Diving Thoughts

Researchers have captured video footage of thresher sharks using their tails to swat smaller fish. This seems to be part of their hunting strategy. It might also explain why thresher sharks are often caught by their tails on long-lines.

Thresher shark tails can be up to half the length of the shark and until now there has been no documented evidence of why they had evolved to such a size. Recently a team from the Pfleger Institute of Environmental Research in California and the University of Massachusetts in Dartmouth towed 2 baited lines behind a boat. Over 27 separate days they filmed 33 common thresher sharks near the bait. 14 of these sharks attempted to hit the bait, with a success rate of 65%. This seems to confirm that the sharks use their tails to stun their prey, making it easier to catch.

So far this has only been observed in common thresher sharks, but it seems likely that the other two species of threshers use a similar strategy.

There is an article and some video from the BBC’s website.

I’ve been lucky enough to dive with them several times down at Malapascua with Trevor and Andreas of Thresher Shark Divers, from whose website I borrowed the photos above.

A sad picture of what must have been a hugely impressive hammerhead shark. It was apparently caught off New South Wales after attempting to eat a smaller shark that had been hooked earlier. Unfortunately the accompanying article from news.com.au doesn’t mention what type of fishing boat it was or what they were fishing for.

The hammerhead apparently weighed 1200kg and was 5 metres long. Experts estimate that it was 40 years old.

Self-styled shark hunter, Vic Hislop, has bought it and has yet to decide whether to exhibit it at his Hervey Bay facility. During the mid-90s after several shark attacks in Hong Kong, he was brought up here for his expertise. I’m not sure it achieved anything more than garnering him some publicity.

Anyway it’s very sad to see this hammerhead hanging up on display rather than swimming around where it belongs.

You can read the full article here.

New Zealand’s National Institute of Water and Atmospheric  Research (NIWA), has been tagging great white sharks around Stewart and Chatham islands for the last 5 years. So far they have tagged 25 with tags that stay on for 6 – 9 months before floating to the surface. The biggest shark they have tagged is nicknamed “Shack” and is a 4.8 metre long. Shack has recently set a world record for the deepest known dive by a great white, of 1200 metres.

NIWA’s research has shown that between April and September the sharks are moving from New Zealand to warmer waters during the winter. One shark was found to have spent 6 months in Norfolk island, which is in the Pacific, north of New Zealand.

The research aims to improve our knowledge of the sharks’ behaviour and habits, which may help to prevent them being accidentally killed by the fishing industry. Although with the price of shark fin being what it is, I suspect it won’t help them being deliberately killed by the shark fin industry.

More on this in the New Zealand Herald.

This is a photo from the Guardian’s website. It is the first of a sequence of 5 of a great white shark investigating three divers in a cage in Mexico. Unfortunately they seem more interested in emphasising that one of the divers is “fending” off the shark, and punching it on the nose, when his hand doesn’t look as though it’s anywhere near its nose. Having said that his hand is definitely touching the shark. The second diver looks as though he would have got some close-up shots too. Meanwhile their mate is bravely hiding behind them both, at back of the cage – just where I would have been!

Anyway it’s quite a good sequence of photos in lovely clear blue water. You can see them on the Guardian’s website.

Scientists have found the fossilized remains of a 10 metre long shark in Kansas. They found a jawbone and huge tooth plates, which they believe it used to crush large shellfish like giant clams. The shark is called Ptychodus mortoni, and sounds as though it is the largest shellfish eating animal ever found.

They only have fragments of the jaw but have estimated that the whole jaw would have been 1 metre long, suggesting that the shark was around 10 metres long. It’s also difficult to tell what the shark would have looked like, but scientists are speculating that it would have been shaped like a modern-day nurse shark such as the one shown here. However it would have behaved more like a sluggish bottom-dweller.

Other remains have been found before, but none that suggested that the shark could grow this big.

The photos were taken from the BBC’s website, where you can read more.

An 18 foot whale shark was found last week in Bahay Kambing, a cove in the municipality of Tingloy, by some divers, including several from Hong Kong. Its dorsal and pectoral fins had all been cut off and there were rope marks on its tail. It was towed to Caban cove which was calmer and volunteers tried to help it. Unfortunately it died in the night from its injuries.

Having been fortunate enough to see a similarly sized whale shark in Thailand, this is very sad.

Whale sharks are classified as vulnerable by the International Union for Conservation of Nature (IUCN), and they are protected under Philippine law. Unfortunately that doesn’t seem to have made much difference in this case.

The photo above was taken from WWF-Philippines website, where they have a long article on this.

Here’s another photo and a link to an article, this time from the Philippine Star’s website.

I’ve also seen a number of photos on Facebook, so the incident is becoming more widely reported worldwide.

There’s also a thread on Scubaboard’s forum.

According to a couple of the articles I read, “locals” have suggested that the whale shark may have become entangled in a fishing net, and had its fins off in order to recover the net! Several years ago in Hong Kong during a shallow night dive a bunch of squid fishermen turned up. Since lights attract the squid, they no doubt concluded that underwater dive lights would be even better at attracting them, so they dropped nets around 3 pairs of divers. It’s a good job they didn’t apply the same logic as these Philippine “locals”, otherwise we’d have got 6 divers back with no arms and legs left.

Presumably if these “locals” are to be believed and saving the nets were the priority, the fins would have been discarded and no-one would dream of selling them for USD 800 per kilo. And I’m not convinced that under Philippine law that it makes any difference why you kill a protected animal, it is still illegal. I don’t think I’m alone in that view as apparently WWF-Philippines and the Bureau of Fisheries and Aquatic Resources (BFAR) have announced a reward of P100,000 to anyone who can provide information leading to the arrest of the people involved.

Two cousins, diving at the Twins near Whitiangi in the North Island of New Zealand, were hassled by a 2 metre bronze whaler on Monday (15 Feb) afternoon. They were about 10 metres from their boat when the shark approached them in what seemed an aggressive manner. As it swam at them for a second time, they threw an old anchor that they had picked up at it. This deterred it briefly, but they then defended themselves with a cray hook, before one of them kicked it in the head with his fin, giving them time to get back on the boat.

The cousins had hung a couple of freshly caught kingfish on the side of their boat, which may well have got the shark’s attention. Clinton Duffy of the Department of Conservation said that the bronze whaler is a large shark, but has quite small teeth as it generally feeds on school fish. They are generally not aggressive but can become excited when there is blood or dead fish in the water.

The above photo is from the New Zealand Herald, and you can read more details here.

In an Auckland aquarium visitors spotted four baby sharks spilling out of a wound in the side of  a pregnant female. The wound was caused by a bite from another shark. Staff removed the female and the four baby sharks, and found four more still inside the mother. All eight of these have now been placed in a nursery tank with some juvenile eagle rays, where people can see them for a while until they are released into the wild. The mother also survived.

It is not unusual for sharks to take chunks out of each other, but this particular bite must have been in just the right place to let the babies out without harming them, and at just the right time in that they were old enough to survive. Bizarrely it may also have saved the young sharks lives, as if they had been born naturally, most likely at night, staff would not have known and they would probably have been eaten by the other sharks and rays in the aquarium.

The above photo comes from the New Zealand Herald, and you can find out more from their website.

The International Union for Conservation of Nature (IUCN) has listed 64 species on their first ever red list for oceanic sharks. These include hammerheads, giant devil rays and porbeagles. Overfishing is getting the blame, and it is particularly problematic for sharks which produce few young and take many years to mature.

Scalloped hammerheads have been categorised as globally endangered. In some parts of the world their numbers have declined by 99% over the last 30 years.

Conservationists are calling for limits to catches and enforcement of strict bans on finning, where a shark’s fins are chopped off before the shark is thrown back to die.

There is a lot more information in an article on the Guardian’s website.

The BBC have an article on the strategies that great whites use when hunting seals. The sharks identify a location, an anchor point, and then search the surrounding area for prey. Researchers found that this anchor point did not necessarily offer the best chance of intercepting prey, but did provide the optimal balance between finding it and catching it.

Apparently this suggests a premeditated hunting strategy akin to that used by human serial killers. The team of researchers did consult a specialist in geographically profiling criminal acts such as those committed by terrorists and serial killers.  Such profiling is used to identify the criminals’ anchor points. But the researchers were using the profiling to analyse the attack patterns and identify location points, not to suggest that sharks are serial killers.

Personally I think the BBC have just taken the opportunity to play up the “sharks are serial killers” angle. In fact when I originally spotted the article yesterday the article title said something like “sharks use same tactics as serial killers” (I don’t remember the exact wording). It’s good to see they’ve now changed it to something less dramatic and more accurate. Sadly Yahoo (“Great white sharks hunt just like Hannibal Lecter“), Bloomberg (“Great White Sharks Said to Hunt Like Serial Killers“) and the LA Daily News (“Great white sharks: serial killers of the sea“), among others, have stayed with exaggeration and hyperbole.

As far as I can what the sharks are doing is adopting a sensible hunting strategy, to wait where you’ve got the best chance of catching what you want. Yes, you could argue that is the same thing that serial killers do. But it’s also what most of us do on a daily basis. If I want to catch a bus, I generally find that the best strategy is to wait at a bus stop. If I want to catch a train or an MTR, then waiting at a station tends to give best results. But sadly headlines such as “Great White Sharks use Same Strategy as Commuters” is presumably not sensational enough for journalists and editors. Instead they take the easy option and continue to villify sharks.

The above photo comes from Neil Hammerschlag and is shown on the BBC website, where you can read the article.

In addition to the basking shark video, the BBC has a few others that are worth a look. Specifically:

• Shark tagging off the west coast of Scotland
• Rare prickly shark captured off California. This is only the second time one has been captured. The video was shot during the shark’s 15 hour stay in Monterey Aquarium. It has now been tagged and released.
• Testing the great white shark’s bite.
• Tagging whale sharks in the Maldives.

The BBC website has a short video from Dan Burton of basking sharks arriving off Cornwall as part of their annual migration. Even though hundreds are now arriving, they are still pretty difficult to spot.

Like their larger cousins, whale sharks, these 7 ton giants are plankton feeders. It is now believed that they come up to the UK to breed.

Go to the BBC website and take a look.