“We don’t really know much about the estuary caves alone, or about them in general,” Molly … [+]
As the sun began to color the sky a kaleidoscope of pastel colours, Newcastle University PhD student Molly Grew and her team prepared for another day of drone missions over the Brisbane Estuary. The morning breeze was gentle, a stroke of luck as they prepared their drone for its daily aerial ballet. “Alright team, today is the day we draft those food pits,” Grew said, adjusting her hat against the early morning glare.
“I hope the weather holds,” someone remarked, adjusting the drone’s settings. “The low tide is on our side so we have to get clear shots.” With one last look at their surroundings, the autonomous flight sequence was engaged, sending the drone up to a precise altitude to capture the best footage for their project. The drone traversed the estuary in calculated passes, its robotic hum mingling with the soft lapping of the water below when it got far enough away from the group. Each sweep meticulously designed to ensure an 80% overlap, capturing every detail of the spike feeding pits that Grew had studied for months. Eighty minutes later, the drone completed its mission, snapping thousands of images and covering an impressive stretch of the estuary floor.
With a satisfied sigh from the scientists and an exchange of hopeful glances, the team packed up their gear and headed back to home base to book the footage. The data they had collected during these intensive days of drone missions promised to reveal new insights into the secret world of stingrays and their crucial role in the estuarine ecosystem. This research, part of a wider investigation in various estuaries including Lake Wallis and Lake Burrill, marks a major effort in using drones, aerial imagery and 3D modeling to map and study the behavior of the stingrays in their natural habitat .
Newcastle University PhD student Molly Grew conducted daily drone surveys of Brisbane Water.
Stingrays, often overlooked but integral to the health of the estuary, actively shape their environment through a process known as bioturbation. As they search for food or burrow into the sediment, they create burrows on the ocean floor. Grew explained to ABC News that these activities are not only about the survival of the spikes themselves, but also significantly affect the ecosystem around them: “When they do this, they help with oxygen penetration, nutrient cycling. […] They can facilitate foraging in other fish and other prey populations.”
The use of drones was instrumental in this study, enabling the researchers to capture detailed images from different angles and depths, providing insight into the frequency and extent of these food pits. “It’s crazy to see what these drones can do,” Grew said. “Just by taking thousands of pictures and at different angles, we can look at the depths of food pits and get a measure of that.”
The findings revealed an estimated 1,090 feeding pits created by the rays during the observation period alone. These pits, distributed across the estuary floor, covered approximately 90.41 square meters, equivalent to 6.2% of the study area. Each pit, on average, had an area of 0.095 square meters and penetrated to an average depth of 0.11 meters. The team also found that the rays moved a total of 4.95 cubic meters of sediment when the fieldwork was done, an average of 0.83 cubic meters per day. This translates into an impressive 575.2 cubic centimeters per square meter of sediment per day, contributing significantly to sediment circulation and nutrient cycling within the estuary.
The spatial distribution of food pits, Grew pointed out in a recently published paper, also displayed intriguing aggregation patterns, indicating density-dependent foraging behaviors among rays. Statistical analyzes confirmed significant spatial autocorrelation, especially on days with higher bioturbation activity. The hotspot further analyzes the defined areas where the largest food sinks accumulated, highlighting the complex interplay between ray behavior and local sediment dynamics.
It is clear that these animals are affecting their environment. In fact, comparisons with global studies showed that there were significant bioturbation rates observed in the Brisbane Estuary, placing these rays among the major contributors to sediment turnover in similar habitats worldwide. Why the variability in bioturbation rates? Grew explains that it can be attributed to species-specific feeding behaviors, environmental conditions and prey availability, emphasizing the need for further species-specific research.
However, despite their ecological importance, estuarine stingrays face a host of threats. Climate change, habitat loss and fishing pressures are increasingly threatening their populations. Dr. Vincent Raoult, a senior lecturer in marine ecology at Griffith University and one of Grew’s supervisors, says estuarine giants are classified as near threatened in Australia because “they tend to live in these types of estuarine and river habitats where they live very close . with people. This means they are bearing the brunt of the impacts of human activities.”
PhD student Molly Grew holding a nail in a net at Brisbane Water
Declines in basket populations can have far-reaching consequences, affecting not only biodiversity, but also commercial fisheries that rely on healthy marine ecosystems. Dr. Raoult highlights the ripple effects of these declines, warning of possible disruption of sediment-dwelling organisms that form the foundation of the marine food chain. “[Their disappearance] means there’s no more oxygen in those sediments and that means there are very few organisms that can now live in those sediments,” he said. “The things that live in those sediments are usually things like worms. They’re a main food source for the lower part of the food chain. […] It’s very worrying for scientists in the long term because there will be consequences down the road if these declines continue to occur globally, and we know they are occurring globally.”
Grew emphasizes the need for more comprehensive research and enhanced conservation measures. “We don’t really know much not just about estuarine caves, or about them in general,” she laments. “There’s hardly any research on it, and we just need more research so we can better conserve and manage them.”