Article: The Atlantic on right whales and ship traffic

The danger to North Atlantic right whales from ship strikes and ocean noise has been discussed here before. The introduction of speed limits for ships in 2008, when there were fewer than 450 of these whales alive, has made a small difference, but the environmentally conscious Bush administration ensured that the law mandating these speed limits expired in 2013 (this year), because, like, time is money. (This really makes me despise politicians.) There are moves afoot to make the speed limits permanent before the end of 2013. This would be a good thing.

The Atlantic quotes Michael Moore of Woods Hole Oceanographic Institution:

According to the New England Aquarium, ship strikes and fishing-gear entanglement until recently were killing the whales faster than they could reproduce. “Laws aside, there are fundamental reasons not to knowingly and willingly destroy a species,” says Moore. “Once it’s gone, it is forever.”

Threats to North Atlantic right whales
Threats to North Atlantic right whales

Read the article here. The best part of it is actually a map/infographic showing the extent of the problem, and that can be found here or by clicking on the clipped version above.

Guest post: Christo on encountering a great white shark

Yesterday we had Craig’s point of view… Here’s Christo van Schalkwyk’s account of the Clan Stuart dive on which he and his fellow divers encountered a white shark. Christo has been diving since March 2012, and in the time since then has logged over two hundred dives, most of them here in Cape Town.

About 30 seconds into the dive, just as I got to the bottom, a little to the north of the engine block, I saw the shark approach from the south. It swam past us towards the north. It turned and swam back down the wreck in a southerly direction, on the inshore side. For a while it was out of sight. We kept looking out for it, while motioning to the other divers to bunch together and stay low on the wreck. A few seconds later we saw it approaching from the south again. I could see both eyes as it swam straight at me. When it was about three metres away it veered off slightly to swim past us, parallel to the wreck. At this point it was only about two metres away from Craig and me. I remember choosing the spot where I was going to hit it if turned back towards us.

Fortunately it kept gliding past and as the pectoral fins came past, something seemed to disturb it. It flicked its tail once and shot away to the north. (Seeing the video taken by Vlad later, it seemed as if one of us exhaling was what disturbed the shark, but this is only speculation.) After a second or two, it was out of sight and we didn’t see it again.

We crouched down low on the wreck, looking around, and repeated the instructions to the other divers to keep close and low down. At this time we saw Sergey coming towards us from a rocky outcrop (or piece of wreckage) about 3-4 metres away from the main wreck, towards the deep side. We beckoned (with some urgency) to him to come closer. He swam quite slowly towards us, but when he got close enough, we pulled him down onto the wreck with us. As he was positioning himself, his weightbelt caught on a piece of the wreck and came off. I had to help him put it back on from underneath.

We stayed where we were (just north of the engine block of the wreck) for about another minute or so. I remember looking at my dive computer which read 2 minutes at that point. It didn’t seem a viable option to surface, even though I knew Tony would be close with the boat. I didn’t fancy the notion of hanging around on the surface, trying to get all 6 divers on the boat, all the while not knowing where the shark was. After another half a minute or so, Craig and I had a hand signal discussion on what to do next. He suggested heading south down the centre of the wreck, in the opposite direction to the shark’s last known heading. I thought we should go for the beach, to the north west. We agreed on the beach and started off in that direction, staying very low.

Just before leaving the wreck, Craig’s weight belt came loose as well. I took the reel from him and held on to his BCD with one hand and the wreck and reel with the other, while he tried to put the weight belt back on. This seemed to take forever – I remember seeing Vlad sliding in under a raised sheet of steel and hiding there (and feeling a bit jealous of his nice cover…). Eventually I gave the reel back to Craig and got him to hold on to the wreck and got in underneath him to try and see what the problem with the belt was. Once the belt was back on, we dropped down onto the sand on the shore side of the wreck.

Then we had to swim over the sand, without cover, towards the beach. It took a while to gather the group together to do this. We stayed very low, flat on the bottom. As we swam the group seemed to fan out, so we stopped once or twice to reassemble. Craig kept watch to the north, while I scanned the southern arc. Once we got into shallower water the surge took us along quite quickly and the group spread out even more, but it wasn’t possible to do anything about that any more. We got tumbled a bit in the breakers on the beach, but in the end managed to help each other to the beach unscathed with only the loss of one mask.

Total dive time: 13 minutes
Boat entry, shore exit.

Christo’s diagram of the dive site, with indications of what happened where, is below. Click on the image to enlarge it!

Christo's drawing of the scene
Christo’s drawing of the scene (click to enlarge)

Article: Slate on ocean salinity

Phil Plait, the Bad Astronomer, writes briefly about images released in June 2011, taken by the earth-observing satellite Aquarius.

Ocean salinity as recorded by Aquarius
Ocean salinity as recorded by Aquarius

As Plait points out,

Observations like this are crucial for us to understand just how our fiendishly complex planet works. Especially now, when our climate is changing, and those changes are evident even year by year.

See the full article here.

More about the wreck of the Brunswick

I attended a talk about the Brunswick, which is wrecked off the end of Long Beach in Simon’s Town, at the Institute for Maritime Technology (IMT), in August. It was given by Jake Harding, who has just completed a thesis on the wreck for his Honours degree at the University of Cape Town. Considering that all I know about the Brunswick (even with reference to Shipwrecks and Salvage in South Africa) can be summed up in two sentences, it was extremely interesting to hear from someone who has researched the ship, and examined the wreck for further information.

The ship

The Brunswick was an East Indiaman, a merchant ship that transported goods between Britain and the East Indies. She was captured by the French vessel Marengo in 1805, and brought to Simon’s Town as a prize of war. She was captured on her sixth voyage and was in quite poor shape. Five or six voyages was considered a good lifespan for an East Indiaman. They travelled very long distances on each voyage and were away from their home ports for over a year at a time, sometimes much longer. During the time she was at anchor in Simon’s Bay, strong winds caused her anchor cables to part, and she ran aground. She was carrying a cargo of sandalwood and cotton, which was sold along with the wreck in an auction. Most of it was salvaged, although archaeologists have spotted the odd piece of sandalwood on the wreck site.

She had 30 guns, which is apparently quite a small number – warships of the time would have had hundreds of guns. At the time when the Brunswick was constructed (late 18th century), iron was used instead of hardwood for the braces or “knees” that held the hull of the ship out in its characteristic shape. She was about 40 metres long and 13 metres wide with three decks. Her hull was sheathed in copper, and the drift bolts holding the parts of the ship together are copper and iron.

The wreck

The wreck was officially discovered and identified as the Brunswick in 1993, although her rudder was found and salvaged in 1967. It can be seen in the courtyard of the Slave Lodge in central Cape Town, formerly the South African Cultural History Museum. It used to be covered with copper sheets, but those were mostly stripped off. It’s huge – over 4 metres long, but with all the original fittings it was likely over 5 metres in length.

The rudder of the Brunswick
The rudder of the Brunswick

It’s clear when you dive the site that there’s quite a lot of wreck hidden under the sand, and even in the few times we’ve dived her we’ve been able to see how the sand shifts and covers and uncovers various parts of the ship. The Simon’s Town harbour wall has led to a lot of sediment build up in the area. When the wreck was surveyed in 1994-1995 for Project Sandalwood, a survey done by IMT and the South African Cultural History Museum, they recorded pieces three metres high sticking out of the sand. There’s nothing like that today. Longshore currents also cause periodic sediment build up and removal, and the cryptic little fish called steentjies annually uncover huge areas of the wreck during their mating displays.

According to Jake, the keelson (like a keel strip), part of the copper strap attaching the rudder to the boat, and a large number of iron knees (we have only seen a couple), and one or two pieces of sandalwood are still on the wreck site. We will be looking carefully for these elements next time we dive there.

Bookshelf: Submerged

Submerged – Daniel Lenihan

Submerged - Daniel Lenihan
Submerged – Daniel Lenihan

Until his retirement, Daniel Lenihan had a dream job, combining diving and archaeology, at the US National Park Service’s Submerged Resources Unit (formerly the Submerged Cultural Resources Unit or SCRU, and renamed in 1999 to include natural resources). He cut his teeth diving during the heydays of the sport, and became a skilled cave diver working with Sheck Exley in Florida in the 1960s and 70s, and deployed many of the principles of that sport when penetrating historical shipwrecks in the United States and around the world.

Submerged, a memoir of Lenihan’s time in the National Parks Service, is a cracker of a book – Clive Cussler wishes he could write like this, and it isn’t even fiction. A competing volume (if you will), Adventures of a Sea Hunter, by James Delgado – a sometime colleague of Lenihan – covers some of the same ground, but with far less impact and immediacy. Lenihan is clearly a doer, and has the requisite ego and charisma to make things happen, even in a bureaucratic setting.

The SCRU team dives and maps wrecks all over the world, from freezing, rough conditions in the Great Lakes in the United States, to a war grave in Pearl Harbour, Micronesia, the Aleutian Islands, and Bikini Atoll, where the US conducted multiple nuclear weapons tests. The chapter that made the greatest impression on me, however, was Lenihan’s account of a body retrieval that he and a buddy did of a diver who had gotten lost and drowned inside an old building that is now submerged in a dam. His account of diving in visibility measured in centimetres, trying to figure out where the diver could have gone in that confined, dark space, is riveting and terrifying. I was also very interested by the tests his team did on submerged motor vehicles, to determine how quickly a car fills up when it is driven into water. Lenihan himself drove a car into a dam, with scuba gear on the seat beside him, and his team attempted a rescue. Because of the air pockets in the vehicle, it was far less stable and much harder to access while submerged than the team initially expected.

The toughness, rigour, safety awareness and innovation that the SCRU team brought to their work is marvellous to me, particularly as they were technically part of an arm of the US government. None of the arms of government are particularly effective in South Africa! This is a fascinating, wide-ranging read that will interest divers and those fascinated by history, particularly its relics that lie underwater.

If you’re in South Africa you can get the book here, otherwise try here or here. For a kindle copy, go here.

Maps about sharks (part II)

For the final assignment in week five of the Coursera MOOC I just completed, Maps and the Geospatial Revolution, we were tasked with creating a map that tells a story. Following on from the discussion I’d had with my classmates in week four, I decided to make the series of maps I’d talked about in the discussion, showing trends in human-shark interactions through time. I downloaded the entire International Shark Attack File database for South Africa from SharkAttackFile.info, and some census data from Statistics South Africa. I used the census data to normalise the ISAF data, so that we can look at rates of interaction and account for increasing coastal populations.

South African coastal provinces and sea surface temperature
South African coastal provinces and sea surface temperature
Colour scale for maps showing number of incidents
Colour scale for maps showing number of incidents

This series of maps shows the number of interactions (fatal and non-fatal, including bites, nudges, etc) between humans and sharks for each decade between 1951 and 2010, as well as the rate of interactions per million people. Its purpose is to determine

  • whether the widely-held perception in South Africa (and in the Western Cape province particularly) that shark attacks are increasing unchecked, is correct;
  • and whether the frequently-provided partial explanation, that increasing coastal populations – putting more people in the water and in the home of sharks – explains the perceived increase or not.

I think these maps are an improvement over the ISAF map for South Africa for a couple of reasons:

  • We can spot trends through time, which a single, static map does not allow.
  • The ISAF data quality has probably improved over the years, and plotting data from 1905 and 2005 on the same map is questionable. At least this way we can just look at the most recent maps if we want to know what things look like right now.
Colour scale for maps showing number of incidents per million people
Colour scale for maps showing number of incidents per million people

From the map series above, you can see that the number of encounters between humans and sharks each decade has remained fairly constant. A naïve reading of the data would suggest that there have been no advances in mitigating the risk of a shark bite. However…

The rate of encounters per decade (normalising the data for increasing coastal populations and water use – orange maps below) has mostly been decreasing, after peaking in 1970. In KZN, this can probably be attributed to increased shark netting at swimming beaches. In Cape Town, the Shark Spotters program (started in 2004) warns swimmers out of the water when a white shark is spotted near the backline of the surf, and notifies them when it is safe to return to the water. In the last 25 or so years scientific research has also shed light on the conditions that sharks tend to prefer, enabling water users to avoid the water at these times.

The water temperature off the Western Cape ranges from 10 to 22 degrees celcius, while off KwaZulu Natal (KZN) it varies between 20 and 25 degrees with the seasons. Warmer water temperatures draw more people into the water, where they meet sharks. For the first 40 years of my data set KZN has a relatively high rate of shark encounters because all it took was a bathing suit to enjoy the ocean there. The Western Cape only caught up to KZN with respect to human-shark interactions once thermal protection from wetsuits became commonplace and more affordable in the 1970s, and people were able to stay in the water for longer.

There is a lot of natural variation in the numbers. Detecting firm trends in time series derived from an ecosystem is tricky, as many factors influence the process under observation. Great white sharks have been protected in South Africa since 1994, but because there was no known baseline population figure for the animals at that stage, it is difficult to assess whether there has been an increase in shark numbers as well as bathers. The presence of a whaling station in Durban, that closed in 1975, probably also contributed to numbers of sharks close to the Durban beachfront (attracted by the offal that was pumped out into the ocean in front of the Bluff) that were larger than there would be otherwise.

Finally, the likelihood of you meeting a shark while in South African waters (unless you go out of your way to, on a cage or baited dive) is very, very small. The most recent numbers put it at a less than six in a million chance over ten years. We dramatically overestimate the risk of a shark incident, because they are so emotive and fear-inducing.

If you want to see these maps laid out nicely in sequence, click here to download a pdf of my assignment. It’s easier to follow, I promise!

Maps about sharks (part I)

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…

Article: Randall Munroe (xkcd) on draining the world’s oceans

After water has been drained to a depth of 2 kilometres
After water has been drained to a depth of 2 kilometres

Randall Munroe, the man behind the xkcd webcomic I love so much, conducts another thought experiment in his “What If?” series, this one related to the ocean:

How quickly would the ocean’s drain if a circular portal 10 meters in radius leading into space was created at the bottom of Challenger Deep, the deepest spot in the ocean? How would the Earth change as the water is being drained?

Check out the Agulhas shelf extending off South Africa in the map above. After even just a few tens of metres of water had gone down the imaginary drain, all the recreational dive sites we frequent would be on dry land.

Complicating the imaginary drainage process is the fact that the ocean is full of mountains and trenches – and many of the mountains are higher than the ones on land. As the water drains, parts of the world’s oceans would become cut off by these mountains and ridges, and stop draining. The step by step illustrations of the progressive changes in the earth’s landmass are surprisingly disturbing. It’s a kind of reverse global warming scenario, where sea levels fall instead of rise.

Read the full article/thought experiment here. Spoiler: we’re all going to be Dutch

Visualising ocean surface temperatures

Here’s another beautiful sea surface temperature visualisation, this one from NOAA. It starts with the Agulhas and Benguella currents that wash South Africa’s shores, and then moves to other locations across the ocean. The dial in the bottom right hand corner of the screen shows what month of the year the data is for. I think it’s beautiful.

You can also view the video on The Atlantic website here, with a bit of a write up.

Article: Wired on shipping pollution

An article on Wired.com led me to this striking composite image created from measurements taken by NASA’s Aura satellite. The satellite measured the amount of nitrogen dioxide (NO2), which is a pollutant created by various forms of human industry (that’s why the coastal areas are so dark) and by ships’ engines. You can see a distinct line between Singapore and Sri Lanka, site of a major shipping lane. There’s more from NASA on the subject here.

Nitrogen dioxide pollution (darker is worse)
Nitrogen dioxide pollution (darker is worse)

Ships’ tracks are also visible at an atmospheric level, as particles from their exhausts float up into the atmosphere and create what looks like the contrails that form behind aeroplanes. There is an explanation of the process, and an image of those kinds of tracks, here (image reproduced below).

Ship tracks visible in the atmosphere
Ship tracks visible in the atmosphere

The image of these trails is actually a stereo one and if you have a pair of 3D glasses… they won’t help at all!

It’s quite sobering. Our fingerprints are all over this planet.

Read the Wired article here.