Fish Facts

Fish Facts: Oysters – have we lost the key to healthy estuaries?

PRIOR to European settlement the estuaries and bays along the southern and eastern coasts of Australia were dominated by reefs of oysters, comprised of the Sydney rock oyster (Saccostrea glomerata) and, in more southerly regions, flat oysters (Ostrea angasi).

Early settlers described the presence of huge populations of these shellfish in intertidal regions as well as up to 12 feet deep below the low tide mark. The existence of huge number of oyster shells in middens confirmed they were a staple part of the diet of coastal aboriginals, who relied on oysters as a key food resource for thousands of years before the Europeans. Early explorers in the Moreton Bay and Tin Can Bay region in the mid 1800s observed on natural oyster banks at low tide that “the astronomical quantity of seed oysters, stretching for miles, had to be seen to be believed”. Such was the abundance of oysters in the mid 1800s, oystering was Australia’s largest colonial industry at that time; their shells even being burnt to produce lime for construction of many of Australia’s earliest colonial buildings. Dredging of subtidal oyster beds was the main method used to gather commercial quantities of oysters at that time.

However, by the late 1800s the oyster industry began to run into problems. Mass mortalities of oyster beds began to be experienced after floods, due to siltation caused by increasing erosion that occurred due to the clearing of river catchments for timber and grazing. The siltation caused the eventual extinction of most of the oyster beds below the low tide mark. Deposition of silt also favoured growth of mud worms, which infested the shells and made the oysters unmarketable. The oyster industry peaked in the early 1900s and has declined since, with harvesting of wild oyster reefs being abandoned in the early 1900s, replaced by aquaculture of oysters on sticks and racks as oyster farmers were forced to grow their oysters higher and higher up the intertidal zone in order to avoid the mudworm and keep the young oyster spat alive.

Today, in areas of northern Moreton Bay near where I live, the continued upwards compression of the distribution of wild rock oysters can be seen by observation of decades old oyster clumps. These clumps, formerly monolithic in shape, are now mushroom shaped, demonstrating that wild oysters are continuing to die from the bottom up. While the cause of death today is “QX disease” (which was first recognised in about 1970), recent scientific sleuthing has confirmed that QX disease is caused by suppression of the oyster’s immune system due to declining water quality.

Indeed, since the decline in our oysters has been happening since 1910, and because commercial harvesting of wild oysters ceased so long ago (replaced by aquaculture), continuing declines in wild oyster populations demonstrates that the problem is not overfishing, but declining water and habitat quality in our estuaries. A combination of stress due to flood waters, chemicals in runoff from the land, and sewerage contamination causes the adult oysters to die from QX, while eutrophication and siltation stops the larval and juvenile oyster spat from being able to settle on clean surfaces to complete their lifecycle. The result is upwards compression of the habitable zone for oysters (those exposed to the water and silt the longest are immunosuppressed the most, and therefore die first, and are not being replaced), a process which has accelerated markedly in correlation with the rapid decline in water quality in Moreton Bay since 2008 (as documented in the “Healthy Waterways” monitoring programme). The reality is, oysters are indicator species, and their gradual disappearance is warning us of declining water quality and, if nothing is done about it, inevitable declines in our fisheries. Indeed, if the current rate of change continues, in a decade or two we may have no oysters left in the western side of Moreton Bay – they will become ecologically extinct.

Unfortunately, this sad story of shellfish destruction is not unique to Australia. A recent paper by Beck et al. (2011) entitled “Oyster Reefs at Risk and Recommendations for Conservation, Restoration, and Management” catalogues declines in oyster reefs around the world, and what can be done about it. Those who are interested in reading more can view the paper at http://www.aibs.org/bioscience-press-releases/resources/Beck.pdf.

Some may ask “So? What have oysters got to do with the quality of our fishing?” Those who target bream around oyster leases will already know the answer, but even they may be surprised how critical oyster reefs are for the health of our estuaries. Oysters (and other filter feeding bivalves) are vitally important for healthy estuarine and bay ecosystems, because they are what scientists call ” ecosystem engineers”. Filter feeders such as oysters not only clean the water by filtration, they also convert the energy of the sun (via them eating phytoplankton) into animal material (zooplankton in the form of oyster eggs and larvae), and oyster shells, which are food and habitat for fish and invertebrates.

When oysters disappear, so does a key ecosystem link tying together primary productivity and fisheries productivity. Their loss results in degraded water quality, and potentially huge losses of abundance in fishes and invertebrates, resulting in structurally changed ecosystems that are prone to nutrient enrichment, domination by algae and microbes and increased risk of algal blooms. Not a good thing.

In summary, the historical decline in oyster reefs and ongoing upwards compression of oyster habitat, provides a biological record of long term declines in water quality and fisheries productivity of our Australian estuaries. These events are alerting us to the slow march of these ecosystems towards domination by the algal/microbial loop. While halting the decline is by no means an easy task, there are some good examples of community based restoration programs from Chesapeake Bay, east coast of USA, where they are combating virtually identical problems to those occurring in Moreton Bay and other estuaries along our east coast. In Chesapeake Bay they are not implementing marine parks, as once you understand the processes at work, it is plain to see that shutting down fishing will do nothing. Instead, the fishing industry is working with government, farmers and the entire community on agreed restoration goals for fisheries, habitat and water quality, and they are working on reducing sediment loads, reducing nutrient loads, and controlling disease in their oyster populations, because they desperately want their oysters back.

Check out http://www.chesapeakebay.net/bayrestoration.aspx?menuitem=13989 for more information on the Chesapeake Bay program.

This is exactly the sort of environmental rehabilitation we need for our estuarine and inshore areas here in Australia. Who is willing to step up and make it happen here?

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