Newsletter: Batten down the hatches

Hi divers

Weekend dive plans

No diving

The weather forecast predicts that we are soon to be lashed with 50 km/h winds, a 7 metre swell, and no small amount of rain. It seldom is as bad as the forecasts claim, however the swell size and direction will hammer False Bay. Despite Sunday’s weather looking peachy, I don’t think the ocean will be, so we will plan for a dry weekend… It is winter after all.

Shark Spotting at Fish Hoek beach
Shark Spotting at Fish Hoek beach

Shark Spotters supporters program

We are proud to announce that we have signed on as official supporters of Shark Spotters. We are Silver partners, and for larger businesses there are higher levels of support on offer. (We are hoping to encourage some of the other dive centres to consider supporting Shark Spotters, too…) Individuals can also sign on to the supporters program, or donate in many different ways.

Shark Spotters is part-funded by the City of Cape Town and the Save Our Seas Foundation, and the rest comes from public donations. The Shark Spotters provide beach safety, a world-first environmentally friendly shark exclusion net at Fish Hoek beach, conduct educational programs at local schools, and conduct shark research in and around False Bay.

We’ll write a blog post with more information soon – but in the mean time, we’re very happy to be contributing to the important work of Shark Spotters. If you’d like to as well, visit their website to find out how to lend your support, or drop me an email and I’ll connect you with the right people.

regards

Tony Lindeque
076 817 1099
www.learntodivetoday.co.za
www.learntodivetoday.co.za/blog/

Diving is addictive!

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Baby (Smith’s swimming) crabs

Last week we met Smith’s swimming crab (Charybdis smithii), a rare visitor from the tropics that arrived in the southern reaches of False Bay during the third week of January 2019. We found some adult individuals at Long Beach, well inside False Bay, on 3 March.

Juvenile Smith's swimming crabs at Kommetjie
Juvenile Smith’s swimming crabs at Kommetjie

At the same time that fishermen were reporting adult Smith’s swimming crab off Cape Point, a huge number of tiny red crabs washed up on Long Beach, Kommetjie. There were great piles of them, but we only managed to visit a day or two later on Friday 18 January.

Juvenile Smith's swimming crab at Kommetjie
Juvenile Smith’s swimming crab at Kommetjie

We had to hunt a little, but after a while we found the crabs – now only a few remaining, but most of them very active and vital. Opinion from George Branch is that these are most likely the final larval stage of Smith’s swimming crab, and the same warm conditions wit anomalous westerlies that brought their adults to the peninsula, brought the juveniles.

Juvenile Smith's swimming crab at Kommetjie
Juvenile Smith’s swimming crab at Kommetjie

This conversation (facebook group, may be closed) suggests that the juvenile Smith’s swimming crabs were also seen on Milnerton Beach around the same time as they appeared on the beach at Kommetjie.

I took a couple of short videos of the tiny crabs, which show them in motion on the wet sand.

Apologies for the loud wave noise and less than perfectly steady camera work. Read more about these pelagic crabs, exceptionally infrequent visitors to our shores, in this post.

Hello, Smith’s swimming crab!

One of the things I love about False Bay, and about Long Beach in particular, is the tendency for creatures from all over South Africa’s coastline to end up here, often tropical marine life that got caught in the warm Agulhas current, and then within the circulation of False Bay, ending up just behind the Simons Town harbour wall.

Smith's swimming crab (Charybdis smithii) Smith’s swimming crab (Charybdis smithii)

Thus it was, early in March, that we discovered several adult specimens of Smith’s swimming crab (Charybdis smithii) at Long Beach when we went for a dive. One or two were already dead, and the rest were struggling either on the sand, or in the shallows, looking unhealthy. The water temperature at the time was about 17 degrees.

The first hint that this unusual and rarely-seen visitor had arrived on our shores was a series of social media posts, from January, in one or two of the fishing groups I follow on facebook. (These are excellent places to keep tabs on what’s happening in parts of the ocean I might not routinely visit, and there’s a wealth of knowledge and experience among the members.) Here’s a conversation between local fishermen about seeing large numbers of adult Smith’s swimming crabs just off Cape Point (also facebook). You can also see some photos of one of the crabs from Sea-Change here (facebook), taken on 22 January in False Bay.

Smith's swimming crab (Charybdis smithii) Smith’s swimming crab (Charybdis smithii)

At the same time as these social media posts, there was an influx of small, red crabs on the other side of the Cape Peninsula, at Long Beach in Kommetjie. (But more on that in another post.) Two Oceans says that Smith’s swimming crabs were first described in False Bay in 1838, and then again in 1978, 1983, and 1993. This facebook thread suggests that they may have been last seen off Muizenberg around 2005-2006.

Smith's swimming crab (Charybdis smithii) Smith’s swimming crab (Charybdis smithii)

I read more about Smith’s swimming crab in two papers: this one (Romanov et al), from 2009, and this one (Van Couwelaar et al) from 1997. The more recent paper updates many of the findings of the earlier one. Both teams of scientists behind these papers used trawl data from pelagic cruises to learn about the distribution and life history of these crabs.

Smith’s swimming crab is a pelagic crab that spends the vast majority of its one year, monsoon-driven life cycle in the water column. They are endemic to the western Indian ocean, and are usually found in the area bounded by the Arabian sea (which is west of India) and the latitude of Dar Es Salaam in Tanzania (about 7 degrees south of the equator), and from the east African coast, across east to the Maldives. They may congregate in huge patches, larger than tens of square kilometres, and may reach a biomass of more than 130 kilograms per square kilometre. These swarms are densest between June and September. During July, their concentration can peak at more than 15,000 individuals per square kilometre.

Smith's swimming crab (Charybdis smithii) Smith’s swimming crab (Charybdis smithii)

The crabs congregate on the seafloor of the continental shelf during the months of September to December, mating late in the year. No adult crabs are usually seen between April and June (Van Couwelaar et al speculate that the adults die after breeding), at which time, after metamorphosis, the swarms again become apparent in the western Indian ocean. The crabs grow to about 7.5 centimetres carapace width.

The crabs seem to perform a diel migration, moving deeper in the water column during the day (down to 350 metres’ depth), and returning to the surface at night. They swim continuously and are voracious predators in order to support the high metabolic demand created by this constant activity. They are able to regenerate all their limbs except for their swimming legs (Van Couwelaar et al deduced this in much the same way as Abraham Wald decided which parts of World War II bombers to reinforce – no crabs with partially grown swimming legs were caught in their trawls).

These crabs are important prey for yellowtail, as for other pelagic fish species such as blue sharks, yellowfin tuna and bigeye tuna. The fishermen of False Bay observed that they made excellent bait.

Smith's swimming crab (Charybdis smithii) Smith’s swimming crab (Charybdis smithii)

So what brings Smith’s swimming crab this far south? False Bay is way out of their range. This paper (Chapman, 1988) suggests that prior arrivals of these crabs on our shores have co-incided with weaker than usual summer south easterly winds (which has definitely been a feature of late 2018 and the start of 2019 – we had a gloriously wind-still summer for the most part) and the westward movement of warm water containing the crabs. We did have a spell of unusual westerly wind just prior to these crabs’ arrival.

A fascinating 1984 paper by George Branch describes a temperature anomaly during the summer of 1982-83. This particular Cape summer was characterised by very little of the typical south easterly winds, leading to reduced upwelling, and relatively high sea temperatures (Duffy et al, Effects of the 1982-3 Warm Water Event on the Breeding of South African Seabirds, 1984). The exceptionally warm water on the south and west coasts of South Africa caused mass strandings of some tropical animals (such as portuguese man ‘o war), mortalities of others (such as black mussels), changes in abundance of some species, and extensions of some species’ geographic range. For example, an exceptional number of juvenile turtles washed up on the beaches of False Bay, several months before the usual start of the usual turtle stranding season (which is, very loosely speaking, March-July). Prof Branch records that large numbers of healthy, adult Smith’s swimming crab washed up at Cape Hangklip, and smaller numbers at Boulders Beach, Strand, Milnerton and Blouberg. About 62% of the crabs were female, and many of them survived in aquaria for some time after stranding.

In short, it seems that we had our own little temperature anomlay, however brief, in early 2019, and the pulse of warm water brought with it these rarely seen (in Cape Town) crabs. What luck to spot this unusual visitor!

There are some lovely pictures of Smith’s swimming crab, healthy and in mid water, taken off Tanzania, here.

Bookshelf: Eye of the Shoal

Eye of the Shoal: A Fishwatcher’s Guide to Life, the Ocean and Everything – Helen Scales

Eye of the Shoal
Eye of the Shoal

This is an absolutely wonderful book about fish. Everything about fish. Helen Scales is a marine biologist and the accomplished author of marine-themed books (I previously wrote about Poseidon’s Steedher book about seahorses).

Here, Scales delves into the world of an animal whose variety seems almost without limit. Her book overflows with wonders, and interweaves science, adventure and mythology to shed light on the under-appreciated inhabitants of the underwater realm.

Unsolicited (this is almost always the case), I read half of this book to Tony while I was busy with it, and it delighted both of us. We learned about bioluminescent fish, poisonous fish, the sounds fish make, and the colours of their skin. We learned about fish that use tools, fish cognition, and about the state of the science regarding whether fish experience pain. We even learned about moray eels and grouper hunting co-operatively.

As a scuba diver, Scales relates tales of dives on which she observed the behaviours and phenomena she describes, and I was inspired to pay more attention to the activities of the fish we see on dives around Cape Town. They may (almost) all be the same colour, but there are certainly things that they do, and fascinating ways of being, that I am failing to appreciate.

Scales provides a bibliography on her website with links to the open access scientific papers that she used to research the book.

Get the book here (South Africa), here (US) or here (UK).

Bookshelf: Sea Change

Sea Change: Primal Joy and the Art of Underwater Tracking – Craig Foster & Ross Frylinck

Sea Change
Sea Change

The Sea Change project may be familiar to you from their large format photographic displays, one of which was for a time along the promenade in Sea Point, and is currently in Lamberts Bay. You may have read about the project, or seen its members – ocean-loving filmmakers, journalists, scientists – diving in the cold water of False Bay year round, without wetsuits. You may even have seen the BBC’s Blue Planet II series, in which the filmmakers, guided by Craig Foster and his fellows, captured an incredible octopus sequence filmed on our doorstep in False Bay.

The Sea-Change book has been a long time in the making, and is the product of hours upon hours upon hours spent in the water, observing the animals that call the kelp forests home. The book contains a story of loss and discovery – that of Ross Frylinck – interwoven with large-format photographs of scenes from the kelp forest, taken by filmmaker and naturalist Craig Foster, a co-founder of the Sea-Change project. Foster also provides the captions.

As a diver, it was immediately clear to me that a great deal of patience and close observation was required to gain the deep understanding that Foster has of the smallest creatures living among the kelp. There is no substitute for time in the water. There is no substitute for swimming slowly, deliberately, and for spending extended time looking at one place. The marks that animals leave on rocks, kelp stipes, the sand, and even on each other’s shells, can tell a story.

I learned a huge amount about animal behaviour from this book, and about the interconnectedness of all the elements of the watery, beautiful world around the Cape Peninsula. The photographs are beautiful and striking, capturing moments that one would be extremely lucky to see during the normal course of things. Diving for more than an hour a day, every day of the year, however, makes such things more commonplace.

Sea Change presents a beautiful opportunity for the wider community of ocean lovers to learn from the unique approach taken by the Sea-Change team (this article gives a good sense of it), and to learn how to understand and observe the animals that surround us when we look beneath the surface of the inshore kelp forests. The project also has something to say about how science happens, and the vital connection between science and storytelling. Identifying animals is fun, but – as any veteran twitcher will tell you – the next level is understanding behaviour. This is a challenge I’m happy to take up.

Get a copy here, or directly from the Sea Change team.

Article: The Atlantic on how much of the sea is fished

The proportion of the world’s oceans that are fished has recently been the subject of two papers in Science, both of which used the same data set to reach different conclusions. One study concluded that 55% of the ocean is affected by fishing, while a follow up (using the original data set, but at finer resolution) concluded that the number is closer to 4%.

A janbruin emerges from a school of fish at Atlantis
A janbruin emerges from a school of fish at Atlantis

Ed Yong of The Atlantic sets out the disagreement, and explains the subtleties and the points of view on each side of the debate. Far from being an insoluble  scientific crisis, the divergent findings show the iterative nature of the scientific process and the manner in which back and forth between scientists leads to progress. It brings to light important questions worth considering, ones that are more important than simply asking the original question of how much of the ocean is fished.

Instead, you have to ask different and more refined questions. How much of that fishing is sustainable? Which species are being targeted? How are they faring? Can they bounce back?

Read the full article here.

Baboons on the beach

Baboon on the beach Baboon on the beach

A recent low tide visit to the beach at Platboom near Cape Point, on the Atlantic coast of the peninsula, enabled us to watch a troop of Chacma baboons (Papio ursinus)  foraging for limpets, mussels and other marine snacks on the rocks at low tide. The baboons bite the tops off the limpets with their formidable incisors, or pry them from the rocks intact to get at the protein-rich flesh. They also eat mussels.

Baboons foraging for seafood at Platboom Baboons foraging for seafood at Platboom

This foraging behaviour is extremely rare among primates. In baboons, it is only observed on the Cape Peninsula and in one other species in Somalia. Matthew Lewis studied this troop of baboons as they foraged around the Cape Point nature reserve, and his thesis makes for fascinating reading. (Wild Card Magazine also featured Matthew’s research.)

Baboon on the beach Baboon on the beach

The amount of time the baboons are able to spend foraging on the shore is largely determined by the height of the tide, and by weather conditions. As a result, the amount of time the baboons spend seeking marine food sources is small compared with the time they spend looking for roots, bulbs, insects, berries, and small animals.

Low tide at Platboom Low tide at Platboom

These baboons are part of the Kanonkop troop which ranges freely in the Cape of Good Hope section of Table Mountain National Park and whose home range does not bring them into conflict with humans (or, as a rule, allow them access to any anthropogenic food sources). They were completely uninterested in us and our vehicle, unlike the baboons we see further up the peninsula around Millers Point, for example.

Concentrating baboon Concentrating baboon

Goose barnacles on the beach

A (lovely, rain-bringing) onshore wind left great rafts of kelp all over Noordhoek beach one weekend in mid May. Finding anything of substance on this beach is unusual; it’s on an exposed piece of coastline and all but the most robust objects are dashed to pieces before they arrive on the sand. Seeing all the washed up kelp also reminded me that frequenting the beaches inside False Bay, that are daily cleaned of washed up kelp by the City of Cape Town, is liable to give one a skewed idea of just how much kelp naturally washes up on the sand.

Kelp stipe covered in goose barnacles
Kelp stipe covered in goose barnacles

This time, there was kelp, and lots of it. Several of the pieces of kelp had been colonised by goose barnacles. There are several species of goose barnacle that occur off South Africa’s coast, but these ones are Lepas testudinata. They are incredibly strange looking animals, and some of them were still alive and writhing slowly in the drying sun.

In parts of the world (I’m looking at you, Iberian peninsula), goose barnacles are an expensive delicacy. I have nothing to say about that.

Goose barnacles, with my paw for scale
Goose barnacles, with my paw for scale

Lepas testudinata larvae most often attach to free-floating pieces of kelp (Ecklonia maxima) and plastic debris, which is why you have probably never seen these mesmerisingly gross-looking creatures while on a dive. In the picture below, you can see that they’re attached to the bottom of a kelp holdfast, where it would ordinarily attach to the rock. This shows that they attached after the kelp broke off.

A kelp holdfast encrusted with goose barnacles
A kelp holdfast encrusted with goose barnacles

Each barnacle is possessed of a long fleshy peduncle, or stalk, which attaches to the kelp holdfast, stipe or fronds. On the end of the peduncle is a carapace (shell) made up of five separate pieces. The large part of the barnacle on the end of the peduncle (what you’d think of as its body), covered by the carapace, is called the capitulum. The apparatus that the barnacle uses for feeding – essentially six pairs of hairy legs – reside inside the carapace, along with the mouth. There’s some more detail and a nice diagram at this link. If you are familiar with other kinds of barnacles – the volcano-shaped ones that live on rocks, ships, whales and piers for example, then most of this (except the peduncle) should sound familiar to you.

Lepas testudinata goose barnacles
Lepas testudinata goose barnacles

Research done around South Africa’s coast (published here) by Otto Whitehead, Aiden Biccard and Charles Griffiths, identified the marked preference of Lepas testudinata for attaching to kelp. The researchers surveyed a selection of beaches around South Africa’s coast, from the west coast of the Cape Peninsula up to northern KwaZulu Natal, between June and October 2009. When they found goose barnacles washed up, they recorded the species of barnacle, the type of material they were attached to, the dimensions of the object, and its location. They also estimated the number of barnacles in each colony they found.

Lepas testudinata was the species they found most commonly, of the six species in total that they identified along the area of coast that was surveyed. (There’s a nice picture of the six species in their paper, which I used to identify the ones I found.)  This species of goose barnacle was found to prefer kelp, as mentioned, and also tended to colonise large objects compared to the other species (this could, of course, be because pieces of kelp are usually larger than items such as bits of plastic, glass, feathers, and shells that some other species prefer).

Kelp fronds with goose barnacles
Kelp fronds with goose barnacles

Lepas testudinata was the only species of goose barnacle that the researchers regularly found to form colonies comprising more than 1,000 individuals. It is also the only species of goose barnacle recorded by the survey that is only found in temperate (cooler) waters, which happens to be where kelp is found, too.

The researchers note that the goose barnacles of the Lepas testudinata species that they found on kelp seemed to have exceptionally long peduncles, some more than 25 centimetres long, and that this seems to differ from what has been previously known about them (which is that they have “short, spiny” peduncles). They suggest that perhaps the variety of Lepas testudinata that colonises kelp may even be a separate species from the one previously described (more research obviously required to ascertain this). You can see from my photographs that the peduncles of the washed up Noordhoek beach goose barnacle colonies are also quite long, some easily 20 centimetres in length.

Clusters of goose barnacles on a kelp stipe
Clusters of goose barnacles on a kelp stipe

They also found that the increasing prevalence of long-lasting and buoyant plastic marine debris and other anthropogenic objects around our coastline, which some species of goose barnacles preferentially attach to, gives these weird little creatures increased opportunities to form colonies, and to spread to new places. This is one of those interesting phenomena to keep in mind, as humans inexorably alter the environment. Some creatures will benefit in strange ways from warming oceans, and others will find new homes in the garbage we leave lying around.

Bookshelf: Shark

Shark – Brian Skerry

Brian Skerry is a National Geographic photojournalist, with whose TED Talk you may be familiar. This book is a collection of articles – about sharks – that appeared in National Geographic magazine, accompanied by one magnificent shark photograph after another. Each chapter’s text is reasonably short. Here, the photos are the primary focus.

Shark
Shark

The chapters focus on four species of shark: great white, white tip, tiger sharks, and mako sharks. Additional text is contributed by several National Geographic writers, and experiencing the familiar editorial quality and stylistic approach of the magazine is like settling down for a chat with an old friend.

The final chapter of the book, written by Skerry, is an appeal for increased understanding of sharks and their vital place in ecosystems, and increased protection for them – in the form of marine reserves, and less fishing, for example. The photographs selected for this chapter makes it clear that in Skerry’s view, science (especially tagging studies) is vital to the endeavour of better understanding sharks, and protecting them.

Get a copy of the book here (South Africa), here or here.

Bookshelf: Antarctica

Antarctica: An Intimate Portrait of the World’s Most Mysterious Continent – Gabrielle Walker

THIS is the book about the Antarctic that I have been looking for all of my life. It’s unlikely that this discovery will stop my obsessive consumption of polar-related literature and documentary material, but this is likely a book I will return to again.

Antarctica
Antarctica

The author, a science writer, has visited Antarctica several times, and is thus able to weave her personal experiences of  life on the driest continent with accounts of the science taking place there, and the scientists doing the work. Walker has had the sort of access to the scientists that most of us can only dream of, and makes reasonably good use of it.

Mixed in with stories of her Antarctic travels and meetings with researchers, Walker also briefly recounts the stories of some of the explorers of last century who opened up the interior of the continent. She is able to visit Western Antarctica, a part of the continent that very few people get to, and where the effects of climate change can be seen most clearly.

There’s a much more comprehensive review from The Guardian here. If you’re interested in the Antarctic, and the science that is being done there, you should read this book.

Get a copy here (South Africa), here or here.