Shark net
From Wikipedia, the free encyclopedia
- For other uses, see Shark net (disambiguation).
A shark net is a submerged net placed around beaches to reduce the occurrence of interactions between sharks and swimmers.
Shark nets do not offer complete protection but work on the principle of 'fewer sharks = fewer attacks'. They reduce shark attack occurrence via shark mortality. Reducing the local shark populations is believed to reduce the chance of an attack. 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 4m 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 also have a high incidence of bycatch including threatened and endangered species such as some turtles, dugongs, dolphins and whales [1]. Animal rights groups suggest alternatives such as surf lifesaving, public education on shark behaviour, chemical and electrical repellents, and drum lines. Drum lines are baited hooks aimed at catching only large sharks (though turtles are sometimes hooked).
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. In the 70 years that the nets have been in operation, there has been only one fatal attack on a netted beach.
In Queensland, Australia, drum lines are used. The Shark Safety Program has been in place since the early 1960s. A fatal attack in January 2006 on a North Stradbroke Island beach, was on a beach protected by drum lines.
In Hong Kong, after the shark-attack death of 3 swimmers over 10 days in 1995, the government installed shark nets on all 32 gazetted beaches. Unlike the long-line and gill-net designs common in Australia and South Africa, these are permanent installations and are barrier nets. They have zero by-catch and are in fact some of the most bio-diverse areas in HK waters, as they form artificial enclosures which act as marine reserves. There have been zero fatalities or by-catch since installation in 1995.
The Hong Kong nets are generally 35 mm square on the surface 2 m and 100 mm square thereafter. They are suspended off 225 mm HDPE pipe or BL14 Marine Float Lines, and anchored strongly to resist the many typhoons and waves up to 10 m. The design philosophy draws heavily on international aqua-culture experience. They are anti-fouled, and spend an average of 9 months a year in the water. An average net enclosure would be 500 m long and either semi-circular or rectangular in shape. They are diver-inspected a minimum of two times a week, and independent verification is required. They exclude larger fish and predators but do not capture any other sea-creatures. They also exclude floating refuse, and clearly define the swimming area. They can be clearly picked out on Google Earth - see "Repulse Bay".[citation needed]
They are operated and maintained by independent contractors, selected under competitive public tender by the Hong Kong government and inspected by a third party. This ensures consistent quality service and value-for-money. The cost of netting Hong Kong’s beaches is a small fraction of total running costs for a public entertainment venue.
