Dugongs pique our curiosity, draw us closer, and ultimately tug our heart strings. Indeed, these are beautiful and fascinating creatures. It is through these wonderful beings that we work to protect seagrass ecosystems which are absolutely crucial to the welfare of humanity, and without which dugongs would surely go extinct.
Seagrass ecosystems are regarded to be as important as coral reef systems because they are critical to the success of coastal fisheries. Seagrass also play an important role in reducing the effects of climate change, yet they are under pressure from human activities. Dugongs are the world’s only vegetarian marine mammal and are reliant on seagrasses for food. Seagrass ecosystems with dugongs indicate a healthy ecosystem.
When seagrass are healthy they play an important role in securing the dietary needs of coastal communities and millions of consumers of fish and seafood globally. They also protect coasts from the impacts of storms, improve the quality of marine water and help prevent climate change acceleration. The dugong that live in these seagrass ecosystems are excellent barometers indicative of the overall health of the ecosystem.
Dugongs & Seagrass FAQs
What is a dugong?
The dugong (Dugong dugon) is the only herbivorous marine mammal. A single adult dugong can grow up to three meters, weigh up to 500 kilograms and live for 70 years. Dugongs can remain underwater for 3 to 12 minutes while feeding and travelling. They can eat up to 40 kilograms of seagrasses per day. Dugongs are seagrass community specialists and their range is broadly coincident with the distribution of seagrasses in the tropical and sub-tropical Indo-West Pacific. Dugongs occur in over 40 countries in the Indian Ocean and western Pacific Ocean and are able to move into different home ranges, travelling hundreds of kilometers in a few days. Dugongs have been important to human cultures in various ways for thousands of years.
What are the differences between a dugong and a manatee?
(1) Dugongs weigh less than manatees. The average adult dugong weighs 250 to 400 kg, while the average manatee weighs 400 to 500 kg; (2) Dugongs cannot tolerate freshwater. Dugongs are strictly marine mammals, but manatees migrate between marine and freshwater; (3) The age of sexual maturity for dugongs is between 4 and 17 years, and for manatees it’s between 5 and 9 years; (4) Dugongs grow to an average length of 3 metres. Manatees usually are between 3 and 3.5 metres in length; (5) The primary threats to dugongs are bycatch and the destruction of seagrass habitats. Manatees are primarily threatened by adverse temperatures and vessel strikes; (6) Dugongs have smooth skin, whereas manatees have rough and wrinkled skin; (7) Dugongs live in the Indian and Pacific Oceans. Manatees live in the Atlantic; (8) the tail of the dugong is fluked (similar to a dolphin tail) the manatee has a paddle-shaped tail.
Why are dugongs vulnerable to extinction?
Despite being able to travel long distances, dugong populations are considered to be declining across their range. It is estimated that populations have suffered a global decline of approximately 20% over the last century, largely due to human activities.
The characteristics and life history of dugongs – long-lived, with low reproductive rates, long generation times and a high investment in each offspring – make their conservation problematic in the face of a range of human threats. A simulation study indicated that, even in ideal natural conditions, without human disturbance, dugong population growth would not exceed 5% per year, making them very vulnerable to over-exploitation or other mortality. Even a slight reduction in adult survivorship can cause a substantial population decline.
Dugongs are classified as vulnerable to extinction on the IUCN Red List of Threatened Species, indicating a high risk of extinction in the wild in the medium-term future.
What are the main threats to dugongs?
The main reasons for the declining dugong populations include:
- Incidental by-catch, which can occur from the use of shark nets, gill nets or tidal traps. In the majority of cases, accidentally caught animals are eaten or sold. Although accidental, most of this capture and trade is illegal. Reports indicate that 56 dugongs were caught intentionally or incidentally between 2005 and 2010: all were caught in nets. Of the 51 that were found alive, all but one was killed for food or sale.
- Destructive fishing practices in coastal waters, such as use of beach seines or push nets and indiscriminate bottom trawling, the use of chemicals such as sodium cyanide and explosives such as dynamite fishing.
- Hunting of dugongs still occurs but is less common nowadays. Dugongs have been taken for meat, blubber and hides, as well for traditional medicine, where the tusks and bones are used for the treatment of asthma, back pain and shock.
- Boat strikes on dugongs occur in heavy boat traffic areas and are usually associated with coastal development. Dugongs usually swim very slowly, and must surface to breathe every 3 to 12 minutes, so it is difficult for them to avoid fast-approaching boats.
- Excessive tourist interaction with dugongs has resulted in operators or local communities disturbing animals and sometimes to physical harm from propeller strikes.
What are Seagrasses?
Seagrasses are a group of flowering plants adapted to exist submerged in shallow marine environments with low turbidity. Globally, there are 60 known species of seagrasses, most of which are distributed along temperate and tropical coastlines. The tropical Indo-Pacific region is the most diverse seagrass bioregion, hosting 24 known species. In many semi-tropical and tropical regions, seagrass habitats are also often closely linked to mangrove and reef ecosystems, which together are essential for the lifecycles of a number of reef and mangrove-dependent species. Seagrass occupy only 0.2% of the world’s oceans.
What are the differences between seagrass and seaweed?
(1) Seagrass produces flowers, seeds and fruit. Seaweed only produces spores; (2) Both seagrass and seaweed are threatened by damaging fishing methods and pollution; (3) Seagrass has roots, whereas seaweed does not. Seaweed uses ‘holdfasts’ to anchor itself to other objects; (4) The recorded number of species of seagrass is approximately 60, but there are estimated to be between 5,000 and 6,000 know species of seaweed; (5) Seagrass is a flowering plant, seaweed is a type of large algae; (6) Seagrass uses its roots to extract nutrients from sediment, but seaweed extracts nutrients from the water around it.
Why is it important to save seagrasses?
Seagrasses provide valuable benefits to people. They play an important role in carbon storage, accounting for 10% of the annual carbon sink capacity of the oceans. It has been estimated that a hectare of the most effective seagrass meadows exceeds by ten-fold the carbon sink capacity of the pristine Amazonian forest.
Seagrass provide habitat and breeding grounds for many marine species, including important fishery species that millions of people around the globe depend on for their livelihoods. They provide nursery grounds for juveniles of commercially important marine species such as shrimp, shellfish and finfish. Subsistence fisheries depend on seagrasses both as fish nurseries and as accessible and sheltered fishing grounds.
Twenty-nine percent (29%) of the world’s seagrass habitat has already been lost largely through human activities and remaining seagrass is disappearing at a rate of 110 km2 per year. These rates of loss are comparable to those reported for mangroves, coral reefs, and tropical rainforests, and place seagrass meadows among the most threatened ecosystems on earth.
What are the main threats to seagrass?
Direct and indirect threats to sensitive seagrass ecosystems from human influences are significant but are largely overlooked by policy makers. These include:
- Direct destruction of seagrass habitats, associated with unsustainable urban and industrial coastal development, as well as some fishing methods (trawling, using explosives and chemicals) causing direct physical damage to seagrass meadows.
- Pollution in coastal waters from inadequate treatment of domestic sewage, untreated industrial liquid and solid waste disposals, run-off caused by deforestation for plantation or property and industrial development leads to loss of seagrasses.
- Natural disasters such as cyclones, storm surges and tsunamis add to human impacts on seagrass habitats through increased turbidity owing to sediment entering the water column, and runoff from the land as surge water recedes. In addition, seagrass plants may be ripped from their holdings due to wave action.
- Climate change exacerbates the impacts of human activities on seagrasses.