Species Guide: Crabs

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Dave with a solid mud crab.

I RECKON that crustaceans are the most popular of all seafood and if not for the prohibitive cost, we’d be chomping down on crabs, prawns and lobsters quite regularly.

Crabs are a favourite summer treat with their sweet white flesh, beautifully marked shells and the hint of danger to keep you on your toes. They are easily purchased but I prefer to catch my own and eat them fresh.

There’s no seafood that my mum loved more than blue swimmer crab (BSC), and whenever I bought a few back, I was the most popular kid in the street…which, during summer was more often than not. Given that I was a fishing-tragic from the get-go, and with our tinnie moored on the flats about 30 m from our front door, I’d be a regular. Hessian sack with handlines and bait over one shoulder and a pair of oars and wire meshed landing net over the other, I’d be loaded and rowing in minutes. It was a short journey to the edge of the seagrass in our crab-filled bay. I saw hundreds of sunrises on the Brisbane Water and I dare say, brought a lot of crabs home. One of my favourite memories as a kid was sitting with Mum at our outdoor table with a pile of warm crabs and a bucket for the “bits”, chatting happily and watching her suck meat out of claws, which was her favourite part! Those days are long gone but they were good days, and oh so simple!

Half a decade later, I’m still chasing crabs. Handlines have been replaced by crab pots and my grown children and grandsons are crew, it’s only the chainsaw that comes anywhere close to the 4 yr. old’s love of catching “nip-nips”. There’s nothing cooler!

So, if you’ve never caught crabs before, this article is for you. I’m no encyclopedia, but I hope you can get a start and catch a feed for yourself. Be warned however, mud crabs and to a lesser degree, BSCs are a pricey delicacy, and they attract crab-thieves black market shamateurs like blowflies to rotting prawns, so expect theft and visits by fisheries officers who will check your gear and remove any unmarked or illegal traps. Play hard by all means, but be honest and stick with the regulations of the state in which you are crabbing.

Whilst spanner crabs are available offshore, we’ll stick to the most accessible species, namely blue swimmers and mud crabs.

A colourful blue swimmer crab.

Blue Swimmer Crabs
Biology and Stock Status: Australian stocks of blue swimmer crab (Portunis armatus), goes by many names, such as the sand crab, blue manna crab, blue crab and swimming crab. They range throughout Southeast Asia, reaching to the southern parts of Japan. In Australia, blue swimmers can be caught from the south coast of Western Australia, north to the Northern Territory, across Queensland, down the east coast and to the New South Wales–Victoria border. They are also found in the warmer, shallow waters of the South Australian gulfs.

On average, BSCs live for 3–4 years, reaching carapace width (top shell) of approx. 200 mm. Sexual maturity is reached at between 6–14 months (they grow faster in warm water), at carapace widths between 86–110 mm. Mating occurs in the first few days after the female sheds her shell, before the new underlying shell hardens. Male crabs hold onto the backs of females just prior to shell-shedding to insert sperm-filled capsules into the female whilst the new shell remains soft, supplying sufficient capsules to enable each female to spawn 3-4 times over a single summer period … not much fun for the males, but an effective reproductive strategy none-the less. Females carry their fertilized eggs under their abdominal flap (which become a bright orange during development), at which time they catch outgoing tides until they leave their estuary. Eggs hatch in the near-offshore waters where the young develop before returning to estuarine waters to mature.

Blue swimmer crabs and other Portunid species are sought after worldwide to the point where many stocks are overfished. For example, the Japanese are so impressed with them that in 2019, they imported 2,095,285 kilograms of frozen swimming crab (Portunus sp.), with more than half being sourced from Bahrain.

Australian stocks are faring much better than most with almost all of our stocks fished sustainably. Strict management controls are recovering the only two “Overfished” stocks (both in WA). The Queensland BSC commercial fishery primarily operates in southern Queensland, with the waters of the Sunshine Coast and Moreton bay being the most productive. Spatial closures within the Moreton Bay, Great Sandy Strait and Great Barrier Reef Marine Parks provide some protection of the Blue Swimmer Crab population. Female crabs cannot be taken and a minimum legal size ensures that a high proportion of male BSC’s have an opportunity to mate before recruitment into the fishery [QDAF 2018]. South Eastern Australia BSC’s occur in coastal and estuarine waters along the length of the New South Wales coastline. New South Wales BSC populations are at the southern end of the species distribution along the east coast and only spawn in the hotter periods between November and February, rather than year-round as occurs further north. A legal minimum size of 60- and 65-mm carapace length is enforced for recreational and commercial fishers, respectively. The most recent estimate of the recreational harvest of BSC in New South Wales was approximately 51 000 crabs (27 t) (West et al. 2016). Commercial catches of this species have tended to fluctuate around a long-term average of about 144 t. Four estuaries account for 85 per cent of commercial BSC landings in New South Wales (121 t in 2016/17), the most important being Wallis Lake (99 t in 2016/17).

Most SA BSCs are taken in the gulfs by hoop and drop nets. The most recent stock assessment by (Status of Australian Fish Stocks (SAFS), 2018) reported a total harvest of 47 t.

Since 2014, the country-wide commercial catch total of blue swimmers crabs hovers around the 1 750 000 kg mark (SAFS, 2018).

Dave prefers to use circular mud crab traps to catch blue swimmers.

Recreational Fishing
Gear Whilst blue swimmers can be targeted in calm inshore bays, estuaries are the most popular location. They can be caught by with dip nets, baited line, dilly nets and in carb traps. Whilst they will eat almost anything, fresh fish bait is by far the fest bait to use, with mullet and luderick being the pick of all. As well as fishing for luderick in season (for food and crab bait), I keep a supply of fish frames, heads and wings in the freezer. BSCs congregate over sand/seagrass shallows in areas that are just out of the main current, but as crabs find the odour of the bait via the current, some current is essential. They are most active at night and during low light periods.

If you use baited lines the idea is to wait until you feel a crab on your bait (a heavy weight or slow drag), and then slowly retrieve it back to you until the crab is within range of your net, when it can be quickly scooped from behind. Areas of strong current are to be avoided because crabs will come to the surface before they are in range of the net, and they usually let go. The advantage of using a line comes in the form of by-catch: expect bream, flathead and once in a while, mulloway!

Dilly nets (or witch’s hats) consist of a cone-shaped net secured around a loop of 5 mm diameter steel. Baits are secured on the middle of the base and the whole lot is tossed into the water via a cord and a labelled float. Crabs become enmeshed as they move over the net to feed on the bait. Check them every 2-3 hrs and rebait as required. Dilly nets are efficient crab catching devices but there are disadvantages with their use. Apart from being susceptible to mud crab damage, big drawbacks are bycatch mortality and the need to untangle each crab you catch, which can be time consuming and awkward.

The most efficient and easiest means of catching BSCs is with a trap as used for mud crabs. They are easy to bait and easy to empty. By-catch stays alive and in the case of protected species (estuary cod around my location), can be released unharmed. Whilst you can make your own traps from wire mesh (and collapsible ones too), I prefer the orange-meshed collapsible traps sold in tackle stores. They are best baited by placing fish flash into a 1 cm × 1 cm plastic mesh bag (protects bait from small fish), which is closed and secured inside the pot with a stainless steel “crab clip”.

As BSCs are caught away from the shore, and often in depths over wading height, boats are perfect. But be warned, always approach you float from down current to avoid catching the trap rope with the propellor.

In the SA gulfs BSCs are targeted with a crab rake during low tide periods. Simply rake the sand and watch for crabs erupting from the sand. Quickly invert the rake to scoop the crab and drop into a suitable floating tub.

I prefer a simple boiled blue swimmer over more complicated recipes. I keep them in cool, fresh saltwater until I’m ready to cook and then ike’ jime them by pushing the spike through their underside, just above the apex of the abdominal flap. An alternate method is to freeze them for 20-30 min. Then it’s simply a case of dropping them into salted boiling for 4-5min. and then cooling then in cold water straight from the pot (so they don’t overcook). When they’re cool enough to handle, take then outside, lift the flat and remove the top shell, gently hosing away the offal. Then sit outside with your Loved-One, eating them warm or chilled.

A big "muddie"!

Mud Crabs
Biology and Stock Status: Two species of Mud Crabs are found in Australian waters – Giant Mud Crab (Scylla serrata), hereafter GMC, and Orange Mud Crab (S. olivacea). The former constitutes more than 99 per cent of the commercial catch of mud crabs in the Northern Territory and Queensland, and the entire commercial catch in New South Wales.

There are two stocks of Australia’s giant mud crab population: the Northern Australian and East Coast biological stocks, respectively. The stocks in WA, NT and QLD are sustainable and surprisingly, NSW stock is undefined.

Most scientific studies have focussed on the Northern stock so less is known about the Southern stock. What we do know is that GMC’ live for 3-4 years and reach sexual maturity at carapace widths between 120 – 150 mm, with a carapace width of 200 mm considered the normal maximum.

Like BSC’s, male GMCs insert sperm capsules into the female during moulting, which enables multiple spawning events of 4.5 mill. eggs (on average) per spawn, females spawn at sea and the planktonic larval stage that can last for several weeks.

The most recent data (SAFS 2018) states that the total commercial catch in Australia in 2017 was close to 1500 t, (NT 275 t, Qld 988 t, NSW 160 t). Recreational catch data is hard to find, but we do know that NT rec anglers are estimated to have taken 24 t (2009-10), Qld 340 t (2010-11), NSW 21 t (2013).

The author with a nice catch of fresh "muddies".

Recreation Fishing
Most crabbers use traps and the most popular traps are the collapsible circular ones found in tackle stores. There are a few key points to consider when trapping muddies, being the use of fresh bait, location of traps near but not on, the shore and moon phase.

Mud crab theft is huge because traps are best left overnight and thievery can be rife under cover of darkness. Loss is minimised by using long, thin ropes and camouflaged floats that can be hidden in trees as opposed to being left floating on the water, although side scan sonar is a problem. The best idea is to set traps late and check them at first light if you can. I look for water that is sufficiently deep at low tide to keep traps out of sight as there will be problems if a trap comes out of the water at low tide. As with BSCs, trap where there’s current so the GMCs can find your bait, at least if the area is quiet enough. Otherwise, you might retain more individuals in calmer, hard to get to locations. Finally, GMCs are more active over full moon periods, so focus of these.

Muddies can be boiled, but chilli mud crab and garlic mud crab are my go-to recipes. Get online and you’ll find one that suits you. The basis of our favourite recipe is: tamarind paste, Chinese white wine, fish and tomato sauce, brown sugar and of course, chilli. We eat on the deck. I spark up a wok and cook away while we chat and enjoy a beverage between courses. It’s surprising how many friends want to join in a crab feast!

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