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: Batten down the hatches

Hi divers

Weekend dive plans

No diving

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 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.

Shark Spotters is part-funded by the City of Cape Town and the Save Our Seas Foundation, and the rest comes from public donations. The Shark Spotters provide beach safety, a world-first environmentally friendly shark exclusion net at Fish Hoek beach, conduct educational programs at local schools, and conduct shark research in and around False Bay.

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.

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!

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.

Bookshelf: Eye of the Shoal

Eye of the Shoal: A Fishwatcher’s Guide to Life, the Ocean and Everything – Helen Scales

Eye of the Shoal
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 Steedher 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).

You need this: the Shark Spotters smartphone app

The Shark Spotters app
The Shark Spotters app

After a successful Back a Buddy campaign, Shark Spotters launched their free shark safety smartphone app in the summer of 2016. The app has had excellent uptake by water users of all stripes, but I’d like to draw it to your attention in case you aren’t familiar with the ways in which it can make your beach life better and safer! Whether you’re a surfer, a diver, an ocean swimmer, a beach comber, or someone who looks forward to long summer days under an umbrella on the sand, the Shark Spotters app can make a difference to you.

The list of Shark Spotters beaches
The list of Shark Spotters beaches

Shark Spotters operate at eight beaches around the Cape Peninsula: four (Muizenberg, St James/Kalk Bay, Fish Hoek & Caves, Kogel Bay) year-round, and another four (Glencairn, Clovelly, Monwabisi and the Hoek, Noordhoek) from October to April, during the warmer months.

The app allows one to set a home beach – you can see from the image above that mine is Fish Hoek – and when you start the app, you’ll get information for your home beach immediately. Tapping on the home beach block at the top of the screen (where it says “Tap for more”) brings one to a series of screens that you can navigate through by swiping the bottom block on the screen.

The information provided is extensive. You can see information on the last three shark sightings at your home beach, the weather and sea conditions (temperature, wind direction, tides, moon phase, swell), and information on what marine life is in the area. This latter information is not a nice to have so you can look out for dolphins from shore, but an important indicator of the likelihood of a shark being in the area. Dolphins, rays and schools of fish can draw white sharks, while sunfish (and even seals) may be mistaken for a shark when viewed from a distance by an untrained observer.

There’s also information on what facilities are available at the beach, and a short video for each beach, usually taken each morning, showing the prevailing conditions.

All this information is available for all the Shark Spotters beaches, not just your home beach, and you can view a different beach by selecting it from the list that appears under your home beach on the home screen.

Digging into the app, you can find a detailed list of all the recorded shark sightings, and in the settings you can turn notifications of sightings on or off according to your preference. I have them switched on, because I like to be reminded of a world outside the office.

Shark Spotters app menu options
Shark Spotters app menu options

The further menu options, shown above, give you access to a large amount of data about the Shark Spotters program, the exclusion net at Fish Hoek beach, how the flag system works, great white sharks, and more. Most of the menu options also provide links to more detailed information online, if you want to know more.

You can download the app for iOS here, for Android here, or use the download links from the Shark Spotters website. The app is completely free of charge, but Shark Spotters is funded by the City of Cape Town, Save Our Seas Foundation, and other generous sponsors, including individuals. Shark Spotters welcomes donations. Click here to donate.

Merry Christmas divers!

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
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.

Newsletter: Better than nothing

Hi divers

Weekend dive plans

Saturday and/or Sunday: Boat dives in False Bay

The forecast today is a little better for the weekend than it was earlier this week. There is some wind and some odd swell and swell direction changes but I believe it should be worth diving both Saturday and Sunday. Sunday will most likely be a little better. I have students on the boat on both days so there is not much space, however, if you are quick you can reserve a spot!

Zandvlei Nature Reserve
Zandvlei Nature Reserve

Things to do

It’s not as if one needs to actively seek out extra commitments at this time of year, but in case you’re at a loose end check out Wavescape’s Slide Night happening on Monday (you need to book in advance for this). You can get some adult education at UCT’s annual Summer School in January, and there’s something for you whether your interest is sharks or shipwrecks.

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!

Article: The Atlantic on how much of the sea is fished

The proportion of the world’s oceans that are fished has recently been the subject of two papers in Science, both of which used the same data set to reach different conclusions. One study concluded that 55% of the ocean is affected by fishing, while a follow up (using the original data set, but at finer resolution) concluded that the number is closer to 4%.

A janbruin emerges from a school of fish at Atlantis
A janbruin emerges from a school of fish at Atlantis

Ed Yong of The Atlantic sets out the disagreement, and explains the subtleties and the points of view on each side of the debate. Far from being an insoluble  scientific crisis, the divergent findings show the iterative nature of the scientific process and the manner in which back and forth between scientists leads to progress. It brings to light important questions worth considering, ones that are more important than simply asking the original question of how much of the ocean is fished.

Instead, you have to ask different and more refined questions. How much of that fishing is sustainable? Which species are being targeted? How are they faring? Can they bounce back?

Read the full article here.

Newsletter: Sorry, kids

Hi divers

Weekend dive plans

No diving

The weather forecast for the weekend is wild and windy, and definitely not diving weather.

Diversnight 2017 at Long Beach
Diversnight 2017 at Long Beach

We had a great night dive last Saturday evening at Long Beach. It was calm but crisp, and we spent a lot of time watching a vast array of fish and invertebrates marauding around on the sand, hunting for their dinner. Clouds of fish fry, so thick that at the beginning of the dive we couldn’t see our feet, provided food for an array of predators. There were a lot of divers in the water, and it was great to see the dive clubs take ownership of this community diving event. Watch out for Diversnight 2019!

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!

Muizenberg’s marine street art

Public art is one of the things that can define a community’s ethos. In the seaside suburb of Muizenberg, there’s a lot on which to feast the eye. What follows is a small tour of some of the most prominent marine-themed artworks I’ve noticed around Muizenberg. I’m ignorant, but I know what I like to look at, and here’s some of it.

Rhodesia road whale pair

Whales on Rhodesia Road
Whales on Rhodesia Road

This humpback whale mother and calf pair feature prominently along the wall of a residential home in Rhodesia road. They were created by Sergio Rinquist (Serge One of the One Love Studio).

Killarney Road fish

Fish on Killarney Road
Fish on Killarney Road

You might have seen these five colourful fish peeking out of Killarney road, visible on your right as you drive along Atlantic Road towards the Main Road. They blend true-to-life forms with colour and playful designs, and are worth closer examination.

Fish on Killarney Road
Fish on Killarney Road

They are also the work of Serge One – the One Love studio is responsible for a lot of the beautification of Muizenberg through their vibrant murals. See his instagram page here.

Rustenburg Pharmacy whale

Humpback whale by Chris Auret
Humpback whale by Chris Auret

It’s definitely worth popping into Rustenburg Pharmacy at 52 Beach Road to check out the massive humpback whale mural by Chris Auret. He calls it Health and “Whaleness” on his website!

If you want to find any of these murals on Google maps, search for the road name and the suburb. None of the roads are very long so you shouldn’t have to look hard to find the artworks. There are also many more incredible public artworks in and on buildings in the Muizenberg area.

If you go exploring on foot, the usual disclaimers associated with movement in a big city in South Africa apply: be aware of your surroundings, don’t flash your valuables around, go in the daytime, and take along a friend or two if possible.