As the sun began to color the sky a kaleidoscope of pastel colors, University of Newcastle doctoral student Molly Grew and her team geared up for another day of drone missions over Brisbane Water estuary. The morning breeze was gentle, a stroke of luck as they prepared their drone for its daily aerial ballet. “Alright team, today’s the day we map those feeding pits,” Grew declared, adjusting her cap against the early morning glare.
“Hope the weather holds,” someone remarked, adjusting the drone’s settings. “Low tide’s on our side, so we should get clear shots.” With one final look at their surroundings, the autonomous flight sequence was engaged, sending the drone aloft at a precise altitude to capture the best footage for their project. The drone swept across the estuary in calculated passes, its robotic hum blending with the soft lapping of the water below when it got far away enough from the group. Each sweep meticulously designed to ensure an 80% overlap, capturing every detail of the stingray feeding pits that Grew had studied for months. Eighty minutes later, the drone completed its mission, having snapped thousands of images and covering an impressive spread of the estuarine floor.
With a satisfied sigh from the scientists and the exchange of hopeful glances, the team packed up their gear and headed back to home base to backup the footage. The data they’d collected over these intense 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 broader investigation across various estuaries including Wallis Lake and Burrill Lake, marks a mammoth effort in employing drones, aerial imagery, and 3D modeling to map and study stingray behavior in their natural habitat.
Stingrays, often overlooked but integral to estuarine health, actively shape their environment through a process known as bioturbation. As they forage for food or bury themselves in the sediment, they create pits on the ocean floor. Grew explained to ABC News that these activities are not just about survival for the stingrays themselves, but also significantly impact 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 various angles and depths, providing insights into the frequency and extent of these feeding pits. “It’s just insane watching what these drones can do,” Grew said. “Just from taking thousands of pictures and at different angles, we can look at the depths of feeding pits and get a mass out of that.”
The findings revealed a staggering 1,090 feeding pits created by rays during the observation period alone. These pits, scattered across the estuarine floor, covered approximately 90.41 square meters, equivalent to 6.2% of the study area. Each pit, on average, measured 0.095 square meters in surface area and penetrated to an average depth of 0.11 meters. The team also found that rays displaced a total of 4.95 cubic meters of sediment when fieldwork was carried out, averaging 0.83 cubic meters per day. This translates to an impressive 575.2 cubic centimeters of sediment per square meter per day, contributing significantly to sediment turnover and nutrient cycling within the estuary.
The spatial distribution of feeding pits, Grew pointed out in a recently published paper, also exhibited intriguing patterns of clustering, indicating density-dependent foraging behaviors among rays. Statistical analyses confirmed significant spatial autocorrelation, particularly on days with higher bioturbation activity. Hot spot analyses further pinpointed areas where larger feeding pits clustered, underscoring the complex interplay between ray behavior and local sediment dynamics.
Clearly, these animals are impacting their environment. In fact, comparisons with global studies showed there were substantial bioturbation rates observed in Brisbane Water Estuary, placing these rays among the top contributors to sediment turnover in similar habitats worldwide. Why the variability in bioturbation rates? Grew explains it can be attributed to species-specific foraging behaviors, environmental conditions, and prey availability, highlighting the need for further species-specific research.
Yet, despite their ecological importance, estuary stingrays face myriad threats. Climate change, habitat loss, and fishing pressures are increasingly jeopardizing their populations. Dr. Vincent Raoult, a senior lecturer in marine ecology at Griffith University and one of Grew’s supervisors, says the estuary stingrays are classified as near threatened in Australia because “they tend to live in these sorts of estuary and river habitats where they live very closely with people. That means they’re bearing the brunt of the impacts of human activities.”
The decline of stingray populations could have far-reaching consequences, affecting not only biodiversity but also commercial fisheries that rely on healthy marine ecosystems. Dr. Raoult underscores the ripple effects of these declines, warning of potential disruptions to 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’s very few organisms that can now live in those sediments,” he said. “The things that live in those sediments are typically things like worms. They’re a key food source for the bottom of the food chain. […] It’s very concerning to scientists in the long run because it will have repercussions down the track if these declines keep occurring globally, and we know they’re happening globally.”
Grew stresses the need for more comprehensive research and enhanced conservation measures. “We don’t really know too much about not just estuary stingrays, or stingrays in general,” she laments. “There’s hardly any research on it, and we just need more research so we can better conserve them and manage them.”