Scientists Tagging Sharks and Rays to Understand Port Phillip Bay’s Ecosystem
In the bustling waters of Port Phillip Bay, a dedicated team of researchers from Deakin University is undertaking a critical study to understand the health of its shark and ray populations. The task of safely handling and tagging these marine creatures is no small feat. It requires the combined effort of three scientists to manoeuvre a two-metre sevengill shark from the water onto their research vessel.
With meticulous care, they work to keep the powerful jaws of the wriggling shark secured while simultaneously administering essential care. Water is carefully channelled through its gills, a vital process as the scientists swiftly insert a tracking device beneath its tough, leathery skin. Alongside this, they collect crucial blood and tissue samples. This entire procedure is conducted with the utmost efficiency before the animal is gently released back into its natural habitat.
Over the past two years, Deakin University researchers have tagged approximately 100 sharks and rays. This extensive tagging program is a cornerstone of their ongoing study, which aims to assess the multifaceted impacts of human activities and the escalating threat of climate change on these species within Melbourne’s iconic bay.
“Our research isn’t solely focused on fishing,” explains Samantha Sherman, a senior lecturer in fisheries. “We are also examining the influence of shipping channels, the presence of abalone farms, and how all these cumulative stressors might be affecting sharks and rays.”
Dr Sherman clarifies a common misconception: sharks and rays inhabiting Port Phillip Bay are not the apex predators many might imagine. Instead, they are classified as mesopredators. This means they occupy a middle ground in the marine food web, preying on smaller sea creatures while simultaneously being vulnerable to larger sharks.
“We are looking at the species that occupy the middle strata of the food web,” she elaborates. “This includes smaller sharks, such as the Port Jackson shark, and various species of rays, like banjo sharks and the Melbourne skate.”
Historically, experts have reported a noticeable decline in shark and ray populations over the last few decades. However, there is a glimmer of hope, with researchers observing signs of recovery following the ban on commercial net fishing implemented in 2022.
Uncovering a Potential Banjo Shark Nursery
Adjacent to the entrance of Port Phillip Bay lies Swan Bay, a tranquil area near Queenscliff. It is here that Louisa Graf, a PhD researcher at Deakin University, believes she has discovered a vital nursery ground for banjo sharks, also known as fiddler rays, and potentially other species.
“What we are beginning to observe is that the juvenile banjo sharks are showing a greater tendency to remain within Swan Bay,” Ms Graf notes. “This residency pattern strongly suggests that they are utilising this area as a nursery.”
Ms Graf has been actively tracking 60 banjo sharks that she has tagged. Her research aims to gain a deeper understanding of how these rays utilise Swan Bay and how their behaviour evolves across different life stages.
The seabed of Swan Bay is richly carpeted with seagrass meadows, providing a habitat for an astonishing diversity of marine life, with over 250 animal species calling it home, according to the Swan Bay Environment Association.
“For an area to function as an important nursery, it is imperative that the marine ecosystem within it is thriving and operating effectively,” Ms Graf emphasises. “The southern Australian coastline is particularly vulnerable. It’s recognised as a global hotspot for ocean warming, making it absolutely crucial that we understand precisely how sharks and rays are utilising these areas right now.”
“This knowledge will enable us to implement effective management protocols to ensure their safety and survival in the future,” she adds.
Fishing is already prohibited in designated sections of Swan Bay, which is classified as a Marine Protected Area (MPA). Within these MPAs, human activities are restricted to conserve biodiversity.
“Marine Protected Areas represent a truly effective management tool for safeguarding sharks and rays,” Ms Graf states. “If we can continue to demonstrate their success, it may pave the way for the designation of more MPAs along our coastline.”
However, banjo sharks are just one of eight species that the Deakin University team is tagging across Port Phillip Bay. This comprehensive research endeavour is designed to provide a holistic overview of the movement patterns and ecological roles of the bay’s diverse shark and ray populations.
Dr Sherman underscores the importance of understanding the entire ecosystem rather than focusing on individual species in isolation. She explains that the decline of one species might not be catastrophic if other healthy populations can effectively substitute its ecological role.
“Sharks and rays play incredibly significant roles within our ecosystems,” Dr Sherman asserts. “They are instrumental in nutrient cycling. Being larger than many fish species, they travel greater distances, facilitating the redistribution of essential nutrients. Furthermore, they serve as prey for larger marine animals, such as bigger sharks, while also consuming smaller organisms, thereby maintaining a delicate balance within the ecosystem.”
Dr Sherman highlights the unique characteristics of Port Phillip Bay, noting its considerable size combined with its relative shallowness and densely populated coastline.
“It’s a marine system that is unlike any other in the world,” she says. “We don’t have comparable systems elsewhere that we can draw upon to fully comprehend what is happening here.”
Climate Change Driving Marine Life Towards Cooler Waters
The pervasive effects of climate change are compelling marine species to seek refuge in cooler waters, pushing them towards the poles, according to Dr Sherman. She explains that scientists are already witnessing tangible shifts in marine ecosystems.
A notable example is the sighting of a bull shark in eastern Victoria in 2024, a phenomenon Dr Sherman attributes to changing ocean currents and temperatures.
“If these species continue to move southward, they will eventually reach Port Phillip Bay,” she warns. “The impact they would have on the existing ecosystem would be immense, as the species currently inhabiting these waters would likely be unable to outcompete these more aggressive bull sharks.”
“Therefore, even though their populations may be healthy and not currently threatened by fishing pressures, they could face significant threats from other species migrating further south in the future,” Dr Sherman concludes.
The warming ocean temperatures will also necessitate adaptation from existing species within the bay. They will either need to relocate to more suitable environments or face the stark reality of extinction, Dr Sherman explains.
However, Ms Graf remains optimistic that the data collected from tracking the bay’s sharks and rays can be invaluable for modelling these distribution changes over the coming decades.
“This data will allow us to identify potential hotspots of abundance and areas where populations may be decreasing along the coastline,” she states. “We can then use this information to designate critical areas for sharks and rays and assess whether existing MPAs are located within these important zones, or if new ones are needed.”
Despite these conservation efforts, researchers have noted an increase in illegal activities, particularly since funding for the Victorian Fishing Authority was reduced. In response to budget cuts, the authority reportedly replaced a significant portion of its legal enforcement officers with a smaller number of engagement officers to patrol the state’s waters last year.
“There is very limited monitoring occurring,” Dr Sherman observes. “We are unfortunately witnessing instances of poaching within our marine protected areas.”
In response to these concerns, a spokesperson for the Victorian government stated that the Victorian Fishing Authority has established the Major Fisheries Crime Unit, a specialised taskforce dedicated to combating organised illegal fishing operations.
“The vast majority of individuals within the fishing community conduct themselves responsibly,” the spokesperson reassured. “We observe approximately 90 per cent compliance with fishing rules and regulations across the state.”
