On the same evening as Alison Kock delivered an update on the Shark Spotters program, we listened to Masters student Kay Welz speak about her analysis of the reams of data that the Shark Spotters have collected. The range of data under consideration stretched over five years, from 2006 to 2011, and was collected at Muizenberg and Fish Hoek beach by spotters who had done 100 or more shifts each. This set of data was selected for its quality, covering the beaches with the most sightings and the highest risk of a human encountering a shark.
The analysis considered a range of variables, and whether any of these increase the probability of a shark being present during a spotting shift (approximately 4-5 hours long). Presence/absence was used instead of counting sightings per shift, because it’s not always possible to identify whether one shark is repeatedly buzzing the beach, or if it’s multiple individual animals. The variables in the study were:
The results are exciting and point to many avenues for future research. Here’s a summary that I jotted down during the talk:
Sea surface temperatures of 16-20 degrees increase the probability of a sighting. The statistical model used spat out a probability of a shark sighting at Muizenberg eight times higher when the water temperature was 18 degrees than when it was 14 degrees.
This warm temperature range corresponds to the preferred temperature range of many of the white shark’s summer prey items, such as steenbras. It follows that when these fish are in the bay, enjoying the warm water, white sharks follow them.
The study found an increased probability of shark sightings during the time from 3rd quarter (waning) to new moon. The chance of a shark sighting at Fish Hoek is four times higher at new moon than at full moon. This is also probably related to prey activity – perhaps there is more fish activity when there is less light to betray their movements.
The finding was that there have been more sightings since 2009 than in the years prior (2006-2008). Unfortunately the study period is not long enough to be dogmatic about this – it’s quite likely a long term fluctuation. Drawing conclusions on this one is dangerous.
A drive around the south peninsula on a windy day will help explain why this variable didn’t turn out to be significant. The beaches along the False Bay coast are oriented quite differently to one another, and surrounded by mountains at different angles which can influence wind speed and direction. Future studies should, it was suggested, incorporate factors such as wind direction, speed and duration, but at a lag so that the effect of “one day of light northwesterly wind” can be distinguished from “five days of strong southeaster”.
Kay pointed out that it is vital to bear in mind that this is not the final word on when you will see sharks near the beaches in False Bay. On a day when the moon is new and the water is 18 degrees no sharks might be sighted, while at full moon with 14 degree water you may see a shark. It’s important to think about this probabalistically; none of this research deals with impossibilities or absolutes, but it enables water users to make smart choices about their activities and the potential proximity of sharks.
Here’s an article from Wavescape on the research.