Fact Sheet: Blue Swimmer Crab

Blue swimmer crab

Portunus armatus spp. image R.Swainston/animafish.com

All about blueys

A powerful swimmer and voracious hunter and scavenger, the blue swimmer crab is also a dinner table favourite and one of the most sought-after recreational fishing species in WA.

A big family

Blue swimmer crabs, sometimes referred to as blue manna crabs in WA, live for up to 3 years, can grow to have a carapace more than 200 mm wide, a claw span up to 800 mm, and weigh up to one kilogram.

There are 2 species of blue swimmer in WA. The primary species is Portunus armatus, with P. pelagicus also found from Broome to the Northern Territory border in small numbers. P. pelagicus is also present across the top of Australia and throughout South-East Asia.

The Portunidae family also includes other large, edible crabs found in Australia such as mud crabs. Crabs from this family can usually be recognised by their flattened hind legs, used as paddles for swimming, and their broad, compressed, laterally streamlined carapace.

locations of major Portunus armatus populations in WA

Distribution

Blue swimmer crabs are found all around Australia except for Tasmania and Victoria, from Esperance in WA around and up the west coast, around the northern and eastern coastlines to the south coast of New South Wales, and in the warmer waters of the South Australian gulfs.

They prefer the sheltered waters of coastal embayments and inhabit inshore and continental shelf ecosystems, from the intertidal zone to at least 50 m in depth. WA blue swimmer crabs are most common in sheltered coastal systems (e.g. estuaries and inshore marine waters) from Nickol Bay near Karratha, to Albany in the south.

Life cycle of the blue swimmer crab

The timing and movements of blue swimmer crabs along the WA coast varies. Estuarine crabs, such as those living in the Leschenault, Peel-Harvey and Swan-Canning Estuaries, tend to move from their estuary into nearby marine waters to spawn, while those in marine embayments such as Cockburn Sound and Shark Bay spend their entire lives within the embayment.

Spawning: Female crabs mostly spawn between spring and early summer. Each female produces a huge amount of eggs – between 100,000 and 1/2 million in a single spawning – and can produce multiple clutches of eggs in each spawning season. However, most of the larvae from these eggs will not survive to become adult crabs.

The eggs are fertilised by the stored sperm and, when laid, they attach to hairs in a spongy mass under the female’s abdomen. The eggs go from orange to black as they mature. The term for a female crab carrying egg clusters in this way is ‘berried’.

Any berried females caught by fishers must be returned to the water. The female incubates the eggs for about 18 days. When the embryos inside are mature she shakes the eggs off her abdomen and they hatch into zoea. A new life cycle has begun!

Zoea: Blue swimmer crabs begin life as tiny larvae, called ‘zoea’. Zoea grow and change shape over a 4 – 6 week period during early summer. Zoea are poor swimmers and drift with the wind and tides, either within embayments or along the coast up to 80 km out to sea. They have a very high mortality rate. Only a few will survive to reach shallow nursery areas by late summer and autumn.

Megalopae: Zoea that successfully return to shallow nursery areas metamorphose into a more crab-like state called a ‘megalopae’, and are about the size of a 5 cent coin. Megalopae settle onto the on the seafloor and live amongst seagrass, algae and rubble. With each moult, they may increase in weight by up to 80 per cent and expand their carapace by up to 20 per cent.

Juvenile: By autumn, most megalopae have formed into juvenile crabs with a recognisable crab shape and carapace 10 – 60 mm wide. They continue growing rapidly.

Young adults: By winter, most of the young crabs have matured and their carapace is about 90 mm wide. During this final moult to reach maturity, female crabs mate with a male for the first time. The newly matured estuarine crabs are likely to follow older adults to the mouth of estuaries or just outside them to avoid freshwater flows caused by run-off and rain and declining water temperatures within the estuaries.

Mating: Blue swimmer crabs in south-west Australia mate between Feb and June. The males moult first, so that their shells have hardened beforehand. A courting male then catches a female and carries her beneath him for 4 – 10 days while fending off other males. The male helps the female to moult and then turns her over to mate while she is still soft-shelled. After mating, he continues to carry her around and protect her for another 3 – 4 days while her shell hardens. A male may mate with several females during one season. The female crabs retain the males’ sperm over winter until their ovaries develop – helped, it is thought, by the rising water temperature in spring.

blue swimmer crabs mating

Predators – and prey

Blue swimmer crabs feed on small fish, molluscs, worms, small crustaceans and even other blue swimmer crabs. They also occasionally eat algae and seagrass. In turn, as larvae, they may be prey to fish. Once juvenile or adult, they are still vulnerable to various fishes and birds eating them, as well as other blue swimmer crabs.

By day, they usually avoid predators by hiding beneath the sand with only their eyes protruding, ready to launch themselves, claws outstretched, at small fish and invertebrate prey. At night, they become mobile, moving around to help them find food.

One crab stock, or many?

In WA, blue swimmer crabs were once thought to be all part of a single breeding population, with areas along the coast re-populated each year by the return of larvae from the ocean.

However, recent genetic studies indicate that in some areas there is limited ‘gene flow’ and a high likelihood that blue swimmer crab populations in areas such as Shark Bay may largely depend on their own breeding stocks to maintain population numbers.

This means that if crab breeding stocks are heavily fished or breeding fails in these areas due to environmental causes, there is a strong risk that the crab population may be adversely affected.

Good and bad years: recruitment

‘Recruitment’ is a term used by researchers to describe the addition of crabs or fish (juvenile or legal size) to a population, either by reproduction or migration.

Levels of recruitment of adult crabs to WA’s blue swimmer crab populations fluctuate considerably. A range of variations – the relative strength of wind and current systems, water temperature, the amount and timing of rainfall, and the primary productivity of an ecosystem – can affect the survival and growth rates of crab larvae and juveniles. This in turn can lead to big fluctuations in the available crab harvest from year to year.

Estuaries along WA’s west coast, such as the Peel-Harvey near Mandurah, are under pressure from rapid population growth, leading to increases in recreational and commercial fishing activity, urban development, and associated environmental change that may also affect crab recruitment.

A major fishery

WA’s annual blue swimmer crab commercial catch usually exceeds 500 tonnes – making up more than half Australia’s total catch of this species. The remainder are caught in South Australia, southern Queensland and New South Wales.

Blue swimmer crabs are fished commercially in bays and estuaries along the WA coast, with the major fisheries including Shark Bay, Nickol Bay in the Pilbara and the Peel-Harvey Estuary.

Hundreds of tonnes of blue swimmer crabs are also caught each year by WA recreational fishers, with popular areas including Geographe Bay near Busselton, Leschenault Estuary near Bunbury, the Peel-Harvey Estuary, the Swan River, Shark Bay and Nickol Bay.

Concern about stocks led to a complete closure of Cockburn Sound and a seasonal closure for Peel-Harvey Estuary to both commercial and recreational blue swimmer crab fishers in 2006. There was a partial reopening of Cockburn Sound in 2009 but the fishery was closed again in 2014 due to declining stock levels. It was discovered that the reduction of human-induced nutrients in Cockburn Sound led to reduced nutrient levels. Monitoring since 2014 has shown a notably smaller crab population than in the 2000s.

A recent assessment indicated the stock could support a low level of recreational catch and Cockburn Sound was reopened on 1 December 2024 with a reduced daily bag limit to support sustainability of the resource.

Management methods

Several protective measures are used to ensure the sustainability of WA’s blue swimmer crab populations, including commercial licences; and for both recreational and commercial fishers – area and seasonal crabbing closures, gear restrictions, and bag and boat limits. The minimum legal size limit ensures crabs are able to spawn at least once before they are caught.

Male or female?

Male blue swimmer crabs, sometimes called ‘blueys’, are usually bigger and more colourful than the females, with a dark-blue carapace, pale belly and rich blue on their legs and claws. Female crabs are a mottled brown. Males also have longer claws in proportion to their carapace than females.

However, the easiest way to check is to turn it upside down and look at the V-shaped abdominal flap on its underside. An adult male’s flap is narrow and angular, while an adult female’s flap is broad and rounded.

Blue swimmer crab male and female underside

Fishy science: protecting blue swimmer crabs

To protect blue swimmer crab populations into the future, DPIRD uses a number of methods to monitor the status of crab populations.

These include fishery independent trap and trawl surveys, which provide researchers with information about the size/age and reproductive condition of crabs in each population.

The graph on the right is an example of the data we collect and how it is presented. Data on commercial catch and catch rate is provided by commercial fishers in statutory monthly returns, while a variety of surveys provide an estimate of the recreational catch and effort.

By analysing commercial and recreational catches, and comparing this information with knowledge about the crabs’ biology and the age and gender structure of the population being fished, researchers can draw conclusions about the state of the stock.

In WA, research is continuing into the relationship between different blue swimmer crab populations. This requires an understanding of the crabs’ growth and movement patterns, and the impact of environmental factors on breeding and growth, as well as fishing and other human activities.

female blue swimmer crab

Length-frequency distribution

Graph: Length-frequency distributions from the November 2023 research trap survey in the Peel-Harvey Estuary compared with the 10-year average.

Did you know blue swimmer crabs also have mineral signatures in their shells?

Crabs incorporate mineral elements from their environment, giving them a ‘mineral signature’ in their shell. By studying their shells, DPIRD research scientists can also learn where they originate from, and how they are connected to other crab populations.

Glossary

Age structure

Number of crabs (or fish) of different ages within a population

Area closure

Closure to fishing in a certain area

Berried

When a female crab carries eggs attached to her abdominal flap

Breeding stock

Mature crabs in a population that are able to breed

Carapace

A protective plate or shell covering the crab’s back

Catch rate

Amount of crabs (or fish) caught in relation to fishing effort

Catch survey

(also called creel survey) Survey of recreational catch, usually involving interviewing fishers at fishing sites

Effort

The amount of time spent fishing by a given group of fishers

Embayment

A semi-enclosed coastal water body

Gene flow

(also called gene migration) The transfer of genetic DNA codes between populations

Harvest

Amount of crabs caught

Megalopae

Post-larval stage in a crab

Moult

Shed or discard old shell in order to grow a new one

Recruitment

Addition of crabs (or fish) to population as a result of reproduction or migration

Seasonal closure

Closure to fishing at a certain time (e.g. for a period of weeks or months)

Zoea

A crab larvae