native shellfish; and improve water quality in bays and estuaries.

The Future of Oyster Fisheries

Another new report, from Environmental Research, reveals a different type of man-made threat. "Ocean acidification is a side effect of human industrial activity," says Environmental Research. "As atmospheric carbon dioxide levels rise, more carbon dioxide is dissolved in ocean surface waters, producing carbonic acid and pushing up the acidity of the water." This change in environment is bad news for a number of ocean species. In particular calcifying organisms, such as coral, many plankton and molluscs, will find it increasingly difficult to construct their shells or skeletons.

According to Environmental Research, mollusc sales currently generate about US$750 million per year in the US, nearly 20 per cent of total US fisheries revenue. US Losses could amount to US$1.4 billion over the next 50 years, says their report. However, if oyster preservation strategies are taken seriously, many competing industries may be eliminated, opening a new window of opportunity for oyster farmers.

Oyster farms could be established in areas where water contamination is high, receiving government support. An idea has also been suggested to have some companies fund nearby oyster farms as a nutrient trade off to excessive water pollution levels. In Europe costs to build a sewage treatment were deferred by supporting the growth of mussels to filter those waters, the same idea can be applied to oysters.

According to the Nature Conservancy, oyster reefs should be managed in ways that consider the value of these systems to surrounding coastal areas, beyond harvests. "Just as important, perhaps, is the need to develop opportunities to support, pay or trade for these services from shellfish reefs."

Adam Anson
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