Disappearing Act

by Seanna Cronin

Tasmania’s dwindling kelp forests

Tasmania has experienced one of the most dramatic marine changes to recently take place in Australia – the disappearance of its iconic kelp forests.
Once home to vast, thick forests of giant kelp, Macrocystis pyrifera, which early explorers marked on their charts as navigational hazards, the island state now only has a few remaining forests around the Tasman Peninsula.
An estimated 95 per cent, or possibly more, of Tasmania’s kelp forests have declined in the past 30 years.
The forests are a unique diving experience and Tasmania is the only place in the world where weedy sea dragons can be found in kelp forests, but that experience could vanish in our lifetime.
There are two main reasons for the rapid degradation –  rising water temperatures and an invasive sea urchin.
The water has warmed here faster than anywhere else in the world, 1.5C in the past 50 years, thanks to a shift in the dominating current.
The warm, nutrient-poor waters of the East Australian Current now travel further down Tasmania’s east coast, replacing the cold, rich waters from the south.
The maximum summer water temperature of 17C has not changed; it is the winter lows that are affecting kelp growth.
Giant kelp grows best in cold water, from 30 to 60cm per day in ideal conditions, but in Tasmania the kelp is not getting its annual winter window of regeneration.
“The water used to get down to 9C, but now it doesn’t get below 11C,” said Mick Baron, co-owner of Eaglehawk Neck Dive Centre.

Tasmania’s remaining giant kelp is growing at the maximum end of its temperature range, and the result are thin, scrawny remnants of formerly dense forests.
Tasmania has one of the highest rates of marine endemism of any temperate area and this makes its ecosystems more vulnerable to introduced species.
A species of kelp-eating black sea urchin, Centrostephanus rodgersii, was first noted in Tasmania in the 1960s.
The urchins are thought to have traveled south on the EAC, thanks to the oceanographic changes just mentioned, from New South Wales, where they are also a problem.
Overfishing of crayfish (rock lobsters), in Tasmania means the urchins have spread unchecked by their natural predator.
The urchins devour kelp and even strip algae from the rocks, turning the sea floor into a barren landscape.
University of Tasmania researcher Professor Craig Johnson has been studying the urchins and estimates they have decimated at least half of the state’s rocky sea floor.
“The big problem is that these barrens are very unproductive, I mean it's a bit like, the analogy is a bit like salinity on land,” he told ABC radio.
“You end up with a barren, unproductive habitat, and that means no rock lobster, no abalone, no other fishery. And that's really the big worry.”
Professor Johnson has been reintroducing crayfish into barren areas and initial trails have been successful.
Commercial fisherman donated crayfish caught in deeper waters to Johnson, who tagged them and introduced them into protected areas.
He then recaptured the crayfish 6 months later and found more than 50 per cent of their stomach contents were urchin – a promising result for possible urchin management using crayfish.
Reigning in the urchins will give Tasmania’s kelp a fighting chance at recovery, but the kelp still have to contend with warm water temperatures.
There are also several other factors thought to affect the kelp – marine pollution from the Derwent estuary and Japanese kelp, which has colonized many areas formerly occupied by giant kelp.
The loss of Tasmania’s kelp forests has had a ripple down effect on the many species which depend on the forest’s complex 3D structure.
From its holdfasts on the sea floor to its canopy on the surface, a kelp forest provides habitat for fish, molluscs, bryozoans, polychaete worms, crustaceans, echinoderms and sponges, and shelter for sea dragons, seals and birds.
“The number of living creatures of all Orders, whose existence intimately depends on the kelp is wonderful,” said Charles Darwin in 1845.
“A great volume might be written, describing the inhabitants of one of these beds of seaweed….I can only compare these great aquatic forests of the southern hemisphere, with the terrestrial ones in the inter-tropical regions.”
To express your concerns about Tasmania’s loss of kelp contact the state’s Department of Primary Industries, Water and Environment at www.dpiw.tas.gov.au and the Tasmanian Fisheries and Aquaculture Institute at www.tafi.org.au