Lecture: Mark Meekan on whale sharks

The Save Our Seas Shark Centre in Kalk Bay has been a hotbed of activity in recent weeks, with a double-whammy of shark-related talks in one week. The “brains trust” of Save Our Seas about – scientists Sarah Fowler, Alison Kock and Mark Meekan, as well as co-CEOs Peter Verhoog and Georgina Wiersma – were all in town and Sarah Fowler and Mark Meekan gave talks on their respective areas of interest. You can find a report on Sarah Fowler’s talk here.

According to Christopher Neff, who introduced Mark Meekan, Dr Meekan describes himself as a “fish biologist”. The rest of the world knows him as the world’s foremost whale shark researcher. He is a researcher at the Australian Institute of Marine Science, on the research team of the ARC Centre of Excellence for Coral Reef Studies. He is a scientist at Save Our Seas and also a member of the National Shark Recovery Group convened by the Australian government department of the environment.

Dr Meekan is involved in several research projects, but the one he spoke about at Save Our Seas was the natural history of whale sharks, and his study of the whale sharks at Ningaloo Reef in western Australia.

Natural history

The whale shark, Rhincodon typus, is the largest fish in the sea, and one of three filter feeding sharks, the others being the megamouth and the basking shark. Basking sharks frequent cooler, temperate waters, whereas whale sharks are commonly seen in tropical oceans. The megamouth is a very rare, deep water species. The Mozambique channel is a hotspot for whale sharks, and they are seasonally seen of KwaZulu-Natal and at Mozambique dive sites. The sharks are able to be identified using the pattern of spots and stripes on their bodies.

Whale sharks have about 3,000 tiny teeth, but feed using a method called ram filter feeding – they swim forwards with their mouths gaping open, or gulp at aggregations of food. Water entering the mouth passes over the gills, allowing them to breathe, and a bolus of food forms which they swallow all in one go. Sometimes their gill rakers get clogged when they are swimming through a particularly dense food aggregation, and the sharks cough in order to expel the excess food. Tony and a group of divers witnessed this once in Ponta do Ouro, Mozambique, and he says that one lady was convinced that the brownish-red substance coming out of the whale shark’s mouth as it coughed was microscopic remains of a scuba diver that the shark had just eaten. The substance was, in fact, plankton and not scuba diver mincemeat, and a whale shark couldn’t eat a diver if he tried!

The first identified whale shark was seen in Cape Town in 1828, after being harpooned in Table Bay. Until 1988 there had only been a total of about 35 sightings of these massive fish (they can grow to up to 20 metres in length), but since then the discovery of various locations (such as off South Africa’s east coast, Belize, Honduras, Ningaloo Reef off Australia, Madagascar, Mozambique, Mexico and the Galapagos Islands), where they gather to breed and feed has occurred, and sightings – and our scientific knowledge of the creature – have ballooned.

Whale sharks have a varied diet, including coral spawn, copepods, sardines, squid, anchovies, mackerel, tuna, albacore, and spawning aggregations of red crabs (such as the one that occurs at Christmas Island), snapper and lantern fish. This video of a whale shark being fed at Okinawa Aquarium shows their gulping behaviour; a bag of plankton is being released at the surface for the shark to eat.

[youtube=http://www.youtube.com/watch?v=BrrmpHWFaEc]

Reproduction

Only one pregnant female has ever been examined, after being fished in the late 1980s. She had (read this carefully – it’s mind boggling!) 300 pups in her uterus, of various ages. Subsequent genetic studies performed on fifteen of the pups (in 2010 – the pups had been frozen for storage in a solid block of ice referred to by Dr Meekan as “the pupsicle”) indicated that they all had the same father, despite being of various sizes. This indicates that female whale sharks are able to store sperm after a fertilisation event, and exhibit aplacental viviparity (apparently a replacement term for what I learned at school, ovovivipary), meaning that they deliver live young. Massive, pregnant females are apparently often seen in the aggregations that occur in the Galapagos Islands.

Whale shark pups have been seen off Pakistan, in markets in the Philippines, and off the Azores. It is presumed that these sharks deliver their young in productive (green! like our local seas in the Cape) waters. When the sharks reach 2-3 metres they head to one of the inshore feeding aggregations mentioned above. Once they’ve grown some more, they simply vanish… into the open ocean. Very large specimens are almost never seen.

Threats

Whale shark meat sells for US$13 per kilogram in Taiwan, which means that a 36 tonne shark would be worth about US$200,000. They are known as the “tofu fish” because of the texture of their meat. There is a picture here (scroll down) of a cross-section of a whale shark that shows how dense its muscle is. Dr Meekan showed us this image and pointed out how little of its muscle is dark red (think of a tuna fish), fast twitch muscle. The top speed of a whale shark is only about 5 kilometres per hour.

Whale sharks are protected in the Maldives (1995), the Philippines (1998), Honduras (1999), Belize (2000), Australia (2001), India (2001), and by CITES. Not yet in South Africa. Whale shark fins are used as signage – giant banners or displays – outside Asian restaurants that serve shark fin soup. The organisation Traffic got whale shark fishing stopped in Taiwan (I may have misheard this – it sounds unlikely given east Asian predilections for eating fish regardless of its conservation status, and I can’t verify it), and the Indian whale shark fishery has also been shut down. Many other fisheries remain, however, in small coastal villages such as the ones on Pamilacal Island, where villagers exploit the seasonal whale shark migrations. Dr Meekan believes that with a bit of effort and education, these fisheries can be replaced by profitable (to the villagers) eco-tourism ventures, which will value the whale sharks while they are alive.

The Ningaloo project

The whale sharks observed at Ningaloo – a population of about 500 – are mostly (80%) juvenile males about 5-7 metres long. 10% are females, and the remaining fraction can’t be identified quickly enough (one has to get underneath the shark to tell if it’s male or female). There is a thriving eco-tourist business that charges visitors about $350 for a swim with the whale sharks. A highly significant decline in the size of the sharks observed by tourist boats (from an average of 7 metres in 1995 to a current 5.5 metres) has been observed, which seems to indicate that larger individuals are still being fished elsewhere.

The research team at Ningaloo have used pop-up archival tags (like the ones used locally on bull sharks) to track the movements of the local whale sharks. These tags, which cost about $5000 each, measure light intensity, temperature, depth and other data. They have an onboard clock, and a timed burn wire causes the tag to pop free of the shark after a predetermined time. When it surfaces, it transmits its location to a satellite. The rest of the data on the tag must be recovered directly from the tag, so the team must get hold of the tag in order to download it. Using the light and time data, clever software is able to determine where in the world the shark was when it was on the surface.

The other kind of tag used is called a splash tag, which broadcasts its location to a satellite every time the shark surfaces. Using a light plane to spot the sharks, and a small boat, about 30 sharks have been tagged. From Ningaloo the sharks move in one of three directions: towards Sri Lanka (Indian Ocean), into south east Asia, or towards Indonesia. During the day the sharks frequently dive to upwards of 400 metres, but spend a lot of time on the surface. In the evening they spend a lot of time at about 100 metres, probably feeding on the plankton that migrates upwards in the water column at night.

There are many mysteries, which always delight (and frustrate) me. One is the staggered surfacing technique that some of the whales display on dives. They descend to several hundred metres’ depth, but then ascend some distance, descend again a little bit, and repeat the process, giving a jagged dive profile that would give a divemaster a serious headache!

The largest creatures on earth – whales, whale sharks, basking sharks (second largest fish) eat the smallest prey. How do they survive? One of the questions that the Ningaloo researchers seek to answer is to explain this apparent paradox, with reference to whale sharks. Survival for them is an acute problem. They live in oligotrophic waters – clean, clear water with little nourishment available. Their metabolic rate is regulated by the temperature of the water they are in, and since they’re in warm seas most of the time, it’s high – meaning they need to eat a lot. Their ram filtration and gulping method of feeding uses a lot of energy.

With this question in mind, the researchers have used a bio-logging approach, with a video/data recorder that is attached to the whale shark and pops off after 24 hours. They found that the whale shark spends up to 20% of its time gliding, compared with active swimming (i.e. using the momentum from prior fin strokes). Its dives are asymmetric, with steep ascents and lazy, slow descents that allow it more time to search for food and consume it while expending very little energy. They also conserve energy by swimming at a slow, constant rate rather than executing many sharp, sudden changes of velocity.

Here’s a news story on Dr Meekan and his research.

[youtube=http://www.youtube.com/watch?v=AZi509qt0R0&w=540]

Dr Meekan hopes that the findings at Ningaloo and elsewhere will indicate a resident population of whale sharks. He is not very positive about the chances of the species should they migrate anywhere near the eastern or western parts of the African continent, and a resident population that doesn’t move about too much on the coast of a first world country would improve their chances of survival. I hope that South Africa is soon able to formally institute measures to protect whale sharks, as an example to the rest of the continent.