Right: a drone moves out to provide live images of any sharks moving along a beach. Useful, say surfers, but not foolproof.
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Background information
(The information given below has been drawn from a number of cities.
Much of the information of shark nets was drawn from the Wikipedia entry titled 'Shark net'.
The full text of this entry can be accessed at https://en.wikipedia.org/wiki/Shark_net
Much of the information on shark barriers was drawn from the Wikipedia entry titled 'Shark barrier'.
The full text can be accessed at https://en.wikipedia.org/wiki/Shark_barrier
The government of Western Australia's Shark Smart internet sites outlines the beach safety and protection initiatives the government has put in place.
This information can be accessed at http://www.sharksmart.com.au/staying-safe/beach-safety-protection-initiatives/)
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 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 up to 1.4 million dollars.
History of shark nets
Shark net meshing was first devised by the New South Wales Fisheries in 1936, after a decade and a half of 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 as 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 New South Wales meshing annual average catch is 143 sharks, quite a number of which are released alive.
Nets were also deployed off certain beaches off KwaZulu-Natal (KZN, formerly Natal) South Africa, in 1952.
Shark barrier
A shark barrier (otherwise known as a 'shark-proof enclosure' or 'beach enclosure') is seabed-to-surface protective barrier that is placed around a beach to protect people from shark attacks. Often confused with shark nets, shark barriers form a fully enclosed swimming area that prevents sharks from entering (nets aim to reduce shark populations). Shark barrier design has evolved from rudimentary fencing materials to netted structures held in place with buoys and anchors. Recent designs have used plastics to increase strength, versatility and to reduce the environmental damage of bycatch.
Shark barriers work on the principle of excluding sharks from a designated swimming area. Shark barriers form an "underwater fence" from seabed-to-surface, beach-to-beach. Shark barriers are seen as a more environmentally friendly option as they largely avoid bycatch, however they cannot protect the same sized area as culling methods.
Shark barriers are not generally used on surf beaches because they usually disintegrate in the swell and so are normally constructed only around sheltered areas such as harbour beaches. It should be noted that where there are surf conditions at Hong Kong beaches, the swell is 'typically small'. Even if they were in place at surf beaches, they would not protect surfers who go some distance from shore.
The shark nets are more suitable for surf beaches as they can be anchored beyond the deepest sandbank and can be easily taken out of the water when ever any big swell is forecast.
Shark net and barrier use in different Australian states
New South Wales
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.
Shark meshing on Sydney beaches began in 1937 to reduce the numbers of sharks and thereby reduce the risk of shark attacks. Originally beaches from Palm Beach to Cronulla were meshed. In 1949 beaches in Wollongong and Newcastle were added, with Central Coast beaches added in 1987. Today 200 km of coastline are meshed. The nets are set for eight months of the year from September to April inclusive.
There have been trials of shark enclosures or barriers proposed for two northern New South Wales beaches - off Seven Mile Beach at Lennox Head and at Ballina's Lighthouse Beach. Neither proceeded because it was judged that conditions made them unworkable.
Queensland
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.
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.
Western Australia
The Western Australian Government has funded a number of protected swimming areas, or beach enclosures to protect swimmers from sharks. Current locations include:
Old Dunsborough, City of Busselton - undertaken as a trial between January and April 2014 to test the suitability of beach enclosures in protecting swimmers from sharks. The enclosure extended about 100m from shore and ran parallel with the beach for approximately 300m. A review of the enclosure found that it was successful, providing a cost effective option for a protected swimming area with no environmental impact.
Busselton foreshore, City of Busselton - installed in November 2015.
Middleton Beach, City of Albany - installed March 2016 for a three year trial.
Sorrento Beach, City of Joondalup - due to be installed in time for the 2016/17 summer.
Quinns Beach, City of Wanneroo - due to be installed in time for the 2016/17 summer.
The City of Cockburn has also funded its own enclosure at Coogee Beach near Fremantle. The enclosure is deployed year round on a three year trial basis to 2017.
Following seven fatalities in the three years from 2010 - 2013, a limited number of drum lines were deployed for a three month trial period between the end of January and April 2014.
There have been no permanent drum lines deployed off the Western Australian coast since 30 April 2014.
Recent shark attacks in New South Wales
2015
February 6, Newcastle (Merewether Beach) - Male bodysurfer bitten on foot by wobbegong shark. Minor lacerations.
February 8, Ballina (Seven Mile Beach) - White pointer attacks surfer Jabez Reitman, 35, about 60m from shore. Chunk of flesh ripped from his back and lacerations to hip.
February 9, Ballina (Shelly Beach) - Japanese surfer Tadashi Nakahara dies after 4m great white severs both legs in an attack close to shore.
May 3, Taree (Saltwater Beach) - Local man treated for serious wounds to his left arm and right hand after attempting to fight off a white pointer about 60m from shore.
July 1, Yamba (Flat Rock) - Great white severs nerves and tendons in male surfer's hand after attacking as he paddled out through the breakers.
July 2, Ballina (Lighthouse Beach) - 32-year-old bodyboarder Mathew Lee mauled by 4m great white. Serious injuries to both his legs but both limbs saved in emergency surgery.
July 31, Evans Head (Main Beach) - Surfer Craig Ison, 52, attacked by a white pointer during morning surf, suffering serious wounds to his leg and arm.
August 22, Port Macquarie (Lighthouse Beach) - Bodyboarder Dale Carr loses 2.5l of blood after being mauled by 3m great white about 150m from shore.
September 4, Forster (Black Head Beach) - White pointer mauls surf-skier David Quinlivan, 65, as he paddles offshore. Serious wounds to his left ankle.
November 10 - Ballina (Lighthouse Beach) - Champion junior surfer Sam Morgan, 20, seriously injured after 3m bull shark mauls his left thigh.
2016
March 30 - Kiama (Bombo Beach) - Surfer Brett Connellan, 23, loses 70% of his upper left leg and suffers deep wounds to his left hand in a white pointer attack about 100m from shore.
September 26 - Ballina (Lighthouse Beach) - 17-year-old surfer rushed to hospital with serious leg wounds after being mauled shortly before 9am.
Apparent increase in unprovoked shark attacks
In the 1990s, the average number of unprovoked attacks in Australia was 6.5 per year. That figure jumped to 13 per year over the last decade. In 2015 alone there were 22 unprovoked attacks, including one fatality and 13 injuries.