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  Gone Fishing  Fish File

Blue Swimmer Crabs


Portunus pelagicus

There is nothing like a day's crabbing with the family. From a boat, drop and cone nets are used. The drop net is about 85 cm in diametre and lies flat on th bottom with the bait tied to the net in the centre. The cone net has a rim of the net on the bottom with the net rising from it and the bait is also in the centre. The cone net tangles the crab in the net. The drop net is pulled so that the crabs fall into the net. I personally prefer the drop net.

Old fish heads make ideal bait. However, these must be replaced or added every 15 minutes so that the smell remains in the water. The crabs are guided to the bait by the smell, so always crab on a fast-flowing tide. Never use fish heads of a crab predator, such as Snapper or Mulloway. Whiting, Bream, Garfish and my Ruff are excellent bait. You should pull the nets in every five minutes, as this is long enough for a crab to find its way in to your net.

I remember many years ago, when I was around 11 yrs of age, I went crabbing with my father off the Semaphore Jetty, and we used cattle bone as bait, with bits of raw meat still left on the bone we had purchased from the local butcher. We tied the bone to the centre of the net with some wire, and that day, we ended up with about 2 dozen nice sized Blue Swimmers.

Dabbing for crabs can also be fun. The dabbing spots are long, shallow sand patches and light mudflats. This is where the crabs bury in the sand. On a calm day, you will be able to see two very small eyes sticking out of the sand, with a slightly disturbed patch of sand where the body is hidden. Rub the dab net or rake along the sand and when it strikes the crab, it will come out of the sand immediately into a defensive posture, claws up. At this point, they are simple to dab. The best time to dab is as the tide is falling, when the crabs are swimming to deeper water.

Blue Crabs are widely distributed throughout tropical Australia but do not extend as far as Victoria and Tasmania. South Australia is probably near the lower limit of the animal's temperature range. Crabs, unlike other crustaceans such as lobster and prawns, have small abdomens (tails or flaps) that are folded up under the body. The females `flap' is broader than that of the male, and bears appendages to which her eggs are attached after spawning. Spawning is believed to take place in early summer, and by late summer the eggs hatch to become minute, transparent larvae which drift in the sea. The larvae pass through many different forms (metamorphosis) before assuming the typical adult shape.

Blue crabs are distributed throughout the inshore waters of South Australia partially in areas with extensive sandy bottoms and seagrass meadows in Gulf St Vincent and Spencer Gulf. Blue Crabs are more commonly taken during the warmer months as they move into the warmer inshore waters of the gulfs.

The fertilized female, or "sponge crab", produces approximately 2 million larvae in the salty Gulf Waters of Sth Australia. The tiny microscopic larvae, called a "zoea" (ZO-ee-ya) are swept out into the ocean where they go through a number of stages before they even resemble a crab. The whole surface of the body is enclosed in an external skeleton (exoskeleton) which the crab must shed in order to grow. This process is called "molting". The "zoea" molt into the next crab form, called a "megalops". This larvae, resembling a small lobster, eventually develops into an immature crab, or "first crab." Eggs hatched become immature crabs in about two months.

All crabs go into semi-hibernation during the winter. When the waters warm in spring, they crawl out and continue eating and molting. An immature female, or "she-crab", will molt a total of about eighteen to twenty-three times before reaching maturity. She will then cease to molt, after mating. Males have the same growth pattern, except that they do not cease growth after sexual maturity but continue to molt into the third summer. Since males continue to grow, a fully mature male crab will be larger in size than a mature female, called a "sook". Most female crabs attain full growth and mate only once, during their second summer.

Mating occurs where salinity preferences of the male and female overlap. The female is fertilized by the male during her last molt and only when she is soft (soft-shelled). During this period while the male is carrying the female, the pair is called a "doubler". The male stays with the fertilized female, or "sponge crab," protecting her from harm, until she is once again able to function as a hard-shell crab.

The fertilized eggs begin development internally and are later extruded out under the apron, producing a spongy, lemon-colored mass. As the embryonic crabs develop, the color of the egg mass darkens to orange, then brown, and finally black. Blue Crabs are serial spawners and release their larvae over a period of one or two weeks. Early reproductive females generally spawn prior to the coming winter, while those maturing later spawn the following spring.

One of the interesting characteristics of the blue crab is the way nature has allowed us to determine its sex. The `flap' on the underside of the female is wide. From the top, the female is a mottled grey/brown colour. The pincer claws of the female are a grey/brown colour and are much shorter than those of the male.

From the top the male is a bright blue colour. The pincer claws of the male are a blue colour. Gender is also determined by the differences in the shape of the abdomen, or "apron", as it is commonly called. The male apron is T-shaped. In the young immature female, the apron is triangular and sealed to the body. However, in the mature female, this apron is broadly rounded (almost semi-circular) and free of the ventral (bottom) shell.

Legal Minimum Length

A Blue Crab is undersize if its length is less than 11cm when measured from side to side at the base of the spines. Size limits now apply in all waters. You are no longer entitled to take undersize blue crabs from any jetty, pier, wharf or breakwater abutting land.

During the spawning there may be an egg mass under the flap of the female. To conserve this species, blue crabs with eggs attached are totally protected in South Australian waters.

Heavy penalties do apply to anyone found taking any fertilsed "sponge" crabs from the ocean.

Bag Limit

A maximum of 40 crabs per person per day may be taken. When fishing from a boat, the personal bag limit applies up to a total of 3 persons, after which a boat limit of 120 blue crabs per day applies.

Baits Used

Fish (Whiting, Bream, Tommy Ruff, Garfish), Squid.

When using fish for bait, do not use fish which are natural predators to the crab, for example, Snapper or Mulloway.

Berley Mix

Not really required. Generally, the smell of the fish bait in the net will attract the crab to the net.

Rigs Used

Hoop Net

  • A net attached to a hoop, including crab nets.

  • Maximum hoop diameter: 107cm

  • Maximum depth of net bag: 92cm

Hand Net

  • May be a dab net, dip net or shrimp net consisting of conical shaped netting joined to a hoop which is attached to a rigid handle.

  • Maximum hoop diameter: 1 metre

  • Maximum depth of net: 1 metre

Drop Net

  • Two hoops joined by a cylindrical or cone-shaped net bag.

  • Maximum hoop diameter: 107cm

Crab Rake or Cockle Rake

  • Hand-held devices for scraping the bed of any waters consisting of a pole, attached to which is a rake and a net or mesh bag.

Mussel Dredge

  • Maximum width: 1 metre.

  • Maximum depth of net: 1 metre.

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