A shark net is a submerged net placed around beaches to reduce shark attacks on swimmers. The majority of Shark nets used are Gillnets which is a wall of netting that hangs in the water and captures the targeted sharks by entanglement. The nets in Queensland, Australia, are typically 186m long, set at a depth of 6 m, have a mesh size of 500 mm and are designed to catch sharks longer than 2m in length. Shark nets are not to be confused with shark barriers.
Shark nets do not offer complete protection but work on the principle of "fewer sharks, fewer attacks". They reduce occurrence via shark mortality. Reducing the local shark populations is believed to reduce the chance of an attack. Historical shark attack figures suggest that the use of shark nets and drumlines does markedly reduce the incidence of shark attack when implemented on a regular and consistent basis. The large mesh size of the nets is designed specifically to capture sharks and prevent their escape until eventually, they drown. Due to boating activity, the nets also float 4 metres or more below the surface and do not connect with the shoreline (excluding Hong Kong's shark barrier nets) thus allowing sharks the opportunity to swim over and around nets. Shark nets can cost A$1 million or A$20,000 per beach per year.
Video Shark net
History
Shark net meshing was thought up by the NSW Fisheries in 1936, after a decade and a half of relentless Shark attacks off Sydney beaches. In March 1935, for example, two people -- one at North Narrabeen and one at Maroubra -- were killed by great white sharks in a single week. The meshing was never designed to enclose a piece of water -- even back then, they knew barrier nets would never survive a surf zone. Instead, it was designed to catch large dangerous sharks as they swam within range of the surf. At first, the catch was huge; over 600 sharks in the first year of operation, off just a few Sydney beaches. But over time, even without adjusting for the spread of the program across almost all Sydney beaches and into Wollongong and Newcastle, the catch declined. Today's NSW meshing annual average catch is 143 sharks, quite a number of which are released alive.
Nets were also first deployed off certain beaches off KwaZulu-Natal (KZN, formerly Natal) South Africa, in 1952.
Maps Shark net
Effectiveness of shark control
Ongoing shark control programs have been very successful in reducing the incidence of shark attack at the protected beaches. In the years from 1900 to 1937, 13 people were killed off NSW surf beaches by sharks; over the next 72 years, the death rate fell to eight, only one of which was at a meshed beach. This in a period when the NSW human population rose from 1.4 million to seven million -- and when more people began going to the beach.
In Queensland, there has been only one fatal attack on a controlled beach since 1962, compared to 27 fatal attacks between 1919 and 1961. Statistics from the NSW Department of Primary Industries indicate that before nets were introduced in NSW in 1936 there was an average of one fatal shark attack every year. There has been only one fatal attack on a protected beach since then and that was in 1951. Similarly, between 1943 and 1951 the South African city of Durban experienced seven fatal attacks but there have been none since nets were introduced in 1952. A more recent comparison shows that in South Africa there were three shark attacks, none fatal, at protected beaches in KwaZulu-Natal between 1990 and 2011, while there were 20 fatal attacks in the same period at unprotected beaches in the Eastern and Western Cape Provinces.
Bycatch
Shark nets also result in incidence of bycatch, including threatened and endangered species like sea turtles, dugongs, dolphins and whales. In QLD in the 2011/12 summer season there were 700 sharks caught, 290 above 4 metres in shark nets and drum lines.
In NSW, the meshing averages one humpback whale every two years; the whale is almost always released alive. In Queensland in 2015, the bycatch included one bottlenose and seven common dolphin (one released alive), 11 catfish, eight cow-nose rays, nine eagle rays, 13 loggerhead turtles, five manta rays (all but one survived), eight shovelnose rays, three toadfish, four tuna, and a white spotted eagle, which was safely released.
The NSW and Queensland also utilse acoustic pingers attached to the nets to reduce bycatch of dolphins, whales and other marine mammals. Use of the pingers has been shown to significantly reduce marine mammal bycatch.
Animal welfare groups suggest alternatives such as surf lifesaving patrols, public education on shark behaviour, radio signals, sonar technology and electric nets. Drum lines are also viewed, by some people, as an alternative: they consist of baited hooks aimed at catching only large sharks (though turtles, dolphins and rays are sometimes hooked).
However, it should be noted that bycatch from shark nets is minor compared to bycatch from other activities. For example, Australia's commercial shark fishing industry is taking over 1200 tonne of shark out of our various fisheries each year: everything from gummy shark to mako, and very likely a few white sharks as well. The NSW prawn trawling industry alone results in 64 tonne of shark as bycatch each year. Six percent of what's caught in the tuna longline fisheries in northern Australia is shark.
Cost
Total cost for the Shark netting program in NSW for the 2009/10 year was approximately AUD 1m, which included the cost of the nets, contractors, observers and shark technician, shark meshing equipment (dolphin pingers and whale alarms etc.), and compliance audit activities. For the 51 beaches protected, this represents a financial cost of approximately AUD$20,000 per beach per year.
Australia
In New South Wales, Australia, 51 beaches are netted. The nets are maintained by the New South Wales Department of Primary Industries. The nets are generally 150 metres long, 6 m wide and "bottom-set" on the seabed in depths of 10 m. The nets can be 500 metres from the beach. The mesh is sized 50-60 centimetres. Nets are lifted every 24 to 48 hours for servicing so as to prevent rotting, to clean out debris and to remove dead sharks and other marine life. It is said that 35-50% of the sharks are entangled from the beach side. Acoustic "pingers" have been fitted to the nets to warn off dolphins and whales and the nets are not in place in winter, the whale migration season. The Department states that the nets have "never been regarded as a means of absolutely preventing any attacks", but help to deter sharks from establishing territories. The netting program began in 1937 and during 70 years while the nets have been in operation, there has been only one fatal attack on a netted beach.
In Queensland, Australia, drum lines are used in combination with shark nets. Queensland's Shark Control Program has been in place since the early 1960s. In Queensland's 2011/12 summer season there were 714 sharks caught, 281 above 2 metres in shark nets and drum lines. Since 1997, the program catches 500-900 sharks annually, including several shark species of conservation concern. They include the following:
A fatal attack in Queensland occurred in January 2006 at Amity Point on North Stradbroke Island. The water at this location drops off to 30 metres depth, and bull sharks are known to frequent the area. Other beaches around the island were protected with drum lines at the time.
South Africa
In South Africa, numerous beaches in KwaZulu-Natal province are protected by the KwaZulu-Natal Sharks Board.
See also
- Shark attack prevention
- Shark Shield
- Shark proof cage
References
External links
- Shark Nets at the Smithsonian Ocean Portal
- Beyond the panic: the facts about shark nets (Coastalwatch - surf website)
- Baby whale dies in shark net (News24)
- ABC story detailing history and science behind shark nets
Source of article : Wikipedia
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