Shark numbers & trends

Monday 17 Sept 2012

A significant link between lunar phase and water temperatures and shark sightings has been shown in the latest research by the Shark Spotting programme, writes Spike / www.wavescape.co.za.

sightings-aug-2012-graph1

Sitting in the Empire Cafe overlooking the crumbling surf of Muizenberg Corner, we discuss sharks, the ocean and surfers. Alison Kock, the shark scientist from Shark Spotters, looks forward to her first surfing lesson immediately after our meeting. She’s keen to get in the water, although from where I sit with a few more years surfing experience, the sea looks murky. But beginners seldom worry about conditions. We’re talking sharks. That’s Alison’s language, and she’s fluent in it. She’s excited that a statistical model they have been working on has crunched some fascinating trends. The starting point was data recorded by their spotters at Muizenberg Corner, Fish Hoek and Saint James over almost a decade. This included name of spotter, time of the day, lunar phase, water temperature, cloud cover, tides, and details on wind and swell.

The most basic output of the model, and that recorded by the press this week, shows that June and July have the least shark sightings, and that by September they’re steadily increasing into summer, when activity is high. These are trends we have come to know. They’re a common part of the seasonal ebb and flow as the resident white sharks of False Bay move more inshore during summer. White sharks look for easy prey, and their movements are specific to that. False Bay white sharks are mostly juveniles and sub adults that are sexually immature. As Alison says, white sharks look for easy prey, and their movements are specific to that. False Bay white sharks are mostly juveniles and sub adults that are sexually immature.

The media has overlooked aspects of the survey that are much more significant than the known fact that more white sharks are seen in summer. One that jumps out like a breaching animal is water temperature. How’s this: according to the data recorded by the spotters, there are four times more chance of spotting a white shark in 18 degree water than there is in 14 degree water. It makes sense. Bait fish are more active in warm water. Another is lunar phase. Though slightly less significant, there is still a trend showing almost twice the chance of a sighting during a New Moon than a Full Moon. The speculation here is that bait fish are more active under cover of darkness, and therefore so are predators. The problem of course, Alison says, is to find a practical use for this information. Immediate steps are underway to place water temperature and lunar phase data on information boards at the beaches. The more knowledge and information, the better for people to make up their own minds. “But we also need to digest this information and think about it a bit more. We need to consider what needs to happen.” She already has some ideas, around how water users assess conditions. Alison urges surfers to look for shark cues when sussing out the surf. Not only should we be checking out the wind, the swell and tides, but let’s expand the ritual to include shark probability. In essence, surf forecasting with a bit of shark forecasting. Does the spot fall within a high shark area? Are there shark spotters there? What time of the year is it? What is the water temperature? What is the moon doing? Is there a lot of activity in the water, such as birds, bait fish, dolphins and seals? Are you near a river mouth? Is the water murky? It’s about probability of spotting a shark given certain conditions. If numerous ‘red flag’ signals converge, surfers need to make a call, just as you might if an onshore is blowing, or the surf is too big at the spot you’re checking out.

Shark sightings on the Cape Pensinsula are increasing. Like it or not, that is inevitable. The way we deal with it is up to us. Finger pointing and acrimony will not resolve a thing. Our challenge is to keep the balance between hype and risk, and taking calcuated risks to enjoy our passion when we feel the time is right. We need to get past the hype, but without losing respect for an apex predator – a tightrope balancing act. Science is inexact. Scientists are the first to admit it. They do not provide the answers. But they do arm us with information that is critical to our understanding, and if we approach the issues as a collective, as a community, this information is invaluable. “We are not trying to foster fear. For us it’s about giving people information they can use.” Oh and by the way, she stood up on her first wave. On the second, she rode all the way to the beach.

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