The city's gentle grazers
Moreton Bay, a large, sheltered body of water next to the metropolis of Brisbane, supports the world's only major dugong population near a capital city. This group of 600 to 800 individuals is the southernmost resident population of Dugong dugon on Australia's east coast. These large mammals, which can grow up to 3 meters long and weigh over 600 kilograms, are more closely related to elephants than to other marine mammals like dolphins. They spend their days grazing on the bay's extensive seagrass meadows, earning them the nickname "sea cows." An adult consumes about 30 kilograms of seagrass daily, often leaving visible feeding trails along the seafloor.
The population's survival depends entirely on the health of these underwater pastures. Moreton Bay contains seven species of seagrass, with the largest meadows located on the Eastern Banks, between Moreton and North Stradbroke islands. Here, the water is clearer, allowing sunlight to penetrate to depths of 12 meters and support the dugongs' preferred food. Their favorite seagrass species is Halophila ovalis, which is high in nitrogen and low in fiber, making it easily digestible. Research shows that the dugongs' grazing patterns actually promote the growth of this specific seagrass, a process sometimes called "cultivation grazing."
Surviving in a busy bay
Life at the southern edge of their range, adjacent to a major urban center, presents unique challenges. The primary threats to the Moreton Bay dugongs are boat strikes and the degradation of their seagrass habitat. Because they are slow-moving and must surface to breathe every few minutes, they are highly susceptible to collisions with watercraft. To mitigate this, "go slow" zones have been established in critical feeding and resting areas throughout the Moreton Bay Marine Park.
The dugongs' food source is vulnerable to changes in water quality. Major flood events can wash large amounts of sediment into the bay, making the water turbid and reducing the light available for seagrass photosynthesis. This has led to significant seagrass loss in the past. To cope with life in sometimes murky waters, dugongs rely on senses other than sight. They have poor vision but acute hearing and use sensitive bristles on their snouts to locate seagrass on the seafloor. Primarily herbivores, studies of the Moreton Bay population have shown they sometimes supplement their diet with invertebrates like sea squirts, possibly in response to seasonal changes in seagrass availability.