It's big, ponderous and ugly, and dwells in the frigid, sunless depths between 300-400 metres on the margins of south-eastern Australia's continental shelf. It has a price on its head, and researchers would like to ask it some questions.

But the giant crab Pseudocarcinus gigas, remains an enigma, and until they can obtain crucial data on its lifestyle, behaviour, growth and reproduction, marine biologists are literally in the dark about whether the $15 million a year crab fishery is sustainable.

Associate Professor Brad Mitchell, a senior researcher with Deakin University's School of Ecology and Environment at Warrnambool, sees the giant crab fishery as a prototype for a new wave of 'boutique' fisheries.

These small-volume, high-value fisheries might have restricted entry, and employ relatively few people when compared with the traditional, high-volume fisheries.

"The giant crab fishery is quite valuable, but with live crabs bringing peak prices of $50 a kilogram in south-east Asia, that $15 million is generated from a catch only a few hundred tonnes," Dr Mitchell said.

Crabs average around 6 kilograms, but at a maximum weight of around 14kg, a single male giant crab could be worth up to $700 in Singapore or Taiwan, where the species is a gourmet item in Chinese restaurants.

Dr Mitchell says if these boutique fisheries are to be sustainable, fishermen may have to develop a portfolio of several different species, caught in different seasons, in different environments, using different gear, and in limited quantities."

By their very nature, boutique fisheries are vulnerable to over-exploitation, so accurate scientific information about the biology and ecology of a species like the giant crab is essential, says Dr Mitchell.

And there lies the challenge. There's no simple way to observe the crab in its deepwater habitat, short of using a multi-million dollar, high-technology submersible equipped with powerful lamps.

The Australian marine research community simply does not have such a craft, which Dr Mitchell says would be an invaluable tool for research in the waters of Australia's 320km exclusive economic zone.

Researchers can only study the crab with the help of crab fishermen.

Andrew Levings, who is studying for a Master of Science degree at Deakin University, is national project officer for a four-state study of the giant crab, which inhabits the huge sweep of Australia's southern coastline between Cape Leeuwin in WA to Jervis Bay on the NSW south coast.

Andrew is well credentialled - as a former president of the Portland Professional Fishermen's Association, his experience and contacts in the industry have helped him to establish a productive working relationship between crab fishermen and scientists.

The project has been running four years, and details of the crab's growth and reproduction are beginning to emerge.

Crab fishermen take measurements with calipers provided through the project. With an associated tagging program, Andrew has already assembled an impressive data set, which includes information from deepwater trawlers that haul up giant crabs as bycatch in their nets.

The giant crab fishery began as a bycatch product, with very large - usually male - crabs turning up in lobster pots set in rocky environments in shallow coastal waters.

As the fishery has expanded, crab fishermen moved to deep water, setting purpose-built crab pots on the silty sea floor at the edge of the continental shelf. In this environment, the catch consists predominantly of medium sized female crabs.

Dr Mitchell says it has been difficult to study the crab's diet in its deepwater environment, because the species is slow to enter pots. Where lobster pots are set overnight, crab pots must be set for 48-72 hours.

"By the time the fishermen lift the pots, the crabs have evacuated the remains of their natural diet, and usually have only the bait in their gut," Dr Mitchell said.

Fortunately for the researchers, cooperation from deepwater trawlers has enabled some crabs as bycatch to be frozen soon after they are brought to the surface, with stomach contents intact. "Our analysis analysis shows that in certain areas, the crabs have a fairly mundane diet of slow-moving prey like molluscs and starfish," Dr Mitchell said.

The project aims to develop a reliable model of the crab's distribution, life cycle and growth rates that, when integrated with information about the effects of catch rates and seasonal or climatic effects on productivity, will allow the fishery to be managed sustainably.

Dr Mitchell says that the limited data available indicate that gravid female crabs move from deeper water up the continental slope, where their eggs hatch. The late larval stages of the crab's growth cycle, called megalopa, probably feed on plankton.

"Our working hypothesis is that the megalopa grow rapidly in this environment, where there tends to be a large aggregation of nutrients and food. After settling to the bottom and starting to grow, they move downslope onto the sea floor, and the reproductive cycle begins again.

Tagging is a tricky business. Larger crabs appear to moult every two or three years, so the tags can only be permanently attached to the shell at a particular spot.

Dr Mitchell and his team have developed a new type of tag, consisting of small cylinder attached to a T-bar that is inserted into the crab's shell in a natural join-line in the shell.

When the crab moults, its shell splits along this line, and the new, soft shell below hardens and seals around the tag, so it remains on the crab.

Too few tagged crabs have been recovered so far to derive any reliable estimate of growth rates, but Dr Mitchell says it appears the animals moult once every 2 to 3 years, increasing their weight by as much as 50 to 80 per cent between moults.

"We thought we were looking at a very long-lived, relatively slow-growing species, but these large growth increments suggest the situation might be otherwise," he said.

Females probably spawn two or three times between moults - and perhaps more than once a year - using stored sperm from mating. However, Dr Mitchell says there are indications that in any given year, only about half the females carry eggs and move upslope to hatch their eggs. This has important implications for sustainable management.

Some northern hemispere crab species form very large mating aggregations on the sea floor. "You may get a pile of crabs two metres high, on a flat, featureless plain - it may be a safety-in-numbers defence against predators at time of high vulnerability," Dr Mitchell said. "We don't know yet if Australian giant crabs display the same behaviour.

"We also have some ideas about what the females do when they are carrying eggs. We have seen gravid females with a line around the back of the body where the shell changes color, suggesting they have been sitting with tails buried in silty sediment.

"We suspect the females dig themselves a foxhole while their eggs develop. That's not unusual, because many crustaceans with eggs tend to be reclusive - it also suggests the females have some mechanism for ventilating the eggs."

Dr Mitchell says that, in the longer term, the plan is to link the crab project, and management of the crab fishery, into a complex model of the marine ecosystems and dynamics in the region.

"We'd like to develop a macro-view of western Bass Strait, to see if we can relate the success of various fisheries to the abundance of other organisms, and other features of the regional environment," he said.

"The CSIRO, for example is collecting information on sea-surface temperatures and measuring the abundance of chlorophyll A (which yields an estimate of the abundance of single-celled marine algae, which anchor marine food webs).

Changes in the abundance of marine algae and other marine species might be expected to ramify through food chains, says Dr Mitchell. Researchers need to understand these complex connections.

They also need to know how long it takes for productivity changes to ripple through ecosystems - in an ecosystem with many different species, with very different life cycles, it might take four or five years before the effects of a marked change in the abundance or productivity of one species become fully apparent.

Information about these long-distance or time-lagged connections is essential for sustained management of fisheries, says Dr Mitchell.

"For example, a profitable squid fishery is developing in Bass Strait. Like all cephalopods, squid aggregate in large numbers to spawn, then die en masse and sink to the bottom.

"Is this massive fallout of organic matter onto the sea floor significant to a species like the giant crab? It's high-quality protein that could be very important to the crab's growth and reproduction.

"So we need to know whether the rapid development of a squid fishery in western Bass Strait will have effects on other fisheries.

"Western Bass Strait seems to be important for A diverse range of marine organisms. Peter Dann, who studies the colony of little penguins on Phillip Island, says there is a net westward movement of penguins from the Phillip Island area to the islands of western Bass Strait, and there seems to be a similar movement by fur seals, which, like penguins, eat squid.

"There are occasional, weak upwellings of cold, nutrient-enriched deep water in western Bass Strait, which may explain why it supports large numbers of marine organisms. So we have to study how these species are linked together.

Dr Mitchell says that unless researchers understand how marine ecosystems work, there is no way of explaining why stocks fluctuate from year to year, and management decisions may be made too late to ensure sustainable management.

"It's very difficult to pursue the ecosystem approach in a marine environment. It requires a huge investment of money and resources, and enormous amounts of information must be drawn together and analysed before valid conclusions can be drawn.

"We must avoid the temptation to speculate. Our decisions must be based on the best available scientific evidence supported by a sound understanding of the fishery."