A recent xkcd cartoon combines statistics (Bayes’ theorem) with an ocean theme, which makes me happy. Bayes’ theorem enables you to calculate conditional probabilities, which is the probabilty of something happening (or being true) given that something else has happened. You can also rearrange the formula to calculate straight probabilities if you know the conditional probabilities. It’s useful.
Conditional probability isn’t a difficult idea. For example, the probability that the next person you see will be female is different (probably higher) to the conditional probability that the next person you see will be female, given that you are standing at a urinal. The condition is that you’re in a men’s restroom.
Seashell
P denotes probability, and the vertical lines denote conditional probability – read “given that”. The way to read the text in this cartoon is as follows: the probability that you are near the ocean, given that you’ve just picked up a seashell, is equal to the probability that you’ve just picked up a seashell given that you’re near the ocean, multiplied by the probability of being near the ocean, divided by the probability of picking up a seashell.
The author (Randall Munroe) of xkcd notes that the equation is roughly equal to number of times I’ve picked up a seashell at the ocean / number of times I’ve picked up a seashell, which is roughly 1, because one almost always picks up seashells at the beach and not elsewhere.
What the caption means is that if you picked up a seashell, you’re probably at the ocean (so you don’t need to hold it to your ear). The explain xkcd wiki may help. Anyway. Methinks I explain too much.
Tony and students on the surface over the wreck of the Brunswick
The Brunswick is a historical wooden shipwreck that lies a few hundred metres off the northern end of Long Beach in Simon’s Town, directly opposite the northern end of the white apartment buildings overlooking the Main Road. Like HNMS Bato, she is infrequently dived. Having lain underwater since 1805, she is heavily overgrown and much of her decking and hull is covered by sand. She used to be a shore entry (with a precipitous climb over the railway line), but in recent years a large number of boulders have been added as a breakwater between the ocean and the railway line, and climbing over in dive gear is no longer possible. For this reason we do the dive from the boat. Close to shore and in shallow water, the Brunswick is an ideal site to get used to boat diving.
Extensive field of wooden decking
The Brunswick was a British East Indiaman, which means she carried men and goods between Britain and the East Indies – (south)east Asia and India. She was carrying a cargo of cotton and sandalwood from China back to Britain when she was captured by some French vessels off Sri Lanka, and brought to Simon’s Bay. In September 1805 her anchor rope parted, and she ran aground during a south easterly gale. Most of her cargo was salvaged, as she lies in shallow (less than six metres deep) water.
Shot line on the sand
The boat from below
An encrusted iron knee
An iron knee in the wreckage
Planking on the Brunswick
Kelp grows on the wooden remains
Puffadder shyshark
Octopus hole
Hermit crabs swarm over the wood
Cape rock crab
Craig looking underneath
Bolt sticking out of the wreck
Tags on the wreck from a prior archaeological survey
Tags on the wreck from a prior archaeological survey
Starfish attached to a bolt
Copper bolt next to a rock lobster
Starfish and copper drift bolt
Pointy bolt
Some of the metal is worn smooth or recently uncovered
Drift bolts in the planking
Corroded iron driftbolt
Side view showing how thick the planks are
Much of the wreck lies under the sand
Another concreted metal item
Octopus surrounded by hermit crabs
Wooden remains from the Brunswick
Christo approaches the boat after the dive
We found the dive site to be similar to HNMS Bato, which was also a sturdily built wooden ship of similar vintage. The Brunswick was 1,200 tons, and her wreckage is spread out quite extensively. There are many thick, wooden planks, laid out as they would have been to form her decks, as well as much evidence of the bronze bolts that secured parts of the ship together. There are also many copper bolts, rivets and what could be small amounts of rolled up copper sheathing in evidence on the site.
Anemone among feather stars and papery burnupena
The highests parts of the wreck are covered with feather stars, anemones, sea cucumbers, and kelp. There are many octopus, and peering under the wreckage with a torch yielded a couple of very large pyjama catsharks. We were lucky to dive the site most recently on a day with lovely visibility, and the shallowness of the water means that there’s a lot of light penetration which improves things enormously.
The highest parts of the Brunswick wreck
Before diving this site, you should call the SA Navy Ops Room on 021 787 3818, to ask for permission and to tell them how long you’ll be. Same procedure as at Long Beach.
This photo was taken on the beach at Espergærde in Denmark, where I photographed these roses, too. There’s a small bulldozer moving sand out of the water up the beach. The water is shallow, so the ‘dozer has driven way out to the end of the jetty. You can see the piled up sand on the water’s edge.
We visited Denmark in July 2011 to spend time with family, and I took this photo at Espergærde (thesetwo photos were also taken in the harbour of this small seaside town). Vegetation like this, right on the sea shore, highlights for me how different the climate of Scandinavia is to what I’m used to in Cape Town. These look like some variety of dog rose, with some rose hips thrown in for good measure.
Here’s something beautiful: a NSRI advertisement that reminds us of the wonder and enchantment to be experienced down by the ocean, and the work done by the NSRI, serving everyday water users and making the ocean a safer place to be. The poem that is read in the voice over is Sea Fever by John Masefield. (The version I quote and the version used in this advertisement differ slightly over the inclusion – here – of the word “do” in the first line of the poem. Both versions appeared in collections of Masefield’s poetry.)
The poem is read by Tom O’Bedlam (not his real name – Roger Ebert suspected that he is someone famous), who has an amazing Youtube channel with readings of hundreds of poems.
Nowhere has the fraught and complex relationship between surfers and sharks played out with as much drama as at Réunion Island, a department of France situated in the Indian Ocean off Madagascar. A surfers’ paradise, the island has seen a succession of gruesome attacks by bull sharks on surfers. A decision was made in August 2012 to cull sharks around the island, which was almost immediately reversed as it contravened French law (the sharks are in a Marine Protected Area, where hunting is forbidden).
An article at GQ.com describes the attacks and the community response, and attempts to understand the reason behind the sudden increase in human-shark interactions. It is interesting, as someone outside surfing culture, to get an insight into the impact of these events on the local surfing community. One can sense the frustration and bewilderment of the surfers, particularly because the explanations for increased shark sightings and bites on humans are hard to grasp.
Frédéric Buyle, a Belgian free diver and shark conservationist, even went so far as to theorise that
… bull sharks’ social units are complex enough that the loss of a single individual could send a group into a tailspin of erratic behavior. It’s also possible, Buyle posits, that if an influential individual were to be injured, the others might help it hunt for easy prey—and nothing could be easier prey than an oblivious land mammal on the surface. It’s a leap of imagination to see the tragedy of the attacks in reverse perspective: a beloved bull (do they love one another?), suddenly wrenched from the water, vanishing into the sky; the grieving survivors (do they grieve for one another?) rallying together, making a necessary change.
It’s important to remember that Buyle isn’t a scientist; he has, however, been passionately involved in the events at Réunion, and writes more about them here (in French – use Google translate).
Christopher Neff writes at Save Our Seas and for The Conversation on the emotive issue of shark hunts and culls. While they satisfy our desire for vengeance on the animal or animals that may have bitten water users, there is no scientific evidence that they work. This excellent article on tiger sharks off Hawaii highlights the same point.
This Friday’s photo is of Fish Hoek beach, taken at sunrise on the day that the shark exclusion net was trialled for the first time. This was a good day for Fish Hoek, for people, and for sharks. Fish Hoek is a very popular swimming beach, with a large number of elderly folk who swim every morning (all year around). There’s also an excellent lifesaving club which is at the centre of much of life in this coastal town.
March weather has not been too kind and we have had very few days of good conditions. Two long weekends of no diving and we once again head into a weekend that does not look promising. Last weekend a pod of false killer whales beached on Noordhoek Beach and again on the Simon’s Town side of the peninsula. More on that here.
Long Beach on Monday
We dived Shark Alley today and had around 3 metre visibility. On the trip there and back I stopped and looked at a few sites and found only dark green and brown water. There is not too much wind around to clean that up. Plus the Navy festival that happens every year around now brings traffic in and out of Simon’s Town to a grinding halt. Parking, even at Long Beach, becomes very scarce. Hout Bay and Table Bay have a similar dark tinge to the water and in fact the water temperature tonight off Kommetijie at the CSIR buoy is just under 15 degrees, the same as False Bay. Warm Atlantic most often means dirty water.
Veronica, Kate‘s mum, arrived a few days ago from the UK and brought me the most amazing piece of dive gear ever. I know you will all want to try it out but the answer will be no so don’t ask!!!!
We’ve been considering marine mammal strandings for the past fewdays. This is a hard subject, but it’s important to think clearly about it, and, as a coastal resident, to be in possesion of some salient facts.
Just after Christmas last year the New York Times City Room regional blog reported on a fin whale that had beached itself at Breezy Point on the shores of Queens in New York city. Fin whales are the second largest species of whale after blue whales, and are endangered. The whale was 20 metres long and estimated to weigh 30 tons. When the animal arrived on the shore it was extremely underweight (it should have weighed about 50-60 tons for its length), but still alive.
The difficulties of assisting whales that beach themselves are made clear here, as well as the reason why whales are not typically euthanased using drugs. The quantity of medication required to put a 30 ton creature out of its misery would create a carcass that was a toxic hazard and would allow limited means of safe disposal.
A follow up article states that the whale had died, and that an open air necropsy would be performed. This would hopefully reveal whether the whale had died of old age, or from other causes such as parasites, ingesting plastic, or a ship strike. A third article describing the necropsy mentions unexplained lesions on the whale’s kidneys and stomach. The whale was male and there was no evidence that he’d been struck by a ship.
A dead whale on its way down Vanguard Drive to a landfill in Cape Town
Disposing of whale carcasses is sometimes controversial. They are typically buried on the beach, or (in Cape Town) taken to a landfill. Beach burial is one way of returning the nutrients in the whale’s body to the earth, but unfortunately not a good idea in locations frequented by sharks and swimmers. Oils from the whale’s carcass leach out into the sand, attracting predators inshore where accidental encounters with humans can be expected.
Ideally, the whale should be dragged out to sea and left there to sink to the bottom and create an ecosystem as a whale fall, but there are the considerations of it causing a hazard to shipping, or washing ashore again (a particular issue in False Bay, where we have onshore winds in summer). Blowing up a whale is non-trivial, dangerous, expensive, and messy. It’s also not kind to the environment. YouTube has several videos demonstrating this.
The first City Room article can be found here. The second, reporting on the death of the whale, is here. The article reporting on its necropsy and subsequent burial is here.
Staying on the subject of marine mammal strandings, here’s an interesting series of events that took place at the Two Peoples Bay Nature Reserve in Western Australia. Initial reports on 2 February this year indicated that between 100 and 150 pantropical spotted dolphins were milling about in knee deep water. One of the dolphins was dead.
In order to herd the pod out into deeper water, and avert a potential mass stranding, officials removed a baby dolphin from the pod and transported it into deeper water. Its distress cries attracted the attention of the remainder of the pod, who then swam out into deeper water to where the calf was, joined it, and subsequently out to sea. They were not seen again.
I think this is a fascinating and incredibly innovative solution to what is usually a very upsetting problem and one that rarely has good outcomes. Do you think the end (saving up to 150 dolphins from stranding on a beach) justified the means (causing temporary distress to a young dolphin, its mother, and the rest of the pod)?
