Swimming through an underwater forest of giant kelp is an awe-inspiring experience. Diving amongst these plants enhances the experience of being able to move in three dimensions underwater. Sometimes it seems as if you can fly.

On a calm day, you can drift effortlessly through the kelp, suspended in the soft light that fills the water beneath the canopy. Graceful blades of kelp gently sway as they follow the slow movements of the ocean, while the main stems of the plants stretch up towards the surface and life-giving sunlight, buoyed up and tensioned by numerous gas filled bladders.

As you swim through a kelp forest the closer plants are well-defined and easy to see, even in the dim light. Further away, plants gradually become less distinct and in the distance, fade into the blue immensity of the ocean.

Giant kelp, or Macrocystis pyrifera, has always been a feature of Tasmania's underwater environment. In Australia, this species only forms significant forests in Tasmanian waters, although large stands can be found in a few other parts of the world such as the cooler waters of South America and North America.

I learned to dive amongst the kelp north of Bicheno on the east coast of Tasmania as a kid in the mid 1970s. As kids, we used to go snorkelling through stands of giant kelp looking for fish to spear and searching under the bullkelp and crayweed for abalone and crayfish, or just explore the marine environment.

Trips to the sea were not all that common, so even when the sea was rough we still used to go diving, even when the waves were big enough to surf. When the swell was up we would dive down to the bottom through the kelp as it washed backwards and forwards across the rocky reef. Once on the bottom, waves would often wash bullkelp over the top of us and we would then have to wait for the surge to reverse direction and carry the plants away so we could fin back to the surface for the next breath of air.

Giant kelp would sway backwards and forwards in the powerful surge and form a constantly moving maze of stems and blades. The stands of giant kelp often had areas so dense that the canopy was almost impossible for a diver to penetrate from below. A breathhold dive beneath these sections of canopy was always a commitment. Once you got under the kelp there was no turning back. It became a challenge to dive down and thread a path through the tangled stems and under the dense part of the canopy.

Getting snagged part way through just added to the excitement and happened regularly. There was no point thrashing around and trying to pull yourself free. You had to stop and calmly unhook the kelp.

A few years later, after I started to scuba dive, and joined the university dive club, kelp forests were always a favourite place to visit. The kelp forests off the Tessellated Pavement and Waterfall Bay Island near Eaglehawk Neck, and at Sleepy Bay and Whitewater Wall at Freycinet, and the smaller stands near Bicheno were one of the best things about diving.

Really dense forests jammed parts of Fortescue Bay, especially in and around Canoe Bay and near the entrance to Bivouac Bay. The densest forest I have ever seen was just off the Fortescue Bay boat ramp. In places the canopy was so thick that even a scuba diver would have had trouble reaching the surface and swimming underneath was at times similar to being on a night dive.

In those days, the early eighties, giant kelp forests seemed to be a permanent part of Tasmania's marine environment, and a unique and spectacular habitat in the Australian environment.

Sadly, giant kelp has been suffering a serious decline, particularly over the last 10 to 15 years. This decline has probably been going on since the Second World War. Most of the forests where I used to go diving in the seventies and eighties have completely vanished. Even the major Macrocystis forests associated with Fortescue Bay have practically disappeared.

Many people have noticed this decline over the years, but only recently has any formal study been completed. Dr Karen Edyvane has just finished the first report to quantify the decline and begin the task of understanding what is happening to Tasmania's coastal ecology. Her report confirms our concerns. Giant kelp beds have been declining for many years.

Giant kelp beds naturally tend to come and go in response to storms and changing nutrient bearing currents. Many beds appear to go through regular boom and bust cycles. What seems to be changing is that beds that become damaged or disappear on the east coast of Tasmania no longer seem able to recover.

The causes for this decline are not obvious. There appears to be a correlation between the decline and the higher water temperatures and lower nutrient levels associated with the East Australian Current, but at the moment this is just speculation. Changing patterns of oceanic water movements may be related to global warming. It is also possible that changes in populations of abalone and rock lobster due to human fishing activities may also be involved, either directly and in isolation, or through their effect on the sea urchin populations as another result of global warming.

The status of giant kelp along other parts of the Tasmanian coastline is also uncertain, as this study was limited to Tasmania's east coast. This means that there is no objective evidence available to suggest that the remaining kelp beds in other parts of Tasmania are unaffected or will be sufficient to ensure that this species does not disappear from our waters.

Apart from the loss of a spectacular habitat, the decline of giant kelp forests may have further impacts on the marine ecology along the Tasmanian coastline. It appears that giant kelp enhances the productivity of rocky reefs. For example, juvenile rock lobster may find it easier to settle out of the plankton onto giant kelp plants than on other parts of our rocky reefs.

If the root cause of this decline turns out to be global warming there may be little we in Tasmania can do about this disturbing decline of giant kelp. However if fishing or other human activities are the cause, or even just part of the cause, then there is a chance that we can do something to arrest the decline of this spectacular Tasmanian marine habitat. To ignore the possibility that something could be done to ensure the long term survival of this habitat would be totally irresponsible.

There is now an urgent need to continue Dr Edyvane's research and monitor the status of giant kelp forests on the east coast of Tasmania and to expand this work to cover the rest of the coastline. It is also essential to begin experimental work with the aim of determining the actual cause of this dramatic decline and better understanding the ecology of Tasmania's rocky reefs.

The TCT has been on the steering committee for Dr Edyvane's kelp project, and the report itself has recently received coverage in the media due to concerns about delays in its public release. Green's politician, Nick McKim, has also nominated Macrocystis for listing under Tasmania's threatened species legislation, and it seems reasonable that this nomination will lead to giant kelp being listed as vulnerable.

This recognition is important, but even more important is that the Tasmanian Government needs to take the findings of this report seriously and act to ensure that giant kelp habitat is maintained in our waters.