NT Fish - Mudcrabs

Male and female mud crabs may be taken in Northern Territory waters and minimum carapace width size limits of 130 mm and 140 mm apply respectively for recreational fishers. Each crabber may take ten mud crabs and a boat limit of thirty mud crabs applies when there are three or more people on board a vessel.

Catching Mud Crabs

Pots and dillies are the most popular type of traps and although each crabber may use up to five traps, each vessel is limited to 10 traps, despite the number of people on board. Crab traps may not exceed a metre in any dimension and half a cubic metre in capacity. A float showing the user's name must be attached to each trap rope. Female mud crabs bearing eggs must be released unharmed and entanglement nets are prohibited.

Most recreational crabbers focus on creeks and rivers but good numbers of mud crabs can also be found on coastal flats, usually adjacent to a creek mouth. Most mud crabs are caught during the dry season and many people find an incoming tide most productive. Fish pieces are the preferred bait.

Why are some Mud Crabs not full of meat?

The mud crab is a crustacean of the Portunidae (swimming crab) family. Its growth is not a continuous process, but results from a series of moults that occur when it grows to the size of its current shell. The phase between each moult is termed the 'intermoult period'.

The onset of moulting is brought about through a hormonal process. A new cuticle is secreted under the old shell. The crab then rapidly absorbs water, splitting its shell along suture lines and then backs out of the old shell.

Inorganic salts stored within the body of the crab are then rapidly redeposited to harden the new cuticle into a larger new shell. The fluid within the body of the crab is then replaced with meat during the intermoult period, when the crab feeds ferociously.

Taking home a crab that has recently moulted may result in a disappointing end to a good day's fishing because the crab has not had a chance to fill its body cavity with flesh and will probably contain mostly liquid or a jelly mass with little edible flesh.

Tagging experiments conducted by Fisheries indicate that growth of mud crabs is rapid and commercial fishers report that when crabs with little flesh are returned to the water, they become full of meat within a few weeks.

How can you tell if a crab is full of meat?

First look at the shell condition. A crab that has recently moulted will have a very clean, shiny and intact shell, which sometimes appears almost translucent. Often, a good indicator of a full crab is the presence of algae or barnacles on the shell. Also, over a short period of time the shell of a crab will start to exhibit some general signs of wear. Inspect the inner 'teeth' of the claws. In an intermoult (full) crab these will wear down from foraging, grinding and crushing shells.

Figure 1. Mud crab claws showing inner 'teeth'	Commercial crabbers check each crab for fullness by pressing the shell at specific locations. There are a couple of areas on the crab shell that provide a good indication of fullness.
Figure 2. A mud crab showing pressure points for testing fullness	For both sexes you can apply pressure with your thumbs to either side of the carapace as shown below in Figure 2. The shell of a crab that is not full will flex a few millimetres in these areas when pressure is applied and may even make a clicking sound.
Figure 3. A mud crab: underside test for fullness of males	If the shell appears new and clean be careful not to press too hard as it is easy to push your thumbs all the way through the shell of a recently moulted crab. If the shell flexes at all, the crab is not full. If you are crabbing, you should consider returning the crab unharmed to the water. If you are buying crabs, you may wish to try another one. If you are still in doubt whether the crab is full, male crabs can be further tested by turning the crab over and pressing with your thumbs firmly on the abdomen plates adjacent to the third leg as shown in Figure 3. Be careful with this test, as the crab is very capable of attacking you with its claws, so make sure it is firmly tied or restrained first. Again, if the shell appears new, be careful not to press too hard, as your thumbs will penetrate the shell.

Whether you are an avid crabber or you buy crabs at the local retailer, you will ensure that every crab you eat is full of tasty flesh by following these simple rules:

1. Check the shell and claws for wear and tear. A clean shell and sharp teeth on the claws means the crab may not be full.
2. Press the top of the shell. If there is movement, the crab is not full.
3. To check male crabs, turn them over and press on the underside adjacent to the third leg. If there is movement the crab is not full.

Cooking Mud Crabs

Mud crabs are often boiled whole and they should be killed beforehand by chilling or piercing their central underside. Steaming them is also a popular cooking method and they should be cleaned and have their claws cracked beforehand to allow steam to penetrate and make them easier to eat. Crabs that are to be boiled should not have their claws cracked as this allows water to be absorbed into the flesh.

Whether steamed or boiled, mud crabs should be placed in a cooking pot once the water is boiling. This will usually stop the water boiling for a few minutes. Once the water is brought back to boiling point, mud crabs should be cooked for ten to twelve minutes then allowed to cool before being eaten.

Addtional Information

• The Australian Mud Crab Fishery Research and Management Forum
• Fishnote: How to handle, tie and store mud crabs
• Fishnote: How to Ensure Your Mud Crab is Full!
• Fishnote: Muddled Muddies
• Research: Mud Crab