The nets are about 656 feet (200 meters) long and about 19 feet (6 meters) wide. They are fixed to buoys anchored to the seabed in water about 40 feet (12 meters) deep. Sharks can swim over, under, and between them.
The nets reduced shark attacks by 90 percent, catching an average of 1,245 sharks per year. But the nets are indiscriminate, catching not only the shark species primarily responsible for attacksZambezi or Bull sharks, Great Whites, and Tiger sharksbut also a large number of harmless shark species, as well as dolphins, stingrays, and turtles.
NSB staff tend the nets 20 days each month, and all live animals, including dangerous sharks, are released. The sharks and some rays are tagged. Most of the dead sharks are taken to the NSB laboratory for research. Still, marine biologists considered the level of by-catch unacceptably high.
Investigations showed that there were simply more nets than necessary. By reducing the number, and by cutting out their overlap, researchers determined it was possible to reduce the by-catch without increasing the risk to bathers. The research was backed by practical findings from Queensland and New South Wales, Australia.
"Nets have been reduced by 25 percent since 1999, to about 18 miles (29 kilometers). This has resulted in fewer animals getting caught, but it is too early to put a figure to it. We have to wait to see if it is consistent," said Dudley.
Shark Net Alternatives
The NSB is also looking at other ways to limit ecological damage without endangering bathers.
One is the use of "drumlines"floats (recycled oil drums) that are anchored to the seabed, with baited hooks attached. The baited hooks are particularly alluring to sharks, and mostly only to the species targeted. Experience in Queensland has shown that some turtles unfortunately also tend to take the bait, but dolphins and stingrays do not.
The NSB is looking at how many drumlines it would take to replace a net, and whether a combination of drumlines and nets would be effective.
An electric device called a Protective Oceanic DevicePOD or "SharkPOD" for shortis useful to individual divers. Since sharks are known to be sensitive to electricity, the SharkPOD creates an electric field around the user. It is expensive, however, and used mainly by commercial divers. The NSB has entered into an agreement with an electronics company to create a smaller device for divers and additional configurations for other user groups.
Another idea under consideration is the possibility of surrounding an entire bathing area with an electrical barrier.
Attaching pingers to the shark nets may help reduce dolphin catches. The battery-operated device sends out a pinging noise that scares the dolphins off. It is already in use in one area frequented by dolphins.
The NSB continues to work closely with local authorities as nets are removed.
"Whatever move we make, we need first of all to ensure it will not endanger the lives of bathers," agreed Graeme Charter, director of the NSB. "Safeguarding bathers is the reason the NSB came into existence."
It's shark week at National Geographic News. Tomorrow we will feature a story on lemon sharks.
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