Friday, July 18, 2014

Scrummy for Scromboids: How Do Great White Sharks Catch Tuna Anyways?

Well all righty then todays post might seem a bit all over the place but bear with me there is a valuable point to be made out of all of it.

Monterey Bay Aquarium. tuna & great white

I have had a bit of a predilection with all things aquatic lately as you may have noticed - plesiosaurs, pliosaurs, ammonites, and all things gishy, gushy, and squidy - have been on my mind of late. And here in the states it is summer and that means one thing and one thing only - SHARKS!! An immature great white shark has already bit a swimmer who blundered into it as the shark was trying to free itself from a fisherman's hook in southern California. And Discovery Channel is set to break new highs (lows?) in shark carnage in their annual Shark Week marathon. The ambassador to sharkdom, great white sharks, have a bit in common with T- rex in that the attention and coverage both animals receive is a bit fantastical and overshadows other shark/theropod species in popular media. That is not to say I personally have or will ever tire of learning new insights into both species. So I say hogwash to notions of overexposure as both species are stupendously cool and actually quite unique in the evolutionary framework of their respective pedigrees. But other types do need more attention and to let you know now, I will be talking about great whites in terms of one specific aspect of their predatory behavior that is little explored. But to set the stage I need to talk about something seemingly completely incongruous with great white sharks - spotted hyena predation.

The book above was one of the foundational pieces of natural history literature that really captured my attention as a youngster. I recently became reacquainted with the book. At the research institution where I work at doing data entry, The Western Foundation of Vertebrate Zoology (despite the name the museum  specializes in birds eggs), was cleaning out old books one of which was Innocent Killers. So I quickly scooped up the free copy of this brilliant text only to discover that it was a copy signed by author Jane Goodall . And yes that is the Jane Goodall I am talking about - the Jane Goodall best known for her work with primates. Less well known is that for several years she worked with wildlife photographer Hugo Van Lawick documenting the social and predatory ecologies of spotted hyenas, wild dogs, and jackals in east Africa. While common knowledge now what Jane revealed concerning spotted hyenas was an intensely social, intelligent, and capable predator that operated largely under the cover of darkness. This data went contrary to previous assertions of spotted hyenas as consummate scavengers - characterizations due to diurnal observations and entrenched negative cultural views of hyena. I know the decline in the quality of natural history documentaries has been well noted (mermaids, Megalodon) but the Nat Geo channel has produced a very interesting film entitled Night Stalkers - Hyena Gangs which is on youtube check it out.

The take home message is that a significant misunderstanding of spotted hyena predatory behavior and ecology went unchecked right up until the 1970's when actual long term field studies, including nocturnal observations, were made. And this is for a particularly well known large predator that lives in open habitats.

Check your privilege, mate

With this example in mind what might we be missing in terms of behavior for great white sharks at night? The challenges of documenting the behavior of this massive, complicated predator are even more  compounded by its aquatic realm - where even during the day turbidity and lack of vision are par for the course. And this is a bias that I think can't be overstated to your typical arm chair naturalist (myself included) - bad visibility, even in tropical waters, is the rule rather than the exception. When you watch undersea documentaries you are seeing the rare, exceptional times where visibility is good. Otherwise tides, currents, plankton blooms, and storms muck up the water a good part of the time - and give predators a decided advantage. Just as hyenas and lions have a decided advantage over their ungulate prey at nightfall.

As a kid in the eighties I watched every documentary I could find on great white sharks in hopes of getting a glimpse of their predatory behavior. But all I saw where rather sedate looking whites gathered around human induced feeding opportunities. I could not figure out how such a languid looking fish captured smart, agile, and quick pinnipeds. Boy was I wrong!! Nat geo captured white shark predatory behavior via stationary underwater cameras in the 90's but it was not until the "air jaws" phenomena off South Africa via Chris Fallows that we really saw unadulterated white shark predatory behavior. What's more not only were white sharks doing acrobatic full breaches, extended chases - they were also hunting this way at night!!

Nat Geo
And I now I want to get to the heart of this post. We have good footage and data on the ways that white sharks go after and capture pinnipeds, even varying their tactics based on the type of pinniped. But how do they capture the other, other white meat? I am talking about those hyper-streamlined, hyper vigilant, super charged, warm blooded super-fish - tuna. Of the genus Thunnus these fish, most notably bluefin, albacore, and yellowfin are super tasty to certain hominids as well as anything else that can get a jump on these most speedy of pescados.

Now I love to quiz fishermen on bits of natural history that they have observed - fisherman's tales notwithstanding they are a treasure trove of knowledge and experience. Lucky enough my uncle regularly goes on charter trips to catch yellowfin tuna at Guadalupe Island off of Baja California, Mexico. And any great white shark groupie should know that island well, it is a preeminent spot for eastern pacific white sharks. Indeed my uncle testifies the white sharks there are a bit of a nuisance, and snatch hooked tuna so often that they are dubbed "the taxman" (experienced surfers on California refer to white sharks as "men in grey suits" alluding to how commonly they encounter them with no incident spooky still though). A quick perusal of you tube clips verifies my uncles' testimony.

Also check out this vid of what is claimed to be an 18 footer at Guadalupe. Based on the girth that might not be an overestimate.

But when I ask my uncle if a great white can catch a free-swimming yellowtail he is quick to say no... only when they are hooked can the whites get a yellowtail as they are too fast.

At first this makes sense, tuna are very fast - faster than whites and faster than the pinnipeds that whites have been documented to hunt. But then again tuna are always put forth as a food of great whites, you have accounts of great white sharks actively following bluefin tuna, especially in the Mediterranean, or getting into tuna aquaculture pens as recently happened in Mexico where a massive shark was killed by tuna ranchers, and then you have loads of accounts of tunas being pulled out of the stomachs of dead great white sharks. Of special significance are the great white sharks of the Mediterranean, said to follow schools of tuna through the straights of Gibraltar and sometimes even being encountered in the traditional matanza tuna killing festivals. Interestingly the rare Mediterranean white sharks, including some of the largest ever captured, may subsist primarily on tuna - seals not occurring there in any large numbers. Here is a good blog post going deeper into this little known population of white sharks. Additionally the Mediterranean might be from a relict Australian population that got misdirected about half a million years ago.

So how does a great white shark go about hunting a tuna? Well I suspect, at least in the case of bluefin tuna, we are not witnessing the white shark in hunt mode because white sharks may only have the advantage at night. I want to draw your attention to this study: Electroretinographic Analysis of Night Vision in Juvenile Pacific Bluefin Tuna (Thunnus orientalis) published in the Biological Bulletin 2009. Basically, although the bluefin tuna has good vision and is a sight orientated predator, it's vision is geared towards diurnal activities and shows a marked decline for nocturnal vision. So marked in fact that collisions at high speed against underwater pen walls is a significant mortality factor in pen raised bluefin tuna. If this decline in nocturnal vision goes across the whole genus of tuna Thunnus, might this be the small window of opportunity that allows the larger, slower but nocturnally capable white sharks to predate speedier tuna? And hopefully now you see why I started with a discussion of hyena nocturnal habits and how for so long that animal was misinterpreted. It is not hard to imaging how the nocturnal habits of a large, rare oceanic predator might go unrecognized as well.

The 19 foot, 5000 pound female great white has been following the school of bluefin tuna for several weeks now but has not attempted an attack. During the day it stays below and behind the school, sometimes letting the school get several miles ahead of it as it slowly maintains course and conserves energy, living off of its fatty liver. But it is gravid and needs a good feed for its growing pups. Tonight is a new moon and under the cover of darkness the white shark will make its move. A few of the larger fish have become aware of something large following the school for several weeks now and are careful not to swim at the periphery of the school as they have experienced attacks before. But other fish are naive to white sharks. They have  ran into small blues and makos but had outgrown these predators in size and strength. This female white shark is experienced in bluefin hunting and knows that only a certain congruence of factors allows her to make a kill of the speedier tuna. Cloud cover, lack of moonlight, and a bloom of phytoplankton gives her the cover to slip into the school of tuna. Although technically not asleep, they are swimming in a sort of waking dream - like race cars at idle. Amazingly the white shark is able to blend into the school by mimicking the swimming rhythm of the school and evade detection. They may sense another large fish but register it as just another tuna. When the shark picks a victim the dash is barely over a distance of 10 feet, but conditions are so murky that even the great white shark misses its mark on a disabling bite to the tail. With the school now alerted it is like a hundred ferraris going from 0 to 50 mph. But the white shark knows that it can still exploit this chaos and when one startled tuna accidentally rams into another tunas gills, momentarily stunning it, the shark has its meal.


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Unknown said...

Thanks....awesome post - I was wondering about that for a while. The Great White's intelligence seems to be their most evolved predatory weapon...watching them bite through a full size tuna with one bite is truly awe-inspiring. .

BK said...

Another idea may be that they wait until the tuna start hunting, and use the chaos to close in.

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