People pressure and Port Phillip Bay

by Mike Jacques

Port Phillip Bay is integral to the lifestyle of millions of Australians. Since European settlement we have been fighting a seesawing battle to deal with all the damage modernity has done to the “The Bay”.

Prehistory

The area around what is now Melbourne was until relatively recently a vast river plain, full of wetlands and lakes. The Yarra flowed down what is now the middle of the bay and formed a lake in the southern reaches near the sea. A big rocky outcrop (now called the Heads) separated the lake from the sea. After rain, the lake would spill over into the sea and gradually carved a river channel through the narrow rocky gorge. Several Aboriginal tribes were hunting across the river valley for perhaps 30,000 thousand years before the climate started to slowly change. Only 7,000 to 10,000 years ago the last Ice Age ended and sea levels gradually rose above the Heads. Sea water increasingly flooded the lake after heavy weather and kept on rising until the sea completely drowned the low-lying river valley. Pt Phillip Bay was created from this flooding.

Port Phillip Bay is now one of the world’s larger enclosed waterways with a surface area of almost 2,000 square kilometres and a total volume of 26 cubic kilometres. The deepest portion is only 24 metres, and half the region is shallower than 8 m. Although it is shallow, most of the bay was navigable by the large sailing ships that arrived from Europe in the nineteenth century.

Aboriginal people were soon driven from the land to make way for farms. Large areas of land were completely cleared and erosion of soil into the bay gradually increased. Before the 1850s gold rush, the population of the catchment had grown to only 75,000 but the mineral discoveries spurred on both industry and a booming population. All those new people started pouring untreated waste into the bay. The catchment draining into Port Phillip Bay is now receiving the run-off from a major urbanised area of almost 10,000 square kilometres occupied by a population of about 3.7 million people. One of the big challenges is that the enclosed shallow water mass is poorly flushed. The average exchange rate for Bay waters is once every 12 months. It flushes more frequently in the southern part closest to Bass Strait and less frequently in the north and in Corio Bay, where the pollution is often the heaviest. Like many other bays in Australia, the mid 1970s saw a peak in industrial pollution and also a renewed commitment to improve water quality.

Sewerage and stormwater

The major marine pollution issues are sediment loads in stormwater and the nutrient levels from stormwater and treatment plants entering the Bay. By the close of the 19th century, the waste and sewage pollution of the Yarra River were causing serious health issues. This led the Melbourne Metropolitan Board of Works to start on a major sewerage program at Werribee in 1893, with the first Melbourne homes being connected to a waste treatment plant in 1897. The technology didn’t always keep pace with demand and by the 1970s the Bay was often described as ‘full of poo’. Then there was a major upgrading of sewage treatment. Nutrient levels have been reasonably stable since 1984, which is a commendable achievement in a rapidly growing city. More recently, in 2005, new technology was introduced at the Western Treatment Plant which cut nitrogen output into the bay by more than 500 tonnes a year. A new target calls for a 1000 tonne reduction per annum.

While we can fairly readily pour money into treating what comes out of a single sewerage pipe, Melbourne’s thousands of kilometres of stormwater drain are a problem on a grander scale. Stormwater drains dump nutrients, litter, debris and silt into the Bay. A stormwater outfall will now often have more harmful nasties in it than a sewerage outfall, especially after summer storms. A sudden flushing of the drain network will wash oil off roads, collect runoff from illegal or broken sewerage connections and collect litter. This often leads to a toxic algal bloom at a nearby creek or beach that can cause beach closures. It’s also no fun to stroll along the beach dodging rafts of litter washed out of nearby creeks and stormwater outfalls. Building and maintaining sediment and litter traps, or trying to treat massive volumes of runoff after a storm, could end up being a huge expense for struggling councils and we are generally reluctant to pay for it in big rates hikes. Ironically, our big water bills have helped by making us collect water for the garden. This has been reducing flows of stormwater nutrients and sediments to the marine environment.

Heavy metals

Our industry is a lot cleaner than it once was, and most of our troubles with industrial pollution are about managing the legacy of past neglect. Unregulated outfalls of nasty chemicals in the last 150 years have contaminated sediments in the Bay and this has entered the marine food chain, especially in the northern part of the Bay, and at Geelong. In the 1980s, mercury, cadmium, lead, Dieldrin, DDT, DDD, and DDE were all found in the Bay. Apart from poisoning the humans who ate the fish, the fish themselves can also have problems with conditions such as fin-rot, neoplasms, lesions, diminished reproduction success and lens cataracts. Mercury concentrations in flathead declined after 1990, when new EPA regulations required industrial effluent to be flushed into the sewer rather than dumped straight into the stormwater drain. In the 1990s, mercury contamination in sand flathead in Port Phillip Bay still exceeded the National Health & Medical Research Council (NHMRC) limit1, but it began to fall. The 1996 CSIRO Port Phillip Bay Environmental Study found that a decline in mercury concentrations has been sustained. The northern end of the bay is still not in great shape. A long-winded government health advisory doesn’t quite say “don’t eat fish in the lower Yarra and Maribyrnong rivers”. I think they should stop mucking around and call the pamphlet “Fishing there!, are you mad?” The main problem in this area is the polychlorinated biphenyls (PCBs) in the sediment. PCBs were used from the 1930s to the 1970s in industrial products. They have now been phased out, but they stay in the environment for a long time. They concentrate in fish tissue and are worse in some species like eels and bream. There have been problems with the fish population in other areas of the bay too. Studies in 1968-71 and 1996 showed a decline in mud ghost shrimp and sand flathead populations. Flathead were once considered to be the most abundant fish species in Port Phillip Bay, but their numbers are estimated to have fallen by 80% since the 1970s. Since the 1970s growth rates in surviving fish have also slowed, particularly in the first four years of life. Some people cite pollution effects as the cause of a drop in fish numbers, but in a bay so heavily fished, I suspect it would be hard to separate pollution impacts from overfishing impacts.

Dredging

If you have a heavy metal problem then you need to avoid stirring up the bottom and remobilising all those buried chemicals. However, more economic growth means bigger ships. We regularly need to deepen Pt Phillip’s shallow shipping channels. Maintenance dredging of the Yarra every 2-3 years, and dumping of sediments in the northern bay, has been ongoing since settlement and has been very active since the 1950s. In 2009, a new six square km Dredge Material Ground (DMG) was constructed to contain 4 million m3 of contaminated dredged sediments. Dumping of dredged sediments taken from the Port of Melbourne to the spoil grounds has resulted in elevated concentrations of cadmium, lead, zinc, and other metals at the dump. The proposal caused a public protest and the EPA monitoring program was expanded for the duration of this Channel Deepening Project at the DMG, and for two years following its completion (from 2008 to 2011). A CSIRO Study stated that spoil dumping shouldn’t result in contamination of surrounding areas, but contaminants have been elevated at times and locals voiced their suspicions that the spoil dumping was involved. It is likely that the local marine life doesn’t react well to these remobilised contaminants. The Environmental Study of Port Phillip Bay in 1968-71 found ghost shrimps to be the second most common bottom-dwelling species in sediments around the Bay, including near the present day DMG. The species was not reported there at all in 2006, but it was still common at the southeastern dredge material ground (offshore of Mt Martha) where the shrimps still get buried, but only by ‘clean’ dredge material from the southern bay.

Fishing

Pt Phillip has a long but declining history as a major commercial fishing ground. The effects of industrialisation have pushed usable fish stocks into the southern bay. More recently, this has been followed increasing pressure from competing recreational and environmental interests. There have been grounds for concerns about past commercial fishing activity. When the Port Phillip Bay scallop dredge fishery commenced in 1963 there was no regular scientific monitoring of the Bay seabed, so we don’t know what the bottom was like before this heavy fishing pressure. Dredges raked over most of the bay in depths greater than 10 metres, causing booms and busts in scallop stocks until the fishery was closed in 1997. This closure decision was the first time a really large commercial fishery had been closed by a single political decision unrelated to stock sustainability. It was driven by the growing recreational and tourism use of the bay, especially dredging’s perceived impact on recreational fishing. Recreational fishermen were particularly alarmed by declining snapper catches. Scientists had also suggested that scallop dredging was reducing the ability of the seabed to process the massive nutrient in-flows from the city. Studies did show that dredging activity was removing 20-30% of bottom-dwelling life each pass, causing some small animal species to crash and others to boom in the vacated space. These numerous burrowing species can be very important in turning over the sediments to reprocess nutrients. While the impacts of dredging may well have been significant, not all animals were affected. The volcano-like mounds of ghost shrimp (up to 10cm high) were flattened by dredges but the shrimp themselves survived dredging and mounds were rapidly rebuilt. Now that dredging has ceased the bay may well recover some of its former ‘natural’ benthic diversity. This doesn’t mean table fish stocks will recover, because the growing pressure on fish stocks in recent decades hasn’t been from the commercial sector. There are 800,000 recreational fishers just in Victoria and most won’t acknowledge that their sector has more than a minor impact on fish stocks. Dr Mark Norman’s recent study confirmed that in Port Phillip Bay, boat-based recreational fishers can catch 2.7 million fish annually. This is almost as large as the commercial catch. Snapper has been the fishery most fought over, but here the recreational harvest arguably has a greater impact on snapper stocks. Commercial snapper harvests are slightly larger by weight, but recreational fishers will catch many more smaller fish. They caught three times more fish (22,000 fish recreationally versus 7,000 commercially in a four month survey). These figures are likely to be underestimated and don’t include night fishing, shore fishing, charter fishing and catch landed outside the survey areas. Recreational fishing pressures are also growing because of increased participation, more efficient gear, and increasing access to areas. According to a CSIRO study the bottom is being denuded of fish, except for stingarees that no-one is eating. “Increased fishing pressure is the most likely explanation for declines in several important commercial and recreational species.”

Impacts of introduced species

To add to the Bay’s woes, we are importing many species that shouldn’t be there, especially on the hulls and in the ballast water of oceangoing ships. Over 3000 commercial ships visited the Port during 1998/99, moving to or from 200 different international and domestic ports. The Port of Melbourne is also highly modified and disturbed, which may makes it more susceptible to successful introductions of pest species. Apart from pollution, the area has been reclaimed and dredged. The delta of the Yarra River was once 90 km2 but this has since been halved. Much of the original aquatic fauna and flora of the area has probably been lost. In this sheltered and nutrient-rich ecological vacuum, pest species can flourish and get a head start. Over 100 species are known to have been introduced to Port Phillip Bay. The major exotic species are European shore crabs, Pacific sea stars, European clams, Asian date mussels, European fan worms, Japanese undaria seaweed, yellowfin gobys, northern sea squirt Ascidiella aspersa and many other small species. Many have become abundant and are contributing to profound changes to the ecology of the Bay. A few North Pacific Seastars (Asterias amurensis) were recorded in the Bay in the mid 1990s. Now they are common in the northern bay. This species virtually eliminated native molluscs when introduced to the Derwent River in the 1980s. These pests outcompete some native animals, including many that have key roles in the ecosystem, like perhaps mud ghost shrimps. They don’t explain every change that has occurred in the Bay. The decline of species like sand flathead was identified well before some of these major pest species were recorded in the Bay. Pests, like weeds on land, aren’t all the same and some do little harm. They can also fall prey to an adaptive predator. Two recently introduced species (European clam Corbula gibba and Pyromaia tuberculata) have become important in the diets of fish as other traditional food sources dry up. Pests generally don’t like competing with better adapted native animals in clean waterways. The news is often bad, but it isn’t hopeless.

Summary

Port Phillip is a central part of the Melbourne lifestyle and, particularly in the southern bay, it’s still a healthy and thriving aquatic playground. Having said that it still has some serious problems. A lot of good work has been done to reverse those impacts, but sometimes we don’t own up to an impact when it interferes with our ‘freedom’ to keep doing something unsustainable. Blaming one source or the other for the ‘major’ impacts on Port Phillip Bay ignores the way our impacts interact and the practicalities of modern life in a place where millions of people live. We can’t ever turn the clock back to that pre-settlement pristine drowned river valley. Closing the port, and that is what no dredging at all would eventually mean, isn’t a viable option. Substantially stopping fishing isn’t necessary, or likely to get popular support. Even the accidental litter from millions of people would be enough to mar our waterways. That doesn’t mean we can’t manage what we do now more intelligently, to the maximum extent of our available resources and technology.