The concept of a shark net seems simple right? Put a net up – stop sharks coming in. However if we fenced our yards like we do with shark nets could that still stop a big dog wandering into your property?
We decided to take a look at the coverage that shark nets give at our local, Manly Beach in Sydney, to see just how big that fence is.
WHAT DO SHARK NETS LOOK LIKE?
According to the Department of Primary Industry website “The nets are 150 metres long by 6 metres deep and have a mesh size of 60 cm. They are a ‘sunk net’ set below the surface in about 10 to 12 metres of water, within 500 metres of the shore” and are “bottom set on bare sand” See full description in 2.3.2 of this report
WHERE ARE THE NETS?
We took out the trusty Garmin GPS watch off Manly Beach and went for an eery paddle along the 3 nets set up off the beach. Below is a map of the nets location and yes the length matched the above description from the DPI website nicely. Here’s a picture of exactly were the nets are currently located.
Now you don’t have to be a marine biologist to see a few not-so-small areas where a large fish or two can swim around (or over) however let’s not just go by visual – lets try and get all mathematical and scientifical.
THE SHARK NET MATH
Based on the stats from the DPI website and a small assumption about average water depth of 10m (instead of the 12m they state nets are placed in – nothing wrong with assumptions right?) – here’s how much coverage shark nets give Manly from Queenscliff to Shelly Beach (1500m):
Shark Net: 150m x 6m x 3 nets = 2,700 m2 total coverage
Manly Beach: 1500m long x 10m deep = 15,000 m2
Shark Net coverage: 18%
THE FISH EYE VIEW
So what would that look like from the water if you were a fish looking for a way into the beach? Here’s the cross section.
Could you say it’s like putting a ping pong net in an Olympic swimming pool to stop a goldfish getting through?
SO DO SHARK NETS WORK AND WHY?
Based on the above you clearly wonder how these things are supposed to work right? We personally have seen whales swim in and around the nets so many times so whats the theory? Is it simply to stop the big sharks taking up residence? Great Whites are migratory anyway so is that even relevant? It’s seems one of the key reasons according to the DPI website is simply that “since the introduction of the Shark Meshing Program in 1937 there has been just one fatal shark attack at a meshed beach in NSW.”
However a lot has changed since 1937 including huge changes in population both human and fish. A detailed report was commissioned in 2009 into the NSW Shark Meshing (Bather Protection) Program. In reading through the report it personally seems no firm conclusions or alternatives were made and politically the mantra was basically “something is better than nothing” to keep the public happy and the fatality stats back the decision. Since 2009 a lot of pressure is growing from many groups on alternative solutions. One thing is certainly clear from the reports and that is many innocent “non-target” species are being killed in the nets every year. That issue combined with the increase in attacks in NSW this year certainly backs a case for better solutions all round.
So sure they work but 78 years is a long time not to have come up with something substantially better.
** Note since writing this we have seen the NSW gov’s announcement to trial new “eco shark nets” at 6 beaches including lighthouse beach, the location of the latest attack in NSW. Let’s hope one of these great new initiatives work and improve the impact on the environment and that the north coast surfer’s get the help they need.
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