Prawning is Western Australia’s third most valuable commercial fishing industry, worth about $25 to $35 million annually. Prawns are crustaceans with a shell, five pairs of swimming legs (pleopods), five pairs of walking legs, three with claws, and a tail.
A variety of species, mostly brown tiger (tiger), western king (king) and banana prawns are harvested, mainly in northern regions. The WA industry has an international reputation for sustainable, high-quality seafood from a pristine environment. The focus is on catching larger prawns, which are more valuable.
Recreational prawning is generally confined to southern rivers and estuaries. The most popular areas include the Swan and Canning rivers and the Peel Inlet near Mandurah. The western school (school) prawn and king prawn are the main species taken.
Prawns must periodically shed their shell (a process called moulting) so they can grow. The shell is made of protein, calcium carbonate and chitin (a substance that resembles the protein in human hair and nails). Before shedding, the prawn re-absorbs most of the protein and chitin from the old shell as the new one forms underneath, and increases water intake to create a space between the body and the new shell.
The soft-shelled prawn emerges from the old shell through a split between the carapace (the shell covering the body area) and tail. The new shell hardens and the prawn grows into it.
When prawns mate, the female must be soft shelled (newly moulted). The male inserts a sperm capsule (spermatophore) into the female. This remains inside the female reproductive organ until the prawn is ready to release her eggs (spawn) at which time the eggs are fertilised.
Distribution and habitat
King, tiger and banana prawns live in nearshore coastal waters. Tiger and banana prawns live mainly in northern areas with tiger prawns found from approximately Shark Bay northwards and banana prawns from Exmouth. King prawns are found along the entire coast of Western Australia. They also live in estuaries, such as the Swan River. School prawns are found in estuaries from Perth southwards.
The following details relate to the king prawn (Penaeus latisulcatus) and tiger prawn (Penaeus esculentus), in Shark Bay and Exmouth Gulf.
Prawns can live for two to three years. They mature at about six to eight months of age, and spawning occurs in offshore waters. King prawns tend to spawn throughout the year while tiger prawns mainly spawn during spring and summer. A single prawn can spawn more than once in any one year. The female releases hundreds of thousands of eggs.
Fertilised eggs hatch within 24 hours and larvae spend time floating in the water, developing through a number of stages, as they drift shoreward to shallow, hypersaline (highly salty) waters. When they reach these shallow waters, they’re called ‘postlarvae’, and are ready to settle on the bottom, where they develop into juvenile prawns.
They remain in the nursery areas for up to six months before they reach a size (near their size at maturity) when physiological changes demand they move back to oceanic waters to mate and spawn, completing their lifecycle.
During this migration, the prawns enter the trawling grounds (mainly sandy or muddy-bottomed areas) where they can first be caught commercially. These areas are generally offshore, in waters 10 to 30 m deep. This migration takes place in summer and autumn each year and is known in the industry as ‘recruitment’.
Prawns eat plant material, decaying organic matter, micro-organisms, small shellfish and worms. King prawns are sensitive to light so they bury themselves during the day and feed actively at night. Tiger prawns tend to be active day and night.
Squid, cuttlefish and demersal (bottom-dwelling) fish prey on juvenile and adult prawns. They are particularly vulnerable during the larval period, when predators are responsible for high death rates.
Illustration © R. Swainston/www.anima.net.au