Reef Report for Tuesday, August 3, 2004
Sea Grass Recovery is Good News for Dugongs
This week’s good local news was the recent announcement of the significant growth and recovery of the sea grass beds off of Cairns.
Perhaps we are a bit unusual, but as we are interested in the environmental health of the Great Barrier Reef, the condition of coastal sea grass beds is a vital link in the Great Barrier Reef ecosystem, and home to some of the more unique wildlife found on the coast.
Sea Grass are broad underwater meadows of grasses grow on the ocean floor. They are found in shallow waters worldwide, and off the Queensland coast they are found at depths of 5-90 meters depth, wherever conditions favor their growth.
Along the Great Barrier Reef sea grass beds are the favored habitat to a variety of species, especially the early growth stages of many fish and prawns, species that play significant roles in the ecology of the reef, and the local economy. Without healthy sea grass beds these nurseries are absent, and fish and prawn populations can decline. While the currents and wave action of violent storms can destroy entire sea grass beds, a more important threat is are plant nutrients, in the form or eroded soil, fertilizers from farms, or treated sewage, all of which can increase algal growth, which blocks the light that sea grasses need.
Probably the most amazing animal found on the sea grass beds is the dugong. Dugongs are one of three species of sea cows. These animals are pretty abundant in the tropical sections of the Indian Ocean, and in Australia are found in the tropical and subtropical waters along the East, North, and West coasts. While abundant in Australian waters, their numbers are in decline elsewhere.
These animals are grazers, and feed almost exclusively on sea grass. They are capable of moving great distances to find their favored food, that being the younger, actively growing sea grasses. Adult dugongs can become pretty large, growing up to 300kg and live for 70 years. As adults these animals are rarely preyed upon, except by killer whales and the largest tiger sharks. They are important food items in the culture of Northern Aboriginal cultures, and are taken in small numbers, which population surveys show has had little effect on their numbers.
While only rarely spotted by divers on the Great Barrier Reef, your best chance of seeing them is on the Frankland Island Dive Trip, where they are sometimes encountered. This island group is just off the coast, and located nearer the sea grass beds these animals utilize.
Along the more populated East Coast the populations of these animals have dropped in recent years. It’s difficult to come up with a set of clear reasons for the cause, especially for animals that can range so widely. But like the manatee in Florida, these animals are killed in collisions with boats, trapped in beach nets, and abandon areas when sea grass beds disappear. Since a female manatee only has a single pup every other year, any additional deaths can seriously impact the species.
In response the Queensland and Australian governments have established dugong reserves to protect critical habitat. Laws also protect them from boat traffic and harassment. Sea Grass beds have also been recognized as vital ecosystems, and a great deal of monitoring and research is leading to their understanding and protection.
About Sea Grasses.
About Dugongs in Australia.
About Protection of Dugongs on the Great Barrier Reef.
The weekly reef report is written by Joel Groberg of DiveTheReef.com,
who compiles them from the many conversations he has with dive staff in the area, as well as many other local sources in the dive
Index of all reef reports.