We have south easterly winds of between 30 and 60 km/h spread out over the next few days, and as a result I am not planning any dives for this weekend.
SAS Somerset at the V&A Waterfront
Diver24
Join UCT Underwater Club and the Two Oceans Aquarium for their annual Diver24 event. This year you can enjoy diving-related talks, snacks, and the vibe of the aquarium at night. More info here (facebook).
On your way, you can check out SAS Somerset, pictured above, as she lists slightly in her berth behind the aquarium. She’s the sister ship of SAS Fleur, a deep dive not too far from Seal Island in False Bay.
I’ve got students in the pool on Saturday, and Sunday looks too windy for dives, so we’ll have to wait to get in some salt water until conditions improve.
Turtle time
It’s the start of that time of year when turtles strand on the beaches of the Western Cape. What to do if you find one? Don’t throw it back into the sea. The Two Oceans Aquarium explains how you should proceed. Read more here.
Student dives over the last week have been reasonably good, with 5-6 metre visibility inshore. The visibility remains good, and in fact has improved with the unseasonal north westerly wind. The downside is a 5 metre, 16 second period swell. We won’t plan dives in this as the surge is often harsh.
Keep an eye out for weekday dives during the next week or two – we’ll publicise in this newsletter, on facebook, and via whatsapp. We would like to take this opportunity to wish you all a very happy and safe holiday season.
The weather has certainly started warming up a little as we roll towards summer. A few days of north west winds will have helped improve the visibility this side of False Bay, however there is some swell heading our way. The swell doesn’t always arrive as big as it is forecast, so I am going to plan dives for Sunday (the better day swell-wise at this point). Depending on what False Bay looks like late on Saturday, I will then decide whether we launch the boat or do shore dives. Let me know if you want to join.
See through fish at the Dubai Aquarium
Dates to diarise
Don’t forget about Shark Night at the aquarium this coming Tuesday – details here.
Diversnight this year is on Saturday 2 November, and the dive clubs are hosting. It’s a super fun evening of night diving, and I suggest you put it in your planner. Facebook event details here.
No diving The weekend is too windy for good diving, with strong south easterly winds starting on Friday evening. We won’t be diving, but I’ve got a boat launch tomorrow and I’ll share the conditions on facebook.
Jellies at the aquarium in Dubai Mall
Worth your time
New Moon beach clean
This Saturday at 9am is the monthly new moon beach cleanup at Surfers Corner, run by The Beach Co-Op. Event details (facebook) here.
Shark Night
As part of this year’s Shark and Ray Symposium, there’s a public event on Tuesday 8 October at the Two Oceans Aquarium that’s ALL ABOUT SHARKS. Looks awesome. Get more info and tickets here.
Talking Trash
As part of First Thursdays, on the evening of 3 October there will be a series of short talks about how waste is managed in Cape Town, the social, environmental and economic impacts of waste, and some strategies Capetonians can implement to better assist the City of Cape Town in its sustainability and resilience journey. The event is in central town, and is just one hour long. Find details here.
Given the time of year, the large swells we are having are not all that unusual. Neither is the odd day or two of south easterly wind. Friday and Saturday feature both these gremlins, so conditions won’t really be great. Sunday’s forecast is a little rosier, however I doubt the visibility will have recovered and the surge will still be a lingering factor. This weekend my choice is to stay home.
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.
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 The forecasts vary wildly this weekend. Windy says howling south easter everywhere, whilst other sites say mild wind suitable for some Atlantic diving. The mountain will break some of the wind so I am sure Table Bay sites will be good, and Hout Bay much the same. I have students for the pool this weekend so there are no launches planned.
Clare is selling her Sony underwater housing that fits the Sony RX100 range of cameras. If you’re interested, drop me a mail and I’ll put you in touch with her.
If you celebrate Christmas, here are our very best wishes to you and your family. This year we’re having a white Christmas in Stockholm (Clare’s first proper brush with snow… I’ll let you know how many snowballs I manage to land before she exacts retribution)!
If you’re in or on the water this week, be safe and have fun, and spare a thought for – and say thank you to – the first responders who work to keep us all safe, even during holidays. These often unseen angels include the NSRI, lifeguards, Volunteer Wildfire Services, police, traffic officers, and law enforcement.
Santa feeding the fish at the Two Oceans Aquarium
Here, also, is a (cellphone) picture of one of the aquarists from the Two Oceans Aquarium, feeding the fish in the I&J Ocean Exhibit this December. The apparent rain of snow is tiny bits of whatever the fish were getting for lunch that day – most likely chopped up squid and white mussel.
