We had a good day’s diving on Wednesday with windless conditions and good visibility. The weekend has some wind in the forecast, at times a little too strong and at times a lot less.
I think it best to do shore dives this weekend on either Sunday or Monday, or both. Sites decided on the day depending on when/if the forecast winds are as they say.
Sunrise at Long Beach
All about False Bay
This weekend (Saturday 14 and Sunday 15 December) features an exhibition of photos and paintings, and two morning talks, one by False Bay’s pre-eminent shark scientist, and another by the lead author of a recent multi-disciplinary study of the bay. All the information you need is here.
It has been a windy week and neither False Bay or Hout Bay look particularly appealing. There is a chance the Atlantic will clean up a little more, but the wind forecast for Saturday and Sunday is a little more than our boat enjoys. We have no diving planned.
Table Mountain seen from Sunset Beach
Diarise
The year is winding down, but there are still good things happening for ocean lovers. Next weekend (14-15 December) features an exhibition of photos and paintings, and two morning talks, one by False Bay’s pre-eminent shark scientist, and another by the lead author of a recent multi-disciplinary study of the bay. All the information you need is here.
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.
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
Beach cleanups
There are at least two happening this weekend:
The regular new moon beach cleanup at Surfers Corner, Muizenberg beach, organised by The Beach Co-Op (info on facebook) on Saturday at 9am
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.
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.
(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.)
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!
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
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
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)
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
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.
The weather forecast predicts that we are soon to be lashed with 50 km/h winds, a 7 metre swell, and no small amount of rain. It seldom is as bad as the forecasts claim, however the swell size and direction will hammer False Bay. Despite Sunday’s weather looking peachy, I don’t think the ocean will be, so we will plan for a dry weekend… It is winter after all.
Shark Spotting at Fish Hoek beach
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.
We’ll write a blog post with more information soon – but in the mean time, we’re very 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.
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.
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).
