Newsletter: Back in Town

Hi divers

Weekend dive plans

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
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.

regards

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

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Newsletter: Electric sea

Hi divers

Weekend dive plans

No diving

We won’t be diving this weekend, and there won’t be a newsletter (or diving) next week, but we’ll be back in business after the public holiday on Heritage Day (24 September). This weekend, Saturday looks good, and Sunday ok for diving.

I launched for divers on Tuesday, and they had fairly good conditions (5-6 metre visibility at Sherwood Forest and Castle Pinnacles) but the surface conditions deteriorated as the day progressed and the wind picked up.

Bioluminescence at Fish Hoek beach
Bioluminescence at Fish Hoek beach

This past weekend, helped by the south easter and then the heat, a thick red tide got caught in Fish Hoek bay and as a result we were treated to some incredible bioluminescence (caused by a type of plankton that fluoresces when disturbed by wave action) on Saturday evening.

This occurs at least a couple of times a year, usually in spring and autumn, on this side of False Bay (and it often makes its way around to the Kogel Bay area on the eastern shore). It is an incredible sight to see. There’s a little video here (pardon the screaming – the beach was packed and the people were excited).

Anyway – see you in 10 days.

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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Shark Spotters in brief

The Shark Spotters centre at Muizenberg
The Shark Spotters centre at Muizenberg

I had to write a short article about Shark Spotters a while ago, for the quarterly magazine of the company I work for. It was fun to write within the constraints of quite a punitive word count, and to try and emphasise the aspects of the program that I think are poorly understood by the public. Here’s the article:

Shark Spotters developed into Cape Town’s primary shark safety strategy out of two similar, informal initiatives. At Muizenberg and Fish Hoek in 2004, surfers arranged with lifeguards, car guards and trek fishermen to warn them when sharks were sighted. Today, Shark Spotters is a NPO funded primarily by the City of Cape Town, Save Our Seas Foundation, and public donations. It operates year-round at four beaches and during summer at another four. These are beaches that have both many water users and relatively common shark sightings.

A team of 30 spotters equipped with binoculars, polarising sunglasses and all-weather gear watch from the mountainside, and when a shark is sighted they notify colleagues at the beach to sound a siren and raise the appropriate flag. The flags indicate the current or recent presence of a shark, that spotting conditions are poor, or that it is safe to swim.

The spotters, all drawn from local communities, are trained in safety, first aid and shark behaviour. A further 10 team members deploy and retrieve the shark exclusion net at Fish Hoek beach during summer. Unlike the gill nets protecting beaches in KwaZulu Natal, this net does not catch sharks. It provides a physical barrier between sharks and swimmers. It is designed to be retrieved at the end of the day, or, to prevent entanglement, when there are marine mammals nearby. The Fish Hoek shark exclusion net is unique worldwide as an environmentally friendly shark attack mitigation measure.

It is the combination of favourable topography and surface-swimming sharks that makes Shark Spotters’ work possible and effective. The land around many of Cape Town’s beaches slopes steeply towards the sea, providing raised vantage points from which to spot. The sharks which pose the primary danger to water users, because of their size and curious natures, are great white sharks. Fortunately these sharks spend much time swimming on the surface, and their distinctive swimming style is readily recognisable.

Shark Spotters also conducts research on sharks to improve safety measures and provide management and conservation recommendations. As a result, the movements of great white sharks in False Bay are well understood. Sharks visit the beaches year-round, but with a distinct seasonal pattern. During winter the sharks congregate at Seal Island to feed on juvenile seals. During summer, sharks head for the backline of Cape Town’s beaches – probably to feed on the fish species found in False Bay at this time, and to rest in the highly oxygenated water close to shore. This is when they pose the greatest risk to water users.

Why support Shark Spotters?

I love the fact that Shark Spotters combines care for people with concern for the environment. The program takes a scientific stance backed by research, and has attracted worldwide recognition. It also provides training and employment for 40 residents of some of Cape Town’s most impoverished communities. I sit on the non-executive committee as a representative of Cape Town’s scuba diving community, and it’s a privilege to work with fellow water users and ocean lovers, and hopefully to provide a benefit to the greater community through our small contribution.

For more information, visit www.sharkspotters.org.za.

(Of course, lately white shark movements in False Bay are slightly less well understood than they have been, probably thanks to a pair of orcas whose irregular visits to Seal Island and Millers Point to hunt white sharks and sevengills seem to clear out the neighbourhood a bit! Fear not, Shark Spotters’ research is aiming to understand these changes, too.)

Bookshelf: Beachcombing in South Africa

Beachcombing in South Africa – Rudy van der Elst

Beachcombing in South Africa
Beachcombing in South Africa

Why so quiet? What have we been doing? Working, mostly. Trying to stay alive. And a bit of reading, and some beachcombing. Enter this is marvellous little book from fish fundi Rudy van der Elst (A Field Guide to the Common Sea Fishes of South Africa).

Chapter by chapter, van der Elst describes the types of debris that one might find on a beach. After a brief orientation chapter covering the ocean current regime around South Africa, relevant regulations, safety, beach ecology, tides, pollution and more, we launch into a tour of washed-up treasures.

Predictably, many of the items to be found are organic in nature – plants, invertebrates of various types, eggs and egg cases, fishes, birds, and shells. There are also items such as oceanographic devices, tags from marine animals, fishing equipment, cyalumes, buoys – some of these (such as tags) should be returned to their owners, and others should be removed from the vicinity of the ocean (such as discarded fishing nets and lines).

The chapter on marine animals (resting, nesting and stranded) is exceptionally useful and it is almost for this alone that I’d like to put a copy of this book in every home in every coastal town in the country. Seals, whales, turtles and seabirds can end up on the beach, sometimes in difficulty and at other times not. It can be hard to tell, and well-meaning members of the public can unwittingly cause great harm while trying to assist. A list of useful contacts in this regard appears at the end of the book, such as the Two Oceans Aquarium and the SPCA (region-specific).

The final two chapters cover miscellaneous “treasures” such as fossilised sharks teeth, sea glass, logs, and actual treasure, as well as beachcombing through the ages in South Africa. Here we learn about tidal fish traps, coastal caves, and other historical coastal dwellers who made their living from the sea.

We’ve found some awesome things on the beach, from shipwrecks to goose barnacles to rare crabs. Beachcombing is an accessible hobby that requires nothing but time, observation skills, curiosity, and a beach to stroll on.

This is a beautifully illustrated, comprehensive little volume that deserves to come with you on your beach holiday. It’ll prompt more careful examination of the flotsam and jetsam on your local beach, and, probably, more early morning low-tide visits to find the best pickings!

Wild Card magazine featured this book when it was published. Get it online here if you’re in South Africa, or here for your Kindle.

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.

One of the Shark Spotters flags at Fish Hoek beachåç
One of the Shark Spotters flags at Fish Hoek beachåç

The Shark Spotters provide beach safety, a world-first environmentally friendly shark exclusion net at Fish Hoek beach, run educational programs at local schools, and conduct shark research in and around False Bay.

All of this work keeps both people and sharks safe, and this is at the heart of what Shark Spotters does.

The shark exclusion net has been around for about five years, and (unlike the gill nets in KZN) is specially designed not to pose a hazard to any marine life. It gets taken in every evening, to eliminate the risk of an animal becoming entangled at night when the spotters are not on duty. It is an extremely successful and popular feature of Fish Hoek beach, and is invariably packed with happy, safe swimmers during the summer months. If you’re curious about it, watch the video above to see what it looks like from underwater, and read more about it here, here and here.

The Shark Spotters info centre at Muizenberg
The Shark Spotters info centre at Muizenberg

Their research continues to illuminate the activities of white sharks in the bay, but is also in the process of shedding light on the interactions between white sharks, sevengill cowsharks and bronze whalers as they share the ecosystem. They have even published a recent paper (open access) on the predation of sevengill cowsharks by orcas in False Bay, which is well worth a read.

The Shark Spotters outpost at Caves (Kogel Bay)
The Shark Spotters outpost at Caves (Kogel Bay)

Currently, they’re partnering with a Swiss firm in a cutting-edge research project to determine how automated shark spotting, making use of cameras and machine learning algorithms, can augment the already impressive skills of the spotting team. By first training a sophisticated algorithm to distinguish between a shark, pieces of kelp, dolphins, wind chop, and all the other visual phenomena that a spotter comes across during the course of a day, it is hoped that a fixed camera system, with some software, could assist the spotters in their work. It is not intended to replace human spotters, but to augment and facilitate their work.

The Shark Spotters hut at Fish Hoek beach
The Shark Spotters hut at Fish Hoek beach

We’re 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.

Newsletter: Is that rain?

Hi divers

Weekend dive plans

No diving

Whalebones on Sixteen Mile beach in the West Coast National Park
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.
 

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.

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!

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