Next to the whale shark, basking sharks (Cetorhinus maximus) are the second largest fish in existence. Fortunately, like the whale shark, these giant fish are harmless filter feeders. Its scientific name comes from the Greek roots meaning "giant big-nosed monster", which seems pretty apt. Found in temperate waters of both the northern and southern hemispheres, these sharks can grow to well over thirty feet in length.
Basking sharks feed on tiny plankton, mainly copepods, using a method called ram feeding. Since it is unable to suck water into its mouth as it feeds and breathes, like whale sharks can, it moves slowly through the water with its giant mouth open "ramming" water over its gills as it swims. Not only are the gills used for breathing, but they are also responsible for capturing the tiny food this fish survives on. The gill rakers, which form the supporting structure for the gill slits, are covered with fine bristle-like combs which trap the plankton. The shark feeds with its mouth agape for a few minutes, then closes the mouth and flaps the gills. This dislodges the trapped plankton from the bristles and the food is swallowed.
Because the prey is so small, the shark must feed nearly constantly in order to sustain its huge body. It also must migrate seasonally in order to find areas of the sea where plankton is plentiful. During the fall and spring plankton is plentiful in coastal waters where upwelling occurs. During these times of year these fish can be found quite close to shore and sometimes even enter fairly shallow bays and estuaries.
During the summer and winter months plankton populations drop drastically. It was once widely assumed that these animals hibernated in the deep sea during these lean times. However, hibernating sharks have never been observed. How then do these sharks survive the steep drop in resources? Well, even during the worst times of year for planktivores, there are still some areas of the ocean where plankton flourishes.
Throughout the oceans flow huge currents. Let’s take the northern Atlantic as an example. Here we have the warm Gulf Stream current moving northward and the Labrador current flowing south from Greenland. Where these two currents meet a thermocline occurs, which is basically an area where the water temperature changes rapidly and dramatically. Like in areas of upwelling (where cold water drops and warm water rises), these thermoclines produce turbulence which suspend nutrients in the water column rather than allowing them to sink to the bottom sediments. This is ideal conditions for plankton to grow.
Now, the other mystery is how do the basking sharks find these thermoclines before they end up starving to death. The most logical answer would be by following temperature gradients. Problem is, sharks do not have temperature sensing organs. They do however have an exceptionally keen sense of smell. I’m sure you’ve heard tales of how a shark can sense blood in the water in minute quantities and is able to follow the olfactory trail to its prey. Well basking sharks, being plankton eaters, are not interested in blood. But they are interested in dimethyl sulfide.
This is a chemical compound that is released in tiny amounts by phytoplankton. It is how zooplankton, like copepods, find their food. So by searching for signs of plant plankton the sharks are sure to find the animal plankton feeding on them. And that’s how they indirectly find the thermoclines.
This also explains why these sharks are found throughout the world in temperate waters but not in tropical areas. As I showed in this diary, tropical seas are like deserts and contain very little planktonic life (which is why tropical waters seem to be so clear). Without thermoclines and upwelling the nutrients are trapped in the sediments and not available for pelagic life.
Basking sharks, along with giant squid, are the main sources of old "sea monster" myths. The decomposing carcasses were mistaken for ancient marine dinosaurs or mythical sea serpents. Above is an old recreation of what the artist believed the beached animal looked like alive. I love the little hind legs.
Basking shark populations are down significantly, mainly due to human activity. Although they are one of only three species protected by international law, illegal hunting for their fins (used in shark fin soup) still takes a toll. You can read about shark finning at this diary. But the leading cause of death of these animals by far is entanglement in fishing gear, especially the thousands of miles of gill nets found around the globe.
In the 1950’s, when their numbers were much higher than today, they were considered to be such a nuisance, because of the damage they caused to salmon netting, that the Canadian government actually had an eradication program in place. At first this consisted of encouraging fishermen to harpoon them on sight. The next step taken is pretty incredible. Baskers, because of their size and lack of natural predators, are not afraid of boats. So the government actually outfitted a ship with a giant steel blade on the bow and the crew's job was to sail around searching for sharks and ramming them, slicing them in half with the blade.
Thankfully that practice has ended. Although the population is slowly recovering in some areas, finning and nets still take a large toll. Eco-tourism, sort of like whale watches for basking sharks, are enlightening the public about the fate of these fish and may help protect them in the long run. Conservation efforts are difficult, as they are with all shark species, because of basic shark reproductive biology. Sharks have small broods and long gestation periods, up to a year for this species.
The reproductive cycle for this fish is pretty fascinating, though. Baskers are ovoviviparous, meaning they develop internally but are nourished by a yolk rather than a placenta. The fetal shark is also oophagous. In addition to the small handful of fertilized eggs, the mother also produces tens of thousands of unfertilized ones. These eggs are eaten by the young within the mother as they develop. When the pups are born live a year later they are around five feet long.
Fun Fact: Although normally very slow moving, basking sharks are known to breach like whales in order to dislodge ectoparasites and lampreys.
Other diaries in this series can be found here.