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.

Baboons on the beach

Baboon on the beach Baboon on the beach

A recent low tide visit to the beach at Platboom near Cape Point, on the Atlantic coast of the peninsula, enabled us to watch a troop of Chacma baboons (Papio ursinus)  foraging for limpets, mussels and other marine snacks on the rocks at low tide. The baboons bite the tops off the limpets with their formidable incisors, or pry them from the rocks intact to get at the protein-rich flesh. They also eat mussels.

Baboons foraging for seafood at Platboom Baboons foraging for seafood at Platboom

This foraging behaviour is extremely rare among primates. In baboons, it is only observed on the Cape Peninsula and in one other species in Somalia. Matthew Lewis studied this troop of baboons as they foraged around the Cape Point nature reserve, and his thesis makes for fascinating reading. (Wild Card Magazine also featured Matthew’s research.)

Baboon on the beach Baboon on the beach

The amount of time the baboons are able to spend foraging on the shore is largely determined by the height of the tide, and by weather conditions. As a result, the amount of time the baboons spend seeking marine food sources is small compared with the time they spend looking for roots, bulbs, insects, berries, and small animals.

Low tide at Platboom Low tide at Platboom

These baboons are part of the Kanonkop troop which ranges freely in the Cape of Good Hope section of Table Mountain National Park and whose home range does not bring them into conflict with humans (or, as a rule, allow them access to any anthropogenic food sources). They were completely uninterested in us and our vehicle, unlike the baboons we see further up the peninsula around Millers Point, for example.

Concentrating baboon Concentrating baboon

Goose barnacles on the beach

A (lovely, rain-bringing) onshore wind left great rafts of kelp all over Noordhoek beach one weekend in mid May. Finding anything of substance on this beach is unusual; it’s on an exposed piece of coastline and all but the most robust objects are dashed to pieces before they arrive on the sand. Seeing all the washed up kelp also reminded me that frequenting the beaches inside False Bay, that are daily cleaned of washed up kelp by the City of Cape Town, is liable to give one a skewed idea of just how much kelp naturally washes up on the sand.

Kelp stipe covered in goose barnacles
Kelp stipe covered in goose barnacles

This time, there was kelp, and lots of it. Several of the pieces of kelp had been colonised by goose barnacles. There are several species of goose barnacle that occur off South Africa’s coast, but these ones are Lepas testudinata. They are incredibly strange looking animals, and some of them were still alive and writhing slowly in the drying sun.

In parts of the world (I’m looking at you, Iberian peninsula), goose barnacles are an expensive delicacy. I have nothing to say about that.

Goose barnacles, with my paw for scale
Goose barnacles, with my paw for scale

Lepas testudinata larvae most often attach to free-floating pieces of kelp (Ecklonia maxima) and plastic debris, which is why you have probably never seen these mesmerisingly gross-looking creatures while on a dive. In the picture below, you can see that they’re attached to the bottom of a kelp holdfast, where it would ordinarily attach to the rock. This shows that they attached after the kelp broke off.

A kelp holdfast encrusted with goose barnacles
A kelp holdfast encrusted with goose barnacles

Each barnacle is possessed of a long fleshy peduncle, or stalk, which attaches to the kelp holdfast, stipe or fronds. On the end of the peduncle is a carapace (shell) made up of five separate pieces. The large part of the barnacle on the end of the peduncle (what you’d think of as its body), covered by the carapace, is called the capitulum. The apparatus that the barnacle uses for feeding – essentially six pairs of hairy legs – reside inside the carapace, along with the mouth. There’s some more detail and a nice diagram at this link. If you are familiar with other kinds of barnacles – the volcano-shaped ones that live on rocks, ships, whales and piers for example, then most of this (except the peduncle) should sound familiar to you.

Lepas testudinata goose barnacles
Lepas testudinata goose barnacles

Research done around South Africa’s coast (published here) by Otto Whitehead, Aiden Biccard and Charles Griffiths, identified the marked preference of Lepas testudinata for attaching to kelp. The researchers surveyed a selection of beaches around South Africa’s coast, from the west coast of the Cape Peninsula up to northern KwaZulu Natal, between June and October 2009. When they found goose barnacles washed up, they recorded the species of barnacle, the type of material they were attached to, the dimensions of the object, and its location. They also estimated the number of barnacles in each colony they found.

Lepas testudinata was the species they found most commonly, of the six species in total that they identified along the area of coast that was surveyed. (There’s a nice picture of the six species in their paper, which I used to identify the ones I found.)  This species of goose barnacle was found to prefer kelp, as mentioned, and also tended to colonise large objects compared to the other species (this could, of course, be because pieces of kelp are usually larger than items such as bits of plastic, glass, feathers, and shells that some other species prefer).

Kelp fronds with goose barnacles
Kelp fronds with goose barnacles

Lepas testudinata was the only species of goose barnacle that the researchers regularly found to form colonies comprising more than 1,000 individuals. It is also the only species of goose barnacle recorded by the survey that is only found in temperate (cooler) waters, which happens to be where kelp is found, too.

The researchers note that the goose barnacles of the Lepas testudinata species that they found on kelp seemed to have exceptionally long peduncles, some more than 25 centimetres long, and that this seems to differ from what has been previously known about them (which is that they have “short, spiny” peduncles). They suggest that perhaps the variety of Lepas testudinata that colonises kelp may even be a separate species from the one previously described (more research obviously required to ascertain this). You can see from my photographs that the peduncles of the washed up Noordhoek beach goose barnacle colonies are also quite long, some easily 20 centimetres in length.

Clusters of goose barnacles on a kelp stipe
Clusters of goose barnacles on a kelp stipe

They also found that the increasing prevalence of long-lasting and buoyant plastic marine debris and other anthropogenic objects around our coastline, which some species of goose barnacles preferentially attach to, gives these weird little creatures increased opportunities to form colonies, and to spread to new places. This is one of those interesting phenomena to keep in mind, as humans inexorably alter the environment. Some creatures will benefit in strange ways from warming oceans, and others will find new homes in the garbage we leave lying around.

Bookshelf: Shark

Shark – Brian Skerry

Brian Skerry is a National Geographic photojournalist, with whose TED Talk you may be familiar. This book is a collection of articles – about sharks – that appeared in National Geographic magazine, accompanied by one magnificent shark photograph after another. Each chapter’s text is reasonably short. Here, the photos are the primary focus.

Shark
Shark

The chapters focus on four species of shark: great white, white tip, tiger sharks, and mako sharks. Additional text is contributed by several National Geographic writers, and experiencing the familiar editorial quality and stylistic approach of the magazine is like settling down for a chat with an old friend.

The final chapter of the book, written by Skerry, is an appeal for increased understanding of sharks and their vital place in ecosystems, and increased protection for them – in the form of marine reserves, and less fishing, for example. The photographs selected for this chapter makes it clear that in Skerry’s view, science (especially tagging studies) is vital to the endeavour of better understanding sharks, and protecting them.

Get a copy of the book here (South Africa), here or here.

Bookshelf: Antarctica

Antarctica: An Intimate Portrait of the World’s Most Mysterious Continent – Gabrielle Walker

THIS is the book about the Antarctic that I have been looking for all of my life. It’s unlikely that this discovery will stop my obsessive consumption of polar-related literature and documentary material, but this is likely a book I will return to again.

Antarctica
Antarctica

The author, a science writer, has visited Antarctica several times, and is thus able to weave her personal experiences of  life on the driest continent with accounts of the science taking place there, and the scientists doing the work. Walker has had the sort of access to the scientists that most of us can only dream of, and makes reasonably good use of it.

Mixed in with stories of her Antarctic travels and meetings with researchers, Walker also briefly recounts the stories of some of the explorers of last century who opened up the interior of the continent. She is able to visit Western Antarctica, a part of the continent that very few people get to, and where the effects of climate change can be seen most clearly.

There’s a much more comprehensive review from The Guardian here. If you’re interested in the Antarctic, and the science that is being done there, you should read this book.

Get a copy here (South Africa), here or here.

Bookshelf: Manta

Manta: Secret Life of Devil Rays – Guy Stevens & Thomas Peschak

I found this book to fill a significant gap in my manta ray knowledge, which was (to be honest) virtually nonexistent. Author Guy Stevens is founder of the Manta Trust and a Save Our Seas project leader, and has spent 15 years in the Maldives studying these enormous, charismatic elasmobranchs. The Manta Trust co-ordinates global manta research efforts, with the aim of protecting and conserving mantas and their relatives.

Manta
Manta

The photographs in this book are by Thomas Peschak, co-founder of the Manta Trust, with whose extraordinary work you should be familiar. (If not, look here, here and here.)

Everything you might want to know about mantas is here, without being glib about the fact that there is still much we do not understand about these animals. The text covers their biology, life histories, threats to their survival, an identification guide, and numerous accounts by field scientists who study mantas and devil rays. (It was hard not to be envious reading some of the day-in-the-life bits!)

This is a beautiful, substantial book. Get it here.

Bookshelf: Pain Forms the Character

Pain Forms the Character: Doc Bester, Cat Hunters & Sealers – Nico de Bruyn & Chris Oosthuizen

Marion Island is one of South Africa’s two sub-Antarctic Prince Edward Islands, technically part of the Western Cape province. The South African National Antarctic Programme runs a meteorological and biological station there, dedicated to research. The researchers study weather and climate, ecosystem studies, seals (southern elephant seals, and Antarctic and sub-Antarctic fur seals), killer whales and seabirds such as albatross, that nest on the island. Researchers usually spend either three or 15 months at a stretch on the island, whose rugged terrain, intimidating wildlife and challenging weather can be said to “form the character”!

Pain Forms the Character
Pain Forms the Character

Marion Island is also infested by rats, introduced from whaling ships in the 1800s. With no predators, they multiplied to the extent that they threatened seabird populations. Cats were introduced in 1949, and by the 1970s there were 3,400 cats on the island. The cats ate mice, of course, and seabirds. An ambitious eradication program – of which our incredible friend Andre was part – eliminated the last of the cats in the early 1990s. The rat problem has resurged since the cats were removed, but work is in progress to get rid of them, too.

The research programs that currently exist on Marion Island are the legacy of Dr Marthan “Doc” Bester’s 40 year career as a scientist and researcher, and this book is a tribute to him. For this book, authors compiled photographs and testimonies from Bester’s colleagues, former cat hunters, and students, and he is the thread that ties this beautifully produced volume together. The focus is less on the scientific findings (you can find those online), and more on what it’s like to live on Marion Island, with the text complemented by many, beautifully evocative photographs.

Get a copy of the book here.

Bookshelf: Between the Tides

Between the Tides: In Search of Sea Turtles – George Hughes

I have been late in coming to this book, which was published about five years ago. George Hughes is a world-renowned, South African turtle scientist whose work has done much to ensure protection for sea turtles in the southern Indian Ocean. He was the guest speaker at an event held at the Two Oceans Aquarium to celebrate the release of Yoshi, the loggerhead turtle who spent over 20 years at the aquarium and is now powering along the Namibian coastline in rude health.

Between the Tides
Between the Tides

Dr Hughes was CEO of the Natal Parks Board and then Ezemvelo KZN Wildlife, but Between the Tides relates his early career as a student looking for turtles along South Africa’s wild north east coast, in places that today support thriving dive and fishing charters. His legacy of turtle research continues.

Turtle surveys were conducted around Madagascar, the Comores, Reunion, the Seychelles, and on the Mozambique coast. The fact that the iSimangaliso Wetland Park now exists, offering a protected and well-regulated breeding environment for three species of turtles (loggerhead, leatherback and green – discovered there in 2014) is thanks to the early and persistent work of Dr Hughes and his colleagues. Turtles were first found nesting on this piece of coast in 1963, when it was still completely wild and mostly neglected by the authorities. In this book Dr Hughes recounts the development of the tagging program that he started, in which over 350,000 hatchlings were flipper tagged and/or marked over a period of 31 years.

Only about two out of every 1,000 hatchlings survive to return to the area in which they hatched, to breed. Female loggerheads are estimated to reach maturity around the age of 36 years, during which time they navigate an ocean of threats. This makes every surviving hatchling incredibly valuable.

The recovery of the number of loggerheads, in particular, has been quite spectacular, with more modest but noticeable gains in the leatherback population. More recently, as technology has allowed it, satellite tagging has shown their movements around the Indian ocean

If you find a baby sea turtle on the beach (this is the time of year when they start washing up), here is what you should do. The most important thing is to keep it dry, and to contact the aquarium as soon as possible.

Dr Hughes also discusses the sustainable use of sea turtles (for example, for food), something which I’d never thought about and which for that reason is fascinating – and very challenging to come at with an open mind, and appreciating the viewpoints of a scientist who has been steeped in turtle research for most of his life.  This is an excellent, proudly South African marine science book, written to be accessible even to those who aren’t turtle fanatics a priori. Highly recommended.

Get a copy of the book here (South Africa), here or here.

What causes the brown water at Muizenberg beach?

Capetonians are familiar with the tea-coloured water that runs in our mountain streams. Most people know that the brown colour comes from tannins, leached naturally from the indigenous fynbos vegetation. Perhaps less well known is the reason for the brown water that is sometimes seen in the surf zone along Muizenberg beach, stretching all the way to Strandfontein, Monwabisi and beyond.

Tea-coloured water at Muizenberg
Tea-coloured water at Muizenberg

The most frequent explanations that are offered on social media are, of course, pollution, “raw sewage”, and the like. This is not the reason for the brown water, and it does not necessarily impact the water’s safety or healthfulness for humans to swim in.

Like False Bay’s famous colour fronts, the reason for the brown waves at Muizenberg beach turns out to have much to do with the topography of False Bay, particularly of the kilometres-long beach at its head (Muizenberg-Strandfontein-Macassar-Monwabisi), and something called a diatom.

View of Muizenberg showing patches of brown water
View of Muizenberg showing patches of brown water

Diatoms

Diatoms are a type of phytoplankton (plant plankton or microalgae). They are single celled, usually symmetrically shaped organisms that multiply by dividing in half at a constant rate. Their cell walls are made of silica, SiO2. Chicken keepers and gardeners may be familiar with diatomaceous earth – this is made up of the fossilised shells of ancient diatoms.

Diatoms are what are called primary producers or autotrophs, meaning that they generate organic material from carbon dioxide and other inorganic nutrients (for example nitrates and phosphates), through the process of photosynthesis, which uses light as an energy source. Primary producers sit at the base of the food chain and all life relies on them, directly or indirectly. Everything else produces organic material from other organic material (such as diatoms).

I am telling you all about diatoms because the brown water at Muizenberg contains an accumulation of a diatom that you can call Anaulus australis Drebes et Schultz the first time you mention it, but usually just Anaulus australis, or Anaulus for short. There are several members of the genus Anaulus, but usually just one tends to be dominant at each beach where these accumulations occur, and Anaulus australis is the main species found along the South African coast.

Analaus are pillow-shaped diatoms. If you wanted to see what an individual Anaulus diatom looked like, you’d use a microscope, but when enough of them are in one place, they can be seen to change the colour of the water. There’s a picture of them under a microscope at the bottom of this webpage (they also occur in Brazil). They occur at beaches with particular topograhical characteristics, which explains why you haven’t seen them at Camps Bay, Kogel Bay, or Scarborough.

At hospitable beaches, the diatoms are always there, spending much of the time lying dormant in the sand behind the surf zone. A proportion of the diatom population is able to survive for relatively long periods (estimated to be more than two months) like this, in the dark on the seabed, not photosynthesising or dividing, until the correct meteorological conditions arise for an accumulation. But first – what sorts of beaches are hospitable to Anaulus?

Brown water in the surf zone at Muizenberg
Brown water in the surf zone at Muizenberg

Topographical conditions

There are five physical features of coasts that are prone to diatom accumulations. They are:

  1. a high-energy sandy – not rocky – shore
  2. a long beach, more than 4 kilometres in extent
  3. the presence of rip currents
  4. a surf zone at least 150 metres wide
  5. a nutrient source close to the surf zone (often an unconfined aquifer overlaid by a dune field)

Muizenberg and Strandfontein beach tick all these boxes. The beach stretches from Surfers Corner all the way across the top of False Bay to Monwabisi, a distance of over 20 kilometres. It is a high energy beach, meaning that it is exposed to large waves and strong winds, and is not protected by any offshore features such as sandbars or headlands that might reduce the force of the waves. Rip currents do occur at the beach, and both these and the exceptionally wide surf zone – wider during south easterly winds in summer – can be observed from the mountainside on Boyes Drive. (A rip current is like a hidden river flowing out to sea from the beach. The Sydney Morning Herald has an excellent visual explainer of rip currents here.)

The head of False Bay where Muizenberg is situated is incredibly nutrient-rich, much of it thanks to urbanisation. The canalised Zandvlei estuary – the only vaguely functional one on False Bay’s coast – is situated a short distance down the beach, and supplies nitrates, phosphates and other nutrients to the surf zone. Many of these nutrients are technically pollutants, added to the river further upstream. The Cape Flats Waste Water Treatment plant at Strandfontein also discharges 200 million litres of treated water per day (under normal, non-drought circumstances) via a canal onto Strandfontein beach. This is essentially an artificial estuary for Zeekoevlei. This waste water has spent some time working its way through the settlement ponds at Strandfontein, but is nevertheless rich in ammonia and other nutrients, and Anaulus accumulations are a very common sight in the surf around this discharge point. The dunes that run along Baden Powell drive overlay a high water table, and groundwater seepage – specially during times of heavy rainfall – may also leach nutrients out of the ground and into the surf zone.

Meteorological conditions

The meteorological conditions required for an Anaulus accumulation involve strong wind and a large swell. These act together to create rough sea conditions, which stir up the dormant diatoms from the ocean floor. The diatoms adhere to air bubbles in the surf zone, staying suspended in the water column, which is when you would notice the water turning brown. Exposed to light, they awaken from their dormant state and start to photosynthesise, take up nutrients, divide and multiply. The presence of rip currents creates an onshore-offshore flow all along the beach. This forms a semi-closed ecosystem, and the diatoms are essentially trapped in gyres in the waves. Longshore currents that run parallel to the beach transport Anaulus cells out of the surf zone at one end, and bring fresh (sea)water in at the other end of the beach.

It may seem surprising that anything manages to accumulate in the waves of a beach, but the surf zone is actually quite retentive, meaning that things that end up there often tend to stay there. (Incidentally, this is why it’s a terrible idea to discharge the byproduct of reverse osmosis seawater desalination –  a super-salty brine – into the surf zone. It must be discharged offshore so that it can disperse and mix with the surrounding water.)

Diatoms in the surf zone at Muizenberg
Diatoms in the surf zone at Muizenberg

You’ll notice that, contrary to what you may have seen when large amounts of plankton are under discussion, I’ve been using the word “accumulation” instead of “bloom” to talk about Anaulus. This is deliberate, because of the constant presence and constant rate of division of the diatoms. When the water goes brown, it doesn’t mean that Anaulus is suddenly multiplying faster than usual. It means that it’s all been gathered together in patches, is exposed to light and therefore photosynthesising (at its usual steady rate), and is thus more visible than it was when it was lying on the ocean floor.

Anaulus at Muizenberg in November 2017
Anaulus at Muizenberg in November 2017

The human factor

You may also be thinking that everything I’ve said about the nutrients that Anaulus requires to survive and thrive points to the fact that humans – and pollution – are ultimately responsible for these brown-water plankton accumulations at Muizenberg. Well yes, in a way. But accumulations of Anaulus australis and related species have been observed and documented for well over 100 years at suitable beaches around the world, and are a natural phenomenon.

Yes, we are providing more nutrients to the False Bay diatom population than they would otherwise have received without human settlement in the greater Cape Town area, but these accumulations would likely occur regardless. They are certainly more intense now than they would have been in the past, but estuaries are nutrient-rich locations even when not surrounded by a large city. Furthermore, the water table is high on the Cape Flats, which would supply nutrients to the surf zone regardless of whether humans lived nearby.

Anaulus is in fact performing a vital and useful function by mopping up the excess nutrients that the city discharges in the ocean. The mass of diatoms – primary producers – also provides a food source to bivalves such as mussels, and other invertebrates. We can be grateful that the excess nutrients that urbanisation directs towards the ocean at the head of False Bay leads only to accumulations of harmless diatoms, rather than to frequent occurrences of harmful algal blooms that can kill marine life and exacerbate respiratory problems in humans.

Muizenberg during a diatom aggregation
Muizenberg during a diatom aggregation

Sources

Most of the original scientific study on surf zone diatoms in South Africa was done by a group of researchers (primarily M Talbot, Eileen Campbell and Guy Bate) from the University of Port Elizabeth, working at the Sundays River Beach in the Eastern Cape. I did quite a bit of reading to research this post, but you can start with this paper for a description of the topographical characteristics of beaches where surf zone diatoms accumulate. The first few chapters of this Masters thesis also provide a good overall survey of what is known about surf zone diatoms.

Putting knowledge into practice

Not every instance of brown, foamy water at the beach will be an Anaulus accumulation. On the west coast of South Africa, for example, there are no beaches where Anaulus occurs, but you may see brownish foam that is the result of heavy wave action frothing up organic matter in the surf (nothing sinister – there is a lot of organic material in the ocean). A clue to help you distinguish diatom accumulations from other brown-water phenomena – apart from running through the checklist of required beach characteristics above – is that an Anaulus accumulation doesn’t stretch much beyond the back of the surf zone. If the brown water stretches beyond the furthest row of waves, it’s probably something else. (And this seems like an apposite time to remind you that sewage looks whitish-grey, not brown, when it’s pumped out into the ocean.)

The number of beaches worldwide where surf zone diatom accumulations occur is so small – less than 100 – that Odebrecht et al could enumerate them in a 2013 paper. I hope this helps to convince you that the brown water at Muizenberg beach (and beyond) is something special and interesting, not to be feared. Go surfing!

Hunting rockcod and moray eel in Sodwana

This is a really cool bit of behaviour that I filmed on a dive to Pinnacles on Two Mile reef while we were in Sodwana last September, and one of my favourite things of all that we saw. A malabar rockcod (Epinephelus malabaricus) – as identified by our dive guides – in dark hunting colours, patrols the reef ahead of a honeycomb moray (Gymnothorax favagineus). At the time I didn’t know what was happening – it looked as though the eel was stalking the grouper – but it turns out to be more complicated and more interesting than that. I had filmed a subset of a total pattern of behaviour, in which the moray and rockcod (from the family of fish also called grouper) were hunting co-operatively.

A researcher at a Swiss university discovered in 2006 that coral groupers seek out giant moray eels (both of these species live in the Red Sea), summoning the eels from their dens with a vigorous shaking of their bodies. The fish and the eel then swim together looking for prey , a deadly tag-team of hunters. The groupers are fast in open water, but the eel can get into crevices to flush out prey. It is this behaviour, executed by a different type of eel and a different type of grouper, that I saw in Sodwana.

The scientists reported that the groupers use a head-stand signal, combined with a shaking of their bodies, to indicate the location of hidden prey to the eels. When the eels see this, most of them swim towards the grouper, and flushed out the prey.

You can read more about the study that revealed the extent of this behaviour here, and the actual paper reporting the research here. The scientists also discovered two other species with complementary skills that hunt co-operatively, on the Great Barrier Reef this time: the coral trout, and octopus.

Moray eels look incredible when they swim freely across the reef. Here’s one doing just that, in the Red Sea.