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    Maps about sharks (part I)

    • 27 August 2013
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    I’ve recently completed an online course via Coursera, called Maps and the Geospatial Revolution, offered by Pennsylvania State University. Coursera is fabulous, and offers things called MOOCs - Massive Open Online Courses. Most (if not all) of them are free. I’m using it as a way to supplement my education with (1) things I’d never heard about when I was 17 and choosing what to do with my future, and (2) things I was interested in back then already, and probably should have done instead of or in addition to what I did study. Also, (3) supplementing the toolkit I use every day at work, specifically with the programming language R (which is nice for statistics, but actually really interesting to me because you can use it to plot amazing maps). I heard about this course, and about Coursera, from this article.

    The Map MOOC I did comprised video lectures, quizzes, practical assignments using the ArcGIS mapping system, and discussion assignments in which we had to share maps we’d found or made, and ideas around the theme of each week’s class. The week four assignment required us to find and discuss maps related to natural hazards around where we live. Our practical assignments that week related to things like earthquakes, tornadoes, volcanic eruptions and floods. I struggled to think of any natural hazards associated with life in South Africa, but eventually came up with this:

    I live in Cape Town, South Africa, a location that is gloriously far removed from the edge of any tectonic plates. We don’t have volcanoes, we don’t have earthquakes to speak of, and there are no tornadoes, hurricanes, typhoons, avalanches, forest fires (although we do have some) or any other massive weather or seismic phenomena. For this I am grateful.

    We do, however, have lots of lovely sharks living in the waters around our coastline. South Africans are outdoor-loving people and some of our most popular swimming beaches (Fish Hoek in Cape Town, for example) have been the site of multiple interactions between humans and sharks, several fatal.

    In Cape Town the Shark Spotters program aims to strike a balance between protecting humans and conserving white sharks, which are the main large shark species found in our waters. Spotters watch the beaches from elevated locations (the Cape Peninsula is nicely mountainous, so this is possible), and since white sharks tend to cruise near the surface when they come inshore, they can be seen if conditions are good. A siren warns swimmers out of the water. When the shark leaves, the swimmers can get back in. A flag system on the beaches also reports whether visibility is good, and whether a shark has been seen. A report by the City of Cape Town on the most recent incident, in which a body boarder was killed, gives a good idea of how the city approaches the problem – the science-based, conservation oriented approach is something I’m very proud of about my city.

    The International Shark Attack File, maintained at the Florida Museum of Natural History, keeps (or tries to keep) a record of all interactions between humans and sharks, worldwide. They have some quite rudimentary, but easy to understand, maps showing how many interactions have occurred in various places. Simple colour scales indicate relative frequency. Here’s the one for South Africa. For comparison, here’s one for Australia, where the issue of shark bites has been often in the news recently. Western Australia has had a number of fatalities (mainly surfers), but the map shows that taking the entire history that the ISAF has on file into account, Western Australia actually lags some way behind Queensland and New South Wales.

    There are various explanations proposed for the uptick in interactions between humans and sharks in recent years – South Africa, Western Australia and Reunion Island are examples of countries that have seen an increased frequency. The two most plausible sounding explanations are (1) natural variation – you’re dealing with a random variable that will fluctuate around some kind of long term mean, which is unknown, and (2) massive increase in the number of water users, and hence more opportunities for people to meet sharks in the ocean. Populations are more mobile, surfboards and holidays are cheaper than they used to be, and swimming in the sea for recreation – which was almost unheard of 100 years ago, is now a “thing”.

    Publishing maps of the shark bite data enable people to make informed decisions based on the risks of swimming or surfing at any given time. Shark Spotters also make additional information, such as the time of year when sharks approach the beaches (the Southern hemisphere summer months) and the conditions (water temperature, phase of the moon, etc) that correlate with more frequent sightings, available to the public.

    Anyway. Long waffle! I am quite passionate about this subject.

    Here’s the ISAF map for South Africa that I shared. Click on the map to go to the ISAF website, where you can see the legend and a bit of supplementary information. This is an example of a choropleth map, in which areas are shaded according to a statistical variable (in this case, the number of shark bites that occurred in the province since 1905).

    ISAF shark attack map for South Africa

    ISAF shark attack map for South Africa

    My fellow students raised some great points about the ISAF maps, which are of dubious utility. Juernjakob said that

    I think the choropleth map on shark attacks in Australia (and probably the one for South Africa as well) could be improved by correcting for the population of the states. Interactions between sharks and humans are more likely where there are more humans and more sharks. Since we know where the humans are, let’s take that out of the equation by normalising the number of shark attacks by the population of humans. That would give a better indication of shark population/aggressiveness/human behaviour /whatever the factors are that lead to shark attacks.

    He also rightly pointed out that:

    There’s still a lot of uncertainty about the data quality (is it all gathered over the same period of time? How many people actually go swimming in the ocean, as opposed to just living in the state?), but it’s a step.

    I replied that:

    It’s not clear whether the data is gathered across the same period of time for each area, and the quality will definitely vary from location to location. Also, sharks and people tend to bump into each other infrequently and with widely different time intervals between bites (sometimes years, sometimes months). Making a map using longer time periods is more instructive in some sense, as you might be able to infer more from a large sample. But if there’s been a big change in the number of people, the way they use the water, or in the number of sharks, a single map might hide some of the information. Something a rolling 10 year number (like “average number of shark attacks per 100k inhabitants over a 10 year period”) shown in a sequence of small maps (like this one - it’s a members only page but the image says it all) would convey a time trend as well as location information.

    Another very cool map related to sharks that came up in the week four discussion about the ISAF map is the one maintained by Ocearch, showing the tracking data for the white sharks tagged during previous Ocearch expeditions. You can check it out here – and spend ages looking at what sort of travels are made by the white sharks tagged around South Africa.

    This discussion gave me some ideas for my week five (final) assignment in the course, which was to make a map that tells a story. More to follow…

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