Key Takeaways:
- Sydney has experienced a series of shark attacks in a short period, with experts attributing the incidents to a combination of factors, including heavy rainfall, warm ocean temperatures, and poor water quality.
- The city’s unique and outdated sewage management system, which only uses primary treatment, may be contributing to the problem by releasing effluent directly into the ocean.
- Experts suggest that the system’s inability to handle heavy rainfall and stormwater overflows may be flushing pollutants and nutrients into the ocean, attracting shark prey and ultimately sharks themselves.
- The use of deepwater ocean outfalls, which pump effluent 2-4km out to sea, may also be playing a role in the increased shark activity.
- Climate change is exacerbating the issue, with warmer ocean temperatures and changing currents trapping nutrient-rich water close to the coast.
Introduction to the Problem
Sydneysiders have been warned about the risks of shark attacks and poor water quality after a series of incidents in recent weeks. The city’s unique sewage management system, which has been linked to "debris balls" washing up on beaches, may be contributing to the problem. Experts say that dirty waters can attract more sharks, as the nutrients attract plankton, which in turn attract the fish that sharks feed on. With the recent shark attacks, the question on everyone’s mind is whether there is a link between Sydney’s sewerage system and the increased shark activity.
The Shark Attacks
The shark attacks occurred on January 18, 19, and 20 at Vaucluse, Dee Why, and Manly in Sydney, and on the mid-north coast. The NSW minister for primary industries and regional development, Tara Moriarty, described the attacks as "unprecedented." Dr. Amy Smoothey at the NSW Department of Primary Industries and Regional Development analyzed images of the bites from the recent attacks and concluded that bull sharks were probably involved. Bull sharks prefer water temperatures above 19C and tend to arrive in Sydney in October, with their numbers peaking in January and February.
The "Perfect Storm" of Factors
Experts have said that the attacks were the result of a "perfect storm" of factors, including more people in the water during summer, a rise in ocean temperatures caused by global heating, and murky waters after weekend storms. The intense rainfall seen in Sydney last weekend, where some areas received more than 50mm in the space of a few hours, is believed to have played a key role in the attacks. The rainfall flushed out brackish water from estuaries into the ocean, which suits bull sharks because they can hunt in low visibility. The murky waters also attract "bait" fish, who feed on microorganisms such as plankton, which themselves thrive on nutrients and bacteria flushed from river, stormwater, and sewage overflows.
The Role of Sydney’s Sewerage System
Prof. Ian Wright, a water scientist at Western Sydney University, believes that Sydney’s unique sewerage system may have played a part in the shark attacks. The system only uses primary treatment, which removes solids before releasing the remaining effluent directly into the ocean. Other cities use secondary treatment, which involves biological processes such as using bacteria to break down organic material. Wright said that stormwater could have flushed Sydney’s diffuse pollution and "sewage from overflows from a sewerage system that does need to be upgraded" into the ocean, ultimately driving shark activity.
The Impact of Deepwater Ocean Outfalls
Another unique feature of Sydney’s sewerage system is its deepwater ocean outfalls, built in the late 1980s and early 1990s. Three treatment plants, at Malabar, Bondi, and North Head, now pump 80% of the city’s effluent through pipes which extend between 2 to 4km out to sea. While some experts believe that the outfalls are not directly responsible for the shark attacks, others suggest that current coastal conditions could mean that the outfalls are having an impact. Climate change means that the eastern Australian current, which normally veers away north of Sydney, is staying close to the coast, trapping nutrient-rich water and potentially the effluent discharged from the outfalls.
What Don’t We Know
Despite the warnings and concerns, there are still many unanswered questions about the link between Sydney’s sewerage system and the shark attacks. Sydney Water has referred questions to the NSW minister for water, Rose Jackson, who has cautioned against over-simplifying the issue as directly linked to the wastewater network. Jackson said that while heavy rainfall is a common occurrence, shark attacks in Sydney and NSW waters are very rare. Sydney Water is undertaking a $34bn network upgrade and expansion over the next 10 years, including to the Malabar, Bondi, and North Head systems.
Conclusion
The recent shark attacks in Sydney have raised concerns about the city’s sewerage system and its potential impact on shark activity. While there are many factors at play, experts believe that the system’s inability to handle heavy rainfall and stormwater overflows may be contributing to the problem. The use of deepwater ocean outfalls and the impact of climate change are also being investigated. As the city continues to grow and develop, it is essential that the sewerage system is upgraded to meet the needs of the community and protect the environment. In the meantime, Sydneysiders are being warned to exercise caution when swimming in the ocean, as poor water quality and shark safety are now effectively the same issue.