The weekend is filled with neither fantastic nor terrible conditions – there’s some wind, a little swell, and very little sun. I plan to launch from Simon’s Town jetty, on Sunday, once I have seen Saturday’s sea state. Want to dive? You know what to do.
The weekend forecast looks really spectacular… If you are a surfer. A nice six metre swell rolls in on a 15 second period, meaning the big wave surfers are busy waxing their boards as we speak.
I think that Friday, Saturday and Sunday should offer a few really good photo opportunities at Dungeons as well as Sunset Reef. The wind will be a factor so I will most likely launch on Friday and Sunday. Let me know if you want to hitch a ride.
Sunday: Boat dives from False Bay Yacht Club to Roman Rock
We are out on the boat tomorrow for a False Bay crossing – a swim – from that side of False Bay to this side. I won’t be swimming. It’s for a good cause and if you want to show support visit the Mad Swimmers facebook page. I don’t expect to be done much before nightfall so we will skip Saturday and launch on Sunday.
It’s been a while since dived the Northern Pinnacle at Roman Rock so that will be the first dive, and the second will be the ledge and channel slightly south of the pinnacle.
We hope you’ll use your time underwater to record some species and log them as soon as you’re on land again, but other very cool sounding events include rock pooling with ocean rockstars George and Margo Branch on Monday late afternoon (details here), and, if Monday is tricky, there’s a tidal pool bioblitz on Saturday afternoon that promises to be a lot of fun (details here).
If this is all Greek to you, check out last week’s newsletter for some links and more detailed background info.
Whalebones on Sixteen Mile beach in the West Coast National Park
The weather forecasts that I watch the most have cycled through a series of odd updates that have shown alternately shown lots of rain on Saturday and Sunday, and none at all. I have students in the pool on Sunday so I won’t venture out into the ocean, but if you do, be aware they may or may not be surge, rain, or sunshine… Depends on your favourite forecast.
Neither day of the weekend looks all that promising for diving. The Atlantic is most likely to have the better visibility, however the swell on Friday and early on Saturday is not really my cup of tea. I will take a look at False Bay on Saturday, and possibly launch there on Sunday if it looks decent. If you’d like to dive, let me know!
Salt marsh at Langebaan lagoon
Turtle time!
We are entering into the busiest time for turtle wash-ups on the Western Cape coastline. On Tuesday, 14 (yes, fourteen) baby turtles stranded themselves in Hermanus. These little animals are in a highly compromised state when they end up on the shore, as they can’t cope with the cold waters this far south.
The Two Oceans Aquarium has a sophisticated turtle rehab facility, and, once they’re fattened up and restored to full health, the little turtles are released in the summer months when the warm current is closest to the Cape Peninsula coastline. Read about what to do if you find a baby turtle here. Local drop off points for tiny turtle guests are at the Two Oceans Aquarium (obvs), and the Shark Spotters info centre in Muizenberg.
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
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
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
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.
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)
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-Changehere (facebook), taken on 22 January in False Bay.
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)
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)
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)
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.
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
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.zawww.learntodivetoday.co.za/blog/Diving is addictive!
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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
Tidal documentary
Don’t forget the screening of Tidal at St James beach on Thursday 7 March. RSVP here.
Eye of the Shoal: A Fishwatcher’s Guide to Life, the Ocean and Everything – Helen Scales
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 Steed, her 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).
