Newsletter: Rolling on

Hi divers

Weekend dive plans

Sunday: Boat or shore dives in False Bay

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
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.

regards

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

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Newsletter: Looks like spring

Hi divers

Weekend dive plans

Sunday: False Bay dives, conditions permitting

The tree in our driveway has a tiny green bud containing a new leaf. Despite the cold this week, I declare spring.

False Bay is quite surgy after some large swell this week, but I hope it’ll have settled down enough by Sunday for some reasonable diving. I’ll only make the call on Saturday afternoon; please let me know if you’d like to be notified of any developing plans.

Table Mountain from Rietvlei Nature Reserve
Table Mountain from Rietvlei Nature Reserve

Beach cleanups

There are at least two happening this weekend:

Sharks in False Bay

There’s a fascinating update this week from Shark Spotters on False Bay’s white sharks (spoiler: they’re awol but we’re not quite certain why yet) – read more here.

regards

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

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Newsletter: Still wintry!

Hi divers

Weekend dive plans

Sunday: Shore dives at Long Beach The water remains cold… But then it is winter. Both Saturday and Sunday should be reasonably decent for diving, but I think Sunday will be best. I will shore dive on Sunday, most likely around 10.00 am, once things have warmed up marginally. Let me know if you want to join us.

Turtle in Mozambique
Turtle in Mozambique

Turtle and seal rescue

The Kommetjie NSRI team rescued a seal and loggerhead turtle, entangled in fishing net, that washed up this week. The seal swam off, relieved, and the turtle is at the aquarium having some TLC and a health check, after which it will be released. Read all about it here.

regards

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

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Newsletter: Windless

Hi divers

Weekend dive plans

Sunday: Boat or shore dives in False Bay (conditions dependent)

Apologies for the lateness of this week’s newsletter… Blame a very delayed Safair flight from Johannesburg last night!

Lighthouse at Kalk Bay
Lighthouse at Kalk Bay

We have three windless days coming up and hopefully some sun to go with that. There has been a lot of swell in False Bay, and coupled with the rain and run off the visibility is not all that great. If anything, Sunday will be the best diving option so based on what we see Saturday, we will plan for Sunday. Let me know if you want to join in.

regards

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

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Newsletter: Wait and see

Hi divers

Weekend dive plans

Sunday: Shore dives in False Bay

Cape fur seals at Kalk Bay harbour
Cape fur seals at Kalk Bay harbour

There is a fair amount of swell around right now, combined with strong winds. Although False Bay looks a little scrappy, the water colour remains good. Sunday looks like the best, or only, option for diving, but we will most likely shore dive. I will wait until Saturday mid afternoon to make the call. Get in touch if you wish to join.

regards

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

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Newsletter: Here’s a challenge

Hi divers

Weekend dive plans

Friday: Shore dives at Long Beach at 8am

Saturday: Shore dives at Long Beach at 12pm

I have students so I will shore dive both tomorrow morning and Saturday afternoon, after the marathon road closures. Sunday and Monday don’t look like good weather days.

Autumn on Fish Hoek beach
Autumn on Fish Hoek beach

City Nature Challenge

Besides a few days of challenging weather for the long weekend there is a different and way more interesting challenge heading your way: the iNaturalist City Nature Challenge 2019. This is a worldwide bioblitz event, happening this year from 26-29 April, during which you get a chance to get outdoors, spot species, and do some citizen science.

Cape Town is participating! If you like competition, we’re pitted against other cities around the world (last year San Francisco had the most observers, who saw the most species, and logged the most observations). Otherwise, it’s a fun opportunity to go diving (or hiking, or paddling, or however you like to get outside), and to share what you see with others.

With the iNaturalist app (for iOS or Android) or on the website, you can photograph (or upload photos you took with your camera) and record all kinds of wildlife and plants. You don’t even have to know what you’re seeing – experts will weigh in with identifications if you are unsure. These citizen science observations are invaluable for mapping species diversity and distribution and are used for all sorts of projects. You can use the iNaturalist app (or website) any time, not just during the City Nature Challenge, and it’s a great tool for recording flora and fauna that you come across, even in your own garden.

On Wednesday 24 April, Georgina Jones is giving a talk at False Bay Underwater Club about the challenge, and the sorts of species you could spot and record. More details on the facebook event page.

We’ll be diving next weekend, conditions permitting, and hope to have some observations to contribute to the City Nature Challenge. We’d love it if you joined us.

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

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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: 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/

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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!

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