Shellfish reefs – the key to regaining the health of our estuaries ?

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Sydney rock oysters: ecosystem engineers (image courtesy of Stirling Cullenward).

ESTUARIES around the world have come under ever increasing pressure over the last century due to human population growth and its associated coastal development. Indeed, in most industrialised countries, degradation of water quality has played a massive role in the decline of estuarine productivity. Fisheries production is one of the first areas to suffer as critical fish spawning and nursery habitats are degraded or destroyed due to catchment clearing, and runoff from agriculture and urbanised areas. These changes interrupt the lifecycle of fishes and play havoc with natural food chains, often simplifying and shrinking the bottom end of the food chain leading to reduced food availability for larval or juvenile fishes at critical stages of their lifecycle. Because of this, an emphasis on restoration of estuary habitats has become an important theme in places like the USA, where scientists in the 1970’s and 80’s linked some fisheries declines with large losses of habitat, particularly seagrasses, saltmarsh and shellfish reefs.

The story in Australia is, sadly, the same. Prior to European settlement the estuaries and bays along the southern and eastern coasts of Australia were dominated by shellfish reefs, comprised mainly 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 50 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 many thousands of years. Early European explorers in the mid 1800s observed natural oyster banks at low tide in east coast estuaries with “astronomical quantity of seed oysters, stretching for miles”. Such was the abundance of oysters in the mid 1800’s, oystering was Australia’s largest colonial industry at that time, their shells even being burnt to produce lime for construction of many of Australias earliest colonial buildings. 

However, by the late 1800s the oyster industry began to run into problems. Almost without exception oyster industries in many parts of Australia peaked in the late 1800s and early 1900s. Overfishing was a problem in some areas, while along the east coast 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. Harvesting of wild oyster reefs was largely abandoned by 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.  Recent scientific sleuthing has confirmed that endemic diseases like “QX disease” in Sydney rock oysters (caused by a protozoan parasite that was first recognised in the 1970s), are expressed mainly through suppression of the oysters immune system due to declining water quality, as well as deposition of large amounts of mud which favours proliferation of some of the intermediate hosts of the parasite. On the other hand, the lethal Bonamia protozoan that infects flat oysters appears likely to have been introduced into Australia sometime in the late 19th or early 20th century, possibly via biofouling on early shipping.

The reality is, shellfish are indicator species and their gradual disappearance from our estuaries is a warning sign. If nothing is done about their loss, the inevitable declines in our estuarine fisheries will continue. Some may ask “So what? What have shellfish got to do with the quality of our fishing?” Those who target bream around oyster leases or snapper around remnant shellfish beds will already know the answer, but even they may be surprised at how critical shellfish 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 (a typical Sydney rock oyster can filter between 2 and 4 litres of water an hour, that’s 50 to nearly 100 litres a day), they also convert the energy of the sun (via them eating phytoplankton) into animal material (zooplankton in the form of oyster eggs and larvae), absorb and recycle nitrogen, and their oyster shells when structured into reefs provide quality habitat for innumerable small fish and invertebrates (crabs and shrimp). When the oysters disappear, so does a key ecosystem link tying together primary productivity and the lower end of the food chain – i.e. we lose the fish food and fish habitat, so its no surprise that fisheries productivity also suffers. Loss of oyster reefs also reduces estuary resilience, resulting in structurally changed ecosystems that are prone to nutrient enrichment, leading to domination by algae, jellyfish and microbes. Not a good thing.

While halting centuries of decline is by no means an easy task, over the last 20 years some good examples of community based shellfish restoration programs have emerged from the USA and New Zealand. These efforts are only now being replicated here in Australia. For more information on what is going on throughout the country, see https://www.shellfishrestoration.org.au/. There you will see several examples of government, scientists, fishers, traditional owners and the broader community working together towards restoration of their local shellfish populations, because they all desperately want their lost shellfish reefs back.

For the estuaries along Australia’s east coast, results from a recent review of the available information on the ecosystem benefits of Sydney rock oysters (SRO) have been illuminating. Despite the fact that the habitat value of SRO reefs has not yet been fully evaluated using modern scientific methods (possibly because so few reefs remain), there are promising preliminary data available that show significant increases in biodiversity associated with SRO reef habitat. While it may be possible to predict the identities of the fish and crustacean species that will benefit (e.g. bream, snapper, and mudcrabs in particular), the magnitude of the potential fisheries production benefits from SRO reef restoration in eastern Australian estuaries remains to be determined.

Filtration abilities of SRO have been described. An average sized adult oyster in the field typically filters between 2 and 4 litres of water per hour, but the filtration data needs to be combined with oyster density data and studies of the areal extent of historic SRO reefs to arrive at an estimation of lost filtration services for estuaries of interest. However, given we know that in many areas less than 5% of historic reefs remain, and that SRO densities of over 3000 oysters per square meter have been recorded on natural remnant reefs, it is clear our estuaries have lost significant filtration capacity. If even a small proportion of that can be restored, this could be highly beneficial to water clarity leading to further flow on benefits (such as seagrass regrowth). It is also known that inhibition of filtration occurs in SRO at comparatively low turbidities, around 10-15% of the turbidities tolerated by the introduced Pacific oyster.  This lost filtration capacity at high turbidity disadvantages the SRO in degraded estuaries, which is one of several reasons why the Pacific oyster is an invasive threat in Australia’s estuaries today. For this reason alone it appears prudent to begin shellfish reef restoration activities as soon as possible in areas where SRO reefs have been lost, in order to try to prevent further environmental degradation and increase the likelihood of successful restoration.

The ability of SRO to fix and cycle nutrients like nitrogen has been studied, with their ability to do so being highly variable due to a range of factors. However, if SRO are anything like oysters overseas, they will be able to make significant inroads into reversing nutrient pollution – scientists in the USA calculated that restoration of less than ¼ of the available bottom with oyster reefs could theoretically meet requirements for water quality targets for removal of excess nitrogen from the entire Chesapeake Bay. Of course, in Australia much more research is needed to fill in these and other data gaps including extent of fisheries production per unit area of intertidal or subtidal SRO reef, their historic contribution to estuary filtration, and quantifying the full extent of SRO roles in ecosystem function.

Despite the limited information currently available, based in part from data collected from overseas shellfish species, preliminary economic analysis suggests that estuary restoration projects in Australia that include shellfish reef restoration will yield full return on investment in c. 5 years, based on improved fisheries yield alone, ignoring the probable longer term economic benefits from increased recreational fishing opportunities, and its associated tourism (e.g. through shellfish reef restoration in recreational fishing havens). Given that there appears to be few, if any, downsides to shellfish reef restoration in the Australian context, scaling up of current small scale field experiments into meaningfully sized “natural reef” construction projects would appear to be the best way of filling in the various data gaps as soon as possible, so that decision makers can more fully assess the ecological, fisheries, socio-economic and cultural benefits of scaling up SRO reef restoration efforts in Australia. It’s certainly an exciting future ahead for estuary anglers Australia wide if these projects get some momentum.

 

 

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