Commercial fishing for prawns in Australia is thought to have commenced in Sydney in the early 1800s and has over the intervening period developed from a local cottage industry to one that is equipped with the latest technology and that encompasses most of the coast of Australia.
In New South Wales (NSW), fishing for prawns is done in many estuaries and in ocean waters from the border with Queensland in the north to that with Victoria in the south.
Average landings in NSW over the five year period to 2008–09 were 1410 tonnes, worth in excess of $18 million.
In addition, in more recent times an aquaculture industry for prawns has developed in NSW which in 2008–09 produced 164 tonnes, worth $3.2 million.
Taking into account the value of boats, land (for aquaculture), gear and associated industries the humble prawn has a significant effect on this State’s economy.
The fisheries:
The common species in the NSW commercial catch are the eastern king, school, royal red and inshore greasyback prawns (Figure 1). There are three commercial fisheries that target prawns; namely the ocean trawl, estuary prawn trawl and estuary general fisheries.
As their names imply, the ocean trawl and estuary prawn trawl fisheries use one type of gear known as a trawl net to catch eastern king, school and royal red (ocean trawl only) prawns. In contrast, the estuary general fishery uses a suite of gears to catch eastern king, school and inshore greasyback prawns.
The recreational fishery operates only in estuaries and uses hand haul and scoop nets. For any given estuary, it catches on average 30% (by weight) of the commercial catch from the same estuary and overall its catches are predominantly eastern king prawns. However, school prawns may make up a substantial part of the catch of the recreational fishery in some estuaries.
In 2008–09 there were 12 permits for prawn farms in NSW. In recent years these farms have been growing almost exclusively the black tiger prawns. Just what is a prawn, what do we know about them and what steps are we taking to further understand these animals?
The basic prawn:
Prawns are invertebrates that live in the sediment, emerging from it for such activities as feeding, moulting and reproducing.
These swimming crustaceans have a head, tail, five pairs of swimming legs (pleopods), five pairs of walking legs (pereopods) and numerous pairs of head appendages (see Figure 2). The front three pairs of pereopods have claws.
Prawns have an external skeleton or shell which must be shed periodically to enable them to grow. The front part of this shell is called the carapace. It covers the head and protects the vital organs such as the heart, brain, gills and stomach.
On top of the carapace is a long, serrated spike called the rostrum. The prawn can use the muscular tail or abdomen as an escape mechanism in sudden backwards-directed flicks, but the usual method of propulsion is via the swimming legs.
Because prawns spend quite a large percentage of their time buried in the bottom sediments, some of the head appendages can be joined together to form a funnel. Water, containing oxygen, can then be drawn down this funnel and over the gills so the prawn can continue to breathe while buried in the sediments.
Colour changes:
Prawns, like most other crustaceans, are able to change colour, depending upon growth, background colouration and time of day. This colouration is due to the colour of the pigment in minute, special cells called chromatophores found in the prawn’s skin beneath the external shell.
Common colours found in these chromatophores are sepia-brown, red, yellow, yellow-white and blue.
The overall colour of the prawn depends upon the relative proportion of each pigment in the cell and the number and arrangement of these cells over the prawn.
Thus the colour bands on tiger prawns when viewed under the microscope appear as thousands of pigment cells, close together in bands and full of colour pigments.
On the other hand, school prawns have pigment cells more widely spaced and the pigment is contracted to the centre of each cell, giving the prawn a pale-spotted appearance. Deepwater prawns have a large number of red chromatophores and appear scarlet or bright red in colour.
This ability to change colour to suit different backgrounds helps the prawns to conceal themselves. In the case of red deepwater prawns such as the royal red prawn, the deep water acts as a filter to remove the red component of light leaving the blue component to reach the bottom. The combination of the red colouration and blue light in this depth of water makes the prawn appear black and therefore more difficult to see.
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