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.

Newsletter: Dive in

Hi divers

Weekend dive plans

Saturday: Shore dives at Long Beach at 9.00 am
Blue gas flame nudibranch
Blue gas flame nudibranch
Weekend diving conditions look good, with Saturday being the better day. I have students, so we’ll meet at Long Beach at 9.00 am. Let me know if you’d like to join us. regards Tony Lindeque 076 817 1099 www.learntodivetoday.co.za www.learntodivetoday.co.za/blog/ Diving is addictive! To subscribe to receive this newsletter by email, use the form on this page!

Newsletter: Pedal pedal pedal

Hi divers

Weekend dive plans

No diving This weekend we are hamstrung by the Cape Town Cycle Tour on Sunday – good luck if you are participating! I’m away in Port Elizabeth on Saturday, so I won’t be running dives even if it wasn’t going to be rather windy (it is). I’ll be diving with students during next week, so let me know if you want to tag along and I’ll keep you posted.
Sea star at Long Beach
Sea star at Long Beach

Ocean/Surf night at the Cape Town Adventure Film Festival

Film buff? Love the ocean? Think popcorn is a food group? This event on Friday, 12 April, may be right up your street. Get tickets here. regards Tony Lindeque 076 817 1099 www.learntodivetoday.co.za www.learntodivetoday.co.za/blog/ Diving is addictive! To subscribe to receive this newsletter by email, use the form on this page!

Newsletter: Turning tides

Hi divers

Weekend dive plans

Sunday: Shore dives in False Bay, or boat dives from False Bay Yacht Club

There are large patches of red tide in False Bay, and no wind to clear them up. For this reason I’ll decide on Saturday whether we’ll be boat or shore diving on Sunday. I have students, so we’ll keep it relatively shallow either way. Let me know if you want to keep in the loop for the weekend dives. There’s been bioluminescence at the Strand beach this week, so keep an eye out for it around Fish Hoek beach during the evenings if the red tide persists.

Spring high tide at Fish Hoek beach
Spring high tide at Fish Hoek beach

Tidal documentary

Don’t forget the screening of Tidal at St James beach on Thursday 7 March. RSVP here.

regards

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

Diving is addictive!

To subscribe to receive this newsletter by email, use the form on this page!

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).

Newsletter: Signs of the times

Hi divers

Weekend dive plans

Sunday: Shore dives at Windmill or A Frame

Sewage spill at Long Beach
Sewage spill at Long Beach

Wildly different forecasts for this weekend make me inclined to go with the safest option, namely Sunday, as the best dive day. I have students to dive so will be shore diving, most likely from Windmill or A Frame, as when I checked earlier this week, the “stay out of the water” sign remains at Long Beach.

regards

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

Diving is addictive!

To subscribe to receive this newsletter by email, use the form on this page!

Bookshelf: Into the Raging Sea

Into the Raging Sea: Thirty-Three Mariners, One Megastorm, and the Sinking of El Faro – Rachel Slade

Into The Raging Sea
Into The Raging Sea

If the article I shared earlier this week made you want to learn more about the 2015 sinking of El Faro, an American cargo ship, this book is for you.

Using the 26 hours of voice recordings recovered from the ship’s deep water resting place after a prolonged search, Rachel Slade is able to reconstruct, in detail, the final voyage of El Faro. Slade also attended the hearings on the sinking held by the US Coast Guard, and interviewed the family and friends of El Faro‘s crew. The result is a detailed and illuminating investigative work that explains the disaster more comprehensively than simply to say that the ship sailed into a hurricane and sank. Slade also emphasises the humanity, connections and personalities of the captain and crew, who otherwise might be lost in the telling as statistics of loss.

The official explanations, and absence of any assumption of culpability for the tragedy, are enraging and frustrating, but illustrate the insidious pressure to take risks that commercial mariners may experience from ship owners and operators. This dynamic plays out at all scales. Even as a small business owner, Tony is sometimes asked to launch his boat in conditions that he deems unsafe. A client may put their own financial gain ahead of the safety of the divers, or of my husband. The risk of such a venture is entirely with the captain and others on the vessel, while the decision-maker (and financial beneficiary of the decision) sits ashore in safety like General Melchett sending his troops to their doom.

Slade’s book is a gripping read, accurately and comprehensively reported, and will appeal to anyone with an interest in maritime drama. It is also of particular interest given that warming oceans will give rise to more storms like Jaoquin, and our ability to forecast their movements with accuracy will, to an increasing degree, impact captains’ ability to keep themselves, their crew and their cargo out of harm’s way.

Do not confuse this book with Into a Raging Sea, the excellent book about South Africa’s National Sea Rescue Institute.

Get Into The Raging Sea here (US), here (UK) or here (South Africa).

Article: Vanity Fair on the sinking of El Faro

William Langewiesche, author of The Outlaw Sea (one of my favourite books) wrote an in-depth article for Vanity Fair, about the sinking of the American cargo ship El Faro, with the loss of all hands on board, in hurricane Jaoquin in 2015. Called the “worst U.S. maritime disaster in three decades”, the loss of El Faro should have been avoidable.

With access to the 26 hours of recordings from the El Faro‘s “black box”, found after an almost year-long search, Langewiesche is able to provide detailed reporting on the hours leading up to the disaster. I found two aspects of the incident incredibly instructive. The sequence of decisions made about where to sail relative to the hurricane, and the culture onboard, seemed worth pondering. Weather forecasting services (a personal obsession) were also key to the fate of the ship to a surprising degree.

It is unlikely that Davidson [the captain] ever fully understood that he had sailed into the eye wall of Joaquin, but he must have realized by now that he had come much too close. As is usually the case, the catastrophe was unfolding because of a combination of factors that had aligned, which included: Davidson’s caution with the home office; his decision to take a straight-line course; the subtle pressures to stick to the schedule; the systematic failure of the forecasts; the persuasiveness of the B.V.S. graphics; the lack of a functioning anemometer; the failure by some to challenge Davidson’s thinking more vigorously; the initial attribution of the ship’s list entirely to the winds; and finally a certain mental inertia that had overcome all of them. This is the stuff of tragedy that can never be completely explained.

Read the full article here. It’s a gripping read by a master storyteller.

Newsletter: From the heart

Hi divers

Weekend dive plans

Saturday: Boat dives from Hout Bay or shore dives at Long Beach

Weekend conditions don’t look all that great. Saturday will most likely be best for launching from Hout Bay , or there may be a slight chance of shore dive from Long Beach for students. I will make this decision late tomorrow depending on sea conditions. Let me know if you’re keen to dive.

A heart for you on Valentines day
A heart for you on Valentines day

Sunday is the Cape Peninsula marathon, starting in Green Point and finishing in Simon’s Town, so expect some road closures.

Film screening on the beach

There’s a free screening of the documentary Tidal at St James beach on Thursday 7 March. Read about the film here, and RSVP on Quicket. There’s no charge.

Tony Lindeque

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

Diving is addictive!

To subscribe to receive this newsletter by email, use the form on this page!

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.