If the seagrass beds go, the dugongs will soon follow. Georgina Lockwood dives into the underwater world of this marine mammal…
Dugong populations (Dugong dugon) are in decline worldwide, and Dr Mario Lebrato, chief scientist at the Bazaruto Center for Scientific Studies (BCSS), says Mozambique holds the last viable population of dugongs in the Western Indian Ocean. This coastal country is home to an estimated 250–300 of these marine mammals. Despite this, dugong sightings in Mozambique are rare.
“Surveys conducted in 1969 suggested that dugongs were abundant along Mozambique’s coast,” Lebrato says. “They occurred in Maputo Bay, Chidenguele, Inhambane Bay, Bazaruto Bay, Mozambique Island, Pemba Bay, and sometimes in the Quirimbas Archipelago, Matimbane Bay and Angoche.”
Today, however, most of Mozambique’s dugongs are concentrated around the Bazaruto Archipelago. The area is dotted with five tropical islands and consists of 1 430km² of beautiful seascape, teeming with coral reefs, mangroves and seagrass meadows.
Sadly, dugongs are threatened by increased coastal development and exploitation, tourism, pollution and food shortages. They shy away from human activity such as speed boats, divers and bathers, and it is now illegal to kill them. However, some are accidentally caught in fishermen’s gill nets. In the past, dugongs were sought after for their bones, meat and oil.
Perhaps the most significant threat to this sirenian is the degeneration of seagrass pastures. The dugong is the only strictly herbivorous marine mammal, and much like its bovine counterpart, the sea cow spends
a great deal of its day grazing.
Previous surveys showed herds of up to 10 individuals, but today dugongs are more frequently seen alone or in pairs. “This can be attributed to declining population numbers, or reduction in the quantity and quality of their desired seagrass species,” Lebrato explains.
In Australia, where seagrass meadows are abundant, herds can reach up to 200 individuals.
“Dugongs are slow breeders because they have a low reproductive rate, long gestation period, and invest heavily in their offspring,” he adds. These ocean-faring heavyweights feed on a low-nutrient food source – seagrass – and like pandas, dugongs spend much of their lives trying to meet their metabolic needs through a vegetarian diet.
“If there is less seagrass available, dugongs may be more concerned with searching for food for survival than mating,” Lebrato explains.
Some studies suggest dugongs can delay breeding during unfavourable conditions. A female will breed every three to seven years and spend 18 months with the calf; the bond between mother and calf is strong.
It is still uncertain whether dugongs migrate or if populations of dugongs in Tanzania and Kenya are connected.
“Studies conducted in Australia revealed dugongs are sedentary but will travel vast distances to find
food,” Lebrato says. It is therefore really important to protect the seagrass meadows.
Additional information provided by Karen Bowles, BCSS research manager.
• 70 years: The lifespan of a dugong.
• Juvenile golden trevally: The little yellow fish often seen swimming with dugongs.
• Predators: Sharks, crocodiles and killer whales are the dugong’s natural predators.
• Elephant: The dugong’s closest living relative.
• Stellar’s sea cow (Hydrodamalis gigas): This giant sea cow was hunted to extinction in 1768. Dugongs are the modern relative of the Stellar’s sea cow.
Seagrass meadows a treasure trove
Green turtles, dugongs and the omnivorous bonnethead shark (Sphyrna tiburo) are all dependent on seagrass beds. These pastures provide shelter to juvenile fish and play an important role in preserving fish stocks.
Sadly, seagrass meadows are threatened globally. In Mozambique, threats like coastal development, dredging, anchoring and human ignorance have led to the destruction of these vital hotspots.
“Of the 439,04km² of seagrass meadows in Mozambique, 27,55km² are estimated to be lost already,” says Karen Bowles, research manager at the Bazaruto Center for Scientific Studies (BCSS).
A decline in seagrass fauna populations also has a negative impact on the health of seagrass pastures. Dugongs and sea turtles act as lawnmowers, and help keep seagrass beds healthy. “Grazing prevents seagrass meadows from overgrowing, which in turn affects tidal currents and decreases the amount of light available to the many organisms that reside there,” Bowles explains. Dugongs prevent less beneficial seagrass species from taking root and dominating the meadows.
These underwater meadows are found in shallow oceans or brackish waters in all temperate and tropical seas. They occur along the east African coastline from Mozambique to Kenya, and also along the coastline of West Africa.
Light exposure, temperature, nutrient availability and wave action determine the diversity and distribution of seagrasses.
“A recent study has found Thalassodendron ciliatum (a type of seagrass) growing at depths of up to 29m,” Bowles adds.
Seagrass is neither a coral, kelp nor seaweed, but is classified as a flowering plant (a marine angiosperm), with roots, leaves, stems, and vascular tissue.
“Seagrasses often compete with algae, as they both depend on sunlight for photosynthesis,” Bowles explains. “That being said, they are often connected to other ecosystems such as mangroves and coral reefs.”
The southern hemisphere is home to 12 species of seagrasses, of which 10 are found in Mozambique, including the globally vulnerable Zostera capensis species. “The Thalassodendron ciliatum seagrass bed between Bazaruto and Benguerra islands is an important location for dugongs,” Bowles adds.
Seagrass beds are an important underwater treasure trove. They benefit endangered marine species and act as carbon sinks; they purify and improve water quality, prevent coastal erosion and reduce wave energy. “New studies suggest seagrass meadows are able to mitigate ocean acidification,” Bowles says.
Additional information by Karen Bowles, BCSS research manager.
Written by Georgina Lockwood
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