Be prepared to hear a fascinating piece of natural history about the Mediterranean great white sharks (Carcharodon carcharias). Story is fascinating for multiple reasons. First, great whites are one of the most charismatic, vulnerable, cryptic and misunderstood predator fish species. Second, the story takes place in a quite unexpected location. Altınoluk is a town on the northern Aegean coast of Turkey. It is located to the south of Troy on a legendary sea route known as the Argonaut route where Jason and the Argonauts went on an epic adventure in search of the golden fleece.
Evolutionarily, sharks are very ancient going back to 400 million years. Great whites however have evolved quite recently during the Miocene period at about 20 million years ago. Shark biologists have debated for more than 150 years whether the ancestral origin of the great whites were from the Megalodons. A recent study of a 4 million year old shark fossil from Peru have shown that great whites were more related to mako sharks and didn’t evolve from Megalodon sharks.
As a biologist who studied gene flow hearing about extreme cases have always inspired me. In July 2008 fishermen of Altınoluk contacted marine scientists about two newborn great white sharks they caught. A research team from the Bosphorus and the Istanbul University collaborated with an international team of shark scientists and made a genetic analysis on the tissue taken from the pups. Results published in 2010 have confirmed that sharks were not siblings. When their genetic make-up were compared against a global database it turned out that they were most related to the Australian population. Further genetic analysis also revealed another very striking fact. The Mediterranean population remained isolated for about 450 thousand years. This is indeed an ancient and epic walkabout from an ocean half a world away literally.
On July 6th 2011, fishermen of Altınoluk caught another great white pup and this time it was alive. Before releasing her back to nature they did a great job of documenting this extremely rare and endangered specimen. Biologists have had some curiosity whether certain locations in the Mediterranean may serve as a nursery for great white sharks. Generally for top predators greatest mortality for the young comes from predation. Females may prefer to give birth to their young in places where adult population is minimal so that their pups find refuge. For example the Sicilian channel, near the Italian island of Lampedusa, is one such location where both pregnant females and newly born great whites have been reported. With three pups caught in trammel nets for two consecutive summers Altınoluk must be marked on the map as a nursery for the great white. For instance, Boncuk Bay in southwestern Turkey is a known nursing ground for the sandbar sharks (Carcharhinus plumbeus):
A journey from New Zealand to the eastern Mediterranean? This is literally half a world away. Lets review possible scenarios for how this ancient shark walkabout might have happened. Strait of Gibraltar is the only natural gate through which species can enter to colonize the Mediterranean. As expected Eastern Mediterranean is the last place for an Atlantic species to colonize (but this hypothesis is clearly not true for the great white). There’s no geological evidence that Mediterranean was connected to the Red Sea within the past few hundred thousand years. Therefore we will have to dismiss the Lessepsian colonization (unless there’s a yet undiscovered underground sea tunnel as Jules Verne famously fantasized in Arabian Tunnel chapter of his book 20 Thousand Leagues Under the Seas) from the Red Sea.
The most likely explanation is that a few individuals probably drifted north in the Atlantic from South Africa. At around 400 – 450 thousand years ago sea levels were 20m higher than today. The well-known strong Agulhas currents can do this trick. Once sharks drift that way up the only seaway they could follow resembling their natural eastbound migration towards Australia is through the Gibraltar. The rest is a classic case of population dynamics known as founder effect in which a population starts with a small number of individuals and as a consequence the genetic diversity is rather poor. Ecologically, the Mediterranean is a very low productivity sea. Narrow base of the food web makes it energetically rather difficult for large apex predators to survive. Perhaps in the past they were able to sustain themselves by preying upon the Mediterranean monk seal (Monachus monachus) an endangered species with a restricted population in the Eastern Mediterranean. Therefore if there is a resident population the adult density must be very low. Great white sightings in the Mediterranean are extremely rare.
Individuals tagged by satellite transmitters have shown that adult sharks migrate pretty regularly between Australia and South Africa. Similar long distance migration along east-west axis has been documented by the Monterey Bay Aquarium where a tagged female great white made a return trip between California and Hawaii. This shark was tagged near Point Reyes on November 30th in 2004. She stayed along the California coast for several months and in February went straight to Hawaii, one of the well-known offshore winter/spring destination for great white sharks. She swam in a very straight line at a constant speed of around 100 km per day completing her journey in about a month. From February to June, the shark remained near the Hawaiian Islands regularly diving to 500 m. In June the tag stopped logging due to battery failure. The tag was found by a beachcomber only 40 miles from where it was tagged around Christmas of 2005. Apparently, the tag came loose after the shark had returned to California. A two year satellite telemetry work done by researchers at Marine Science Research Institute has shown that pregnant females travel between the mating area at Guadalupe Island and nursery in Baja California.
Satellite tagging data collected from adult great whites began to reveal a 3 dimensional picture of their movement patterns. We are mostly accustomed to their surface swimming which has become a stereotype for all sharks. In the less deep waters along the coast great whites could be found at any depth. One of those curious patterns is “yo-yo swimming” where individuals have been recorded to make repeated dives and climbs from surface to seabed. They also have been recorded swimming along the ocean floor. They may do “spike dives” often around dawn and dusk. In the open ocean, on the other hand, they may swim on the surface or several hundred meters deep, sometimes diving as deep as 1000 meters.
Despite their low numbers, sharks have an interesting reproductive biology and great whites can maintain a population for a long time. We know almost next to nothing about the reproductive behavior of great whites but a video of nurse sharks may give us an idea (see below). In the absence of males female sharks can make babies by doubling the chromosomes of their eggs through a process known as parthenogenesis. Sharks have XY genetic determination system just like humans. Therefore pups born this way will always be female and will not re-stock the depleted male population.
There has been quite a few attempts to tag juvenile great white. In 2001 Pfleger Institute of Environmental Research managed to attach a tag and collected 27-days-long recording of a baby great white. In 2008 a very detailed report of juvenile great white sharks tagged in Australia was published by CSIRO. Virtually nothing is known about juvenile sharks in other seas including the Mediterranean. Therefore as a nursery site Altınoluk requires immediate attention of shark biologists. First week of July is a critical time for catching live pups for satellite tagging.
Squished among three continents the Mediterranean is a special sea with very different ecological and geological properties from other temperate seas. It has a quite adventurous geological history. To illustrate, imagine this extreme event that took place 5.6 million years ago. During a period known as the Messinian Salinity Crisis the Mediterranean lost its connection to the Atlantic. The crisis lasted about 300 thousand years and by the end of it the entire basin dried completely. Then a crack appeared in Gibraltar and the basin re-filled rapidly in just a few thousand years.
Productions made about great whites almost invariably are obsessed with the size of this charismatic fish. Here in Nature Documentaries we are proud to present you the opposite end of the spectrum: the smallest great white ever documented on film.
Finally, enjoy this most awe-inspiring breach sequence from BBC in slow motion and be informed that contrary to the common belief sharks do get cancer: