A great white shark charges forward.
My only encounters with a great white were on a cage diving trip, many years ago. Two of us at a time would enter the cage and wait, cameras poised. This was around Isla Guadalupe, 250km west of Baja Peninsula, Mexico. The water was around 30metres deep and the water clarity stunning. As we stood in the cage, breathing through hookah (surface-supplied) diving regulators, heavily weighted to keep us firmly planted, I would get in to a rhythm, look left… look right…. look left…look right, gently swinging my head and shoulders. Shoals of yellowfin tuna would constantly glide past, so there was always something to see. Occasionally a massive great white would cruise in to view, and we would both try and position ourselves to get the perfect shot. That’s where my problems began. The guy I was normally paired up with – a great guy who’s name I’m afraid I now forget – was easily 6’5″. Those who know me know I am not especially tall. No matter which way I turned it seemed like his arms and camera were stretching out in front of me, always edging in to the frame of my pictures. Time and again my shots included a forearm or a big, meaty fist and camera in front of the shark. My frustration grew and silently I seethed! I watched the great whites slowly cruise and then gently turn and wheel. If only I could get outside this (expletive deleted) cage and get a couple of clean shots. This thought went around and around in my head. As the days past the novelty of seeing great whites close up, and standing still in cool water for two hours at a stretch, faded a little for some. This meant that those of us – the ones who were there to try and get the shot – were able to spread out a little. So in the latter part of the trip I would often be in a cage on my own. But still I was not satisfied. Those damn cage bars! When a shark cruised past, checking me out, and I would track its path – only to have a couple of cage bars creep into the edge of the picture. So once again, standing in the cage looking left, looking right, looking left …. staring out in to the incredibly clear blue water for maybe an hour, and seeing nothing. If only I could have gotten out of the cage, to have a little look around with no obstructions. Look left, look right, look le… and there – directly in front of me, maybe two metres away, from out of nowhere, was a massive great white. Suddenly I lost all interest in getting out of the cage. Seconds previously I had looked in that direction and seen nothing, then suddenly this enormous fish was right there – in my face. I could not believe it had moved so fast. Great whites are famous for their vertical attacks on fur seals, where they swim directly up, slamming in to the intended victim which such speed and power that both predator and prey breach clear of the water. This got me thinking recently, just how fast are these sharks, and how do they achieve such speeds?
When we think of sharks tend to think of sleek, powerful predators that appear to cruise effortlessly, but are capable of dazzling bursts of speed when they attack prey. This image of the shark is exemplified by the shortfin mako (Isurus oxyrinchus). We know makos are fast,they are often described as the fastest of all sharks, but how fast? Reliable measurements of swimming speeds of large fish are notoriously difficult to achieve; most scientific data out there are estimates based on things such as tail beat frequency, white muscle contraction speeds and sometimes measurements of speed of juveniles under test conditions. Trouble is, we known maximum speed tends to increase with overall length but white muscle twitch speed decreases with size, so extrapolations are full of caveats. Look online and you will find incredible speeds ascribed to makos, often on reputable websites; 60mph is cited on some pages, 74km per hour on many others. Trouble is, finding out where these figures actually come from is pretty damn tricky. Wikipedia until recently quoted 74kmph as the maximum speed of makos; so I kinda suspect that many article authors in a hurry simply googled and went to Wikipedia (ahh, we’d all done it). In the current Wikipedia version (2020/06/06) the figure is revised down to 68km per hour (42mph). Unfortunately, the referenced paper for this figure (Graham, et al., 1990) doesn’t mention this speed, or indeed any maximum speed, for full grown makos (sorry Wikipedia, you are terrific most of the time). Other pages cite anecdotal accounts of makos, hooked by fishermen, covering 30m in 2 seconds, but … we all know about fishermen’s tales. So the bottom line is, we don’t really have much in the way of solid data on how fast they can swim, but they are pretty damn fast.
A Mako shark make a half-hearted attempt to grab a cape petrel.
So let’s look a bit more widely at the problem. It is generally believed the fastest of all fish in the ocean are billfish (sailfish, swordfish etc.). Esteemed organisations such NOAA (The U.S. National Oceanic and Atmospheric Administration) on their Ocean Facts web page, state that the sailfish (Istiophorus platypterus) reaches a blistering 70mph (113kmph) (https://oceanservice.noaa.gov/facts/fastest-fish.html accessed 7th June 2020). However, a 2015 study lead by Stefano Marras, from the Institute for Marine and Coastal Environment, Oristano, Italy, and Takuji Noda, Kyoto University, Japan, suggested the truth was a little more sedade. Using high speed videography and data loggers with built in accelerometers attached to the fish (rather than the adrenaline-fuelled anecdotes of big-game fishermen) they found that the maximum speeds burst speeds recorded by sailfish chasing prey were between 8.19 and 9.77 metres per second (that’s 29.5 – 35kmph), with average burst speed between 19 and 26kmph (these ranges for top and mean speeds reflect differences between the two recording methods).
As it turns out, a second study the following year, also looking at sailfish maximum speeds, but using very different techniques, produced very similar results. Morten Svendsen and others (Svendsen et al 2016) looked at four different species of fish, all noted for their speed, including sailfish. The limiting factors, they determined, are maximum tail speed movement based on muscle contraction values, and bubble cavitation. Bubble cavitation is essentially the water boiling as pressure drops. Physics tells us that the temperature at which the vaporisation point of a body of liquid is reached varies as the pressure varies. This is why, were an astronaut take a glass of water on a spacewalk, the water would instantly vaporise because there is almost no pressure in space (it does not, unfortunately, tell us why he would do something as pointless as that). It is also why mountaineers cannot make a decent cup of tea at altitude, because at the lowered atmospheric pressure water will boil at less than 100 degrees C, which as any tea drinker will tell you is far from ideal. Bubble cavitation is a phenomenon well known to small powerboat drivers. As a propeller turns it creates dramatic pressure drops behind the blades. This, in turn, causes microbubbles to form (the plume of bubbles one can see behind the prop of a speedboat) as some of the water vaporises. As these bubbles hit higher pressure water they collapse, often violently, creating pressure waves. These pressure waves will, over time, destroy the material of the propeller. So what has all this got to do with fish swimming? Well exactly the same physics apply. Between 10-15 metres per second (36-45kmph) significant cavitation will start to occur around the tailfin of fast swimming fish or marine mammals, but pain is likely to set in a little below that speed. Interestingly tuna, another fast swimming group of fish, have bony tails without nerve endings. It is therefore possible they may be able to exceed the speed threshold that other fish species cannot, and in fact lesions have been on the tails of tuna, which it is thought might be cause by bubble cavitation. Bubble cavitation lessens at depth (as temperature drops and pressure increases) but maximum speed will still be limited by muscle contraction speed and ‘tail stall’ when the pressure differential is too great.
So where does this leave us with mako sharks? Well, a reasonable assumption is that their top speed is probably only slightly slower than that of sailfish. So that puts them just under 30kmph mark. That may not sound quite so exciting, but it’s still almost four times faster than a top Olympic swimmer. The French champion swimmer Frédérick Bousquet set a 50 metre dash world record in 2009, with an average speed of 8.6km per hour.
Now swimming fast requires a lot of energy; and it requires muscles to move fast. That’s pretty self-evident. But we know that cold blooded (ectothermic) animals can’t move fast when their muscles are cold, because the chemical reactions, e.g. the production and utilisation of Adenosine triphosphate (ATP) for muscle contraction, occur more slowly. That’s why no reptiles are active during winter months in temperate regions, and in summer lizards will bask in the morning sun to warm up before becoming active. But you can’t do that underwater, the sea temperature does not heat up daily, nor sunlight’s warmth penetrate beneath the waves. And, as Herman Melville wrote ‘Of all nature’s animated kingdoms, fish are the most unchristian, inhospitable, heartless, and cold-blooded of creatures‘. So how do these ultra-fast fish species overcome this problem? Well we now know that quite a few different fish species have evolved their own way to become warm-blooded, after a fashion. Shortfin mako, porbeagle, salmon and great white sharks have all been found to be capable of maintaining their body temperature several degrees above that of the surrounding water. In 1969, Francis Carey and Jim Teal, of Woods Hole Institute, published a paper showing that porbeagles and makos were able to maintain their body temperature 7-10 degrees Centigrade above the surrounding water. They are able to do this through a mechanism known as the rete mirabilia (from Latin, meaning wonderful net). We now know that not only these shark species, but several other fast swimming species such as tuna, all have this mechanism for raising body temperature. A rete mirabilia is essential a network of arteries and veins lying close to one another and acting as a countercurrent exchange system. In these species of shark, large powerful red muscle generate heat deep within the body as they swim. The rete mirabilia surrounds these muscles, with many side branches looping down into the muscle, and heat is transferred. Bands of alternate arteries and veins transfer heat, which is carried to the white ‘fast twitch’ muscle. This ability to warm the body, or parts of the body, above that of the surrounding water temperature probably serves multiple purposes. For salmon sharks, porbeagles and great whites, it probably helps them to function and hunt in chilly waters; porbeagles occur off Northern Norway; great whites congregate around Fiordland and Stewart Island, Southern New Zealand. Having been snorkelling in both areas without a wetsuit I can tell you that after 15 minutes I was barely functioning and only just able to pull myself back in to the boat. Recent studies have shown that blue marlin, swordfish and makos and porbeagles, heat the blood supply to their eyes and brains. This has been demonstrated to dramatically improve the response of their retinas to light stimuli, and so probably improves their visual acuity for hunting at depth.
I’ve yet to encounter a living porbeagle at sea. Despite ranging widely around the western shores of the British Isles, they are essentially an offshore, deeper water species, and so rarely encountered by divers. The closest I have come was working, many years ago, on a Cornish fishing vessel. This was a Newlyn Netter, setting bottom nets for highly prized hake around deep wrecks in the middle of the Irish Sea. In two teams the crew would work around the clock; travelling is a great circle we would set nets for twelve hours then, arriving at our start wreck, haul nets for twelve hours. Hauling nets at night, we pulled up porbeagle, unfortunately. This happened years ago but, unfortunately, accidental bycatch remains one of the biggest threats to porbeagles. Broad and powerfully built, they look much like small great whites, although they are predominantly fish eaters and not considered dangerous. Porbeagles are thought to be second only to the closely related salmon shark in terms of thermoregulation. This probably explains their ability to tolerate colder water better than most shark species. A recent study off Nova Scotia found that most were caught at depths where the water temperature was only 5-10 deg. C. That’s a colder preferred range than pretty much any other highly active shark species.
Porbeagle shark, Lamna nasus, caught as bycatch, on the deck of a fishing vessel, Irish Sea, UK.
My first encounter with a mako was an entirely unexpected one, along the East Coast of New Zealand, off Kaikoura (this story is told in more detail on my photography blog here). Kaikoura, is a small town on the north east coast of New Zealand’s South Island. It is famous as one of the best places in the World for whale watching, especially sperm whales. Kaikoura may be most famous for whales, but it is also a fantastic place to see many species of albatross up close. Whilst the big whales grab most of the international headlines, the sheer drama of seeing several species of albatross up close – really close – soaring, wheeling and plunging down to feed, is pretty hard to beat. Once well out to sea, the water was chummed to bring the albatross in. however, it not just the albatrosses and giant petrels that noticed the food in the water. The scent of chum attracted in predators from below. A dark triangular fin broke the surface and began weaving through the wary seabirds. The shark was a juvenile mako, approximately 5-6ft (1.5-1.8m) long. Whilst clearly drawn towards us by the fish scraps in the water, it then became interested in the birds splashing around.
The great albatrosses eyed the shark with a mixture of wariness and belligerence; with a wingspan probably exceeding the length of the shark they may have seemed a little large to tackle. The smaller petrels were more anxious. It made a grab for one cape petrel that did not move out of its path fast enough, but the attack seemed have hearted and the petrel skittered away easily enough. There was probably enough fish remains floating in the water to keep the shark happy. Makos will occasionally take seabirds, but mostly feed on pelagic fish species such as mackerel, herring and anchovies. Larger individuals have been found to have young seals and even common dolphins in their stomachs, as well as billfish such as marlin. Common dolphins and marlin are both renowned for their speed, so whilst it is possible that these were injured individuals snapped up by the mako, it is also these fell prey to the makos lightning speed. I have yet to get in the water with a mako; however, should I be lucky enough to find myself snorkelling or diving with one, I’m not going to try and outswim it.
So to finish off, lots return to the beginning. I still haven’t answered the question about great whites. All other things being equal, absolute speed tends to increase with overall length. So how do the much larger great whites compare with makos? When they turn the power on, and come swim vertically up, what speed is a 2.5 tonne shark doing when it bursts clear of the water’s surface like a cruise missile? A 2011 paper by R. Aiden Martin and Neil Hammerschlag looked at exactly that. They were interested in the predator prey relationship between great whites and cape fur seals (Arctocephalus pusillus pusillus). Great whites use sheer speed and surprise to kill or incapacitate fur seals. If they are unsuccessful in the first attack, the greater maneuverability of the fur seal favours them. Using towed bouys to entice sharks to strike, they then analysed video footed to determine exit speed. One such attack was calculated at 35kmph. Analysis of underwater footage of actual attacks on seals suggests even faster speeds, (though the margin for error is probably greater). So on that basis, the great white shades out the mako, maybe even the sailfish. But, until even is compared using similar methodologies, the jury is still out.