The Basking Shark

On this episode of Secrets of the Animal Kingdom, we delve into the life and characteristics of the Basking Shark, one of the weirdest and largest fishes on earth

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Transcript

Imagine a cross between a fish, and a giant baleen whale, swimming, slowly, menacingly, at the surface of the ocean. That's pretty much what the basking shark is. A giant, mostly passive filter feeder, but one who is quite terrifying to see up close.

Hello everyone and welcome to the 2nd episode of Secrets of the Animal Kingdom! Today we will be looking at one of our mascots, the basking shark. The basking shark or Cetorhinus maximus is the second largest species of living fish, behind only its more famous relative the whale shark, reaching lengths of around 20-30 feet and 4-5 tons, on average. It is part of the order lamniformes, which also happens to be the order that includes great whites. Basking sharks are mostly greyish, while the skin is covered with small denticles arranged with their points facing backward so that the skin feels smooth to a hand passing over it from front to back, but very rough to a hand passing in the reverse direction. Their mouth has a white interior, and it exhibits many different adaptations for filter feeding. It's not exactly a very well-studied species, like most other sharks, and not much is known about several aspects of the basking shark. 

The basking shark is one of three species of plankton-feeding sharks, the other two being the whale and megamouth sharks. First of all, it is gigantic when open, and its teeth are quite small and not really used for very much, furthermore, they have huge gills that almost encircle their entire head. It has a very boring feeding method, which essentially entails swimming very slowly (Around 3 kmph) with its huge mouth wide open to capture small prey, primarily zooplankton, but also very small fish and some other invertebrates. As water flows into its mouth and through its gills, plankton gets stuck on its gill rakers covered in mucus and then gets eaten. Incidentally, this is also how they get their name, because they do this near the surface of the water, making it seem as if they are basking in the sun. They are not suction feeders and cannot suck or pump water into their mouths, relying on only their forward motion to take water in. Still, even a relatively small basking shark (15 ft in length) was found to be able to filter 400-500 tons of water per hour. They are not indiscriminate feeders of plankton, instead, they actively search for parts of the ocean with the largest concentrations of their prey items. Basking sharks have the smallest brain-to-body size ratios among all sharks, probably as a reflection of their rather lethargic lifestyle. However, they can perform a breach, which is basically when an aquatic jumps completely out of the water, which in this case is maybe a mating display or an attempt to get rid of parasites. I don't know about you but this is what surprised me when I was researching for this video. The basking shark is quite chill and tolerates divers and boats that approach it, making it an important drive for eco-tourism in some areas. Another cool fact about the basking shark that I couldn't fit anywhere else is that many basking shark carcasses have been mistaken as carcasses of plesiosaurs or sea monsters by self-proclaimed scientists.

Basking sharks are a more or less cosmopolitan or global species that have been found in coastal as well as open waters. They seem to prefer temperate and colder waters, often being seen in the northern and southern regions of the Atlantic and the Pacific, but they do often cross equatorial waters as a part of their migratory paths. Speaking of migration, basking sharks are prolific migrators, cruising along at whirlwind speeds of around 3 km an hour, covering distances of up to 2400 Km. Different basking sharks seem to take different migratory paths: some have been observed crossing the equator, others seem to prefer to stay in and around the North Atlantic for example. They are also quite good at diving, with some diving up to 750 m deep, probably in search of patches of their prey. They can dive deeper than baleen whales, which could be a competitive advantage that they have over said whales. Diving behavior seems to be more prevalent in the winter than in the summer, probably because surface zooplankton levels fall at this time. 

When it comes to their social behavior, they are usually solitary yet have been seen to aggregate in groups. Some individuals have been observed annually forming groups, which might be linked to courtship as they seem to perform behaviors consistent with courtship rituals in other shark species. Basking sharks also seem to form loose aggregations when feeding with other individuals, but this is probably just a case of different individuals tolerating each other while exploiting the same resources, in this case, high-density patches of zooplankton. The basking shark is viviparous, with an estimated gestation period of 12-36 months and a likely resting period of two or so years between litters.

Now we come to the depressing part of the episode where we talk about how the basking shark is affected by human activity. It was once a very abundant species, but now it is listed as endangered by the IUCN and probably numbers over 20,000 or so individuals. They are not targeted by any fisheries in the present, but historically they were fished relentlessly for their meat, fins, skin, cartilage, and/or liver oil, made worse by the ease of catching them. Basking sharks have slow growth rates and slow reproductive rates, causing them to have low productivity and thus making them especially vulnerable to population loss as dead individuals cannot be replaced easily. Basking shark catches and sightings in a particular area are often really common for a short period of time, but soon become very uncommon as they get killed either intentionally or unintentionally. For example, in the North Atlantic, the basking shark population peaked in the 60s and 70s but then dropped off sharply in the 80s, probably as a result of unsustainable fishing practices. A similar trend was seen in New Zealand waters, except in the 90’s instead of the 70’s. The overall population trend may be stabilizing and may even be beginning to recover in some areas, but recovery is expected to take centuries due to its low productivity. 

The basking shark is primarily threatened by accidental bycatches, especially in trawling nets. A bycatch is a fish or other aquatic animal that unintentionally gets caught in a fishing net while the fisherman is specifically targeting some other species of fish. Trawling is a method of fishing in which a net that is heavily weighted to keep it on the seafloor is dragged along by one or more boats. While it is very efficient from a commercial perspective, it is extremely harmful to the environment on a number of levels, not least of which is that endangered and threatened species can easily get caught as bycatch in trawling vessels. The basking shark is especially vulnerable to bycatch as it is very slow and thus can't exactly swim away and is also very big. While targeted fisheries were mostly banned in the 2000s, its large fins are very valuable in the fin trade, fetching up to $57,000 for a single fin. Being hit by boats is another concern for basking sharks, because of their slow speed, tendency to rest near the surface, and their tolerance of boats.

Now we come to what we have and should do to save the basking shark from extinction. In view of its rapidly declining numbers, it is a largely protected species across most of the world. It was added to Appendices I & II of the Convention on the Conservation of Migratory Species of Wild Animals (CMS), which requires all parties to protect the species and to work regionally toward conservation. Most of the countries that make up the Basking Sharks range are signatories to this convention, with the notable exception of the US and Canada. It is also added to Appendix II of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) requires Parties to ensure that exports are accompanied by permits based on findings that parts are sourced from legal and sustainable fisheries. The North-East Atlantic Fisheries Commission (NEAFC) bans all targeted basking shark fisheries while the General Fisheries Commission for the Mediterranean (GFCM) banned retention and mandated careful release for the Basking Shark, but the IUCN notes that implementation of this has been slow. Unfortunately, the implementation at the domestic level for most of these treaties and agreements has been lacking, severely comprising their effectiveness. Basking shark conservation is further hampered by how little we know about its population size. The estimate I said earlier comes from the IUCN but is very speculative. In the future, more studies of its population size and composition are needed, 

The basking shark is truly one of the most weird and wonderful behemoths to have graced our oceans. It is perhaps one of the most unique species to exist in our modern-day oceans,  IN recent times, we have made significant strides in understanding the behavior of this giant, which undoubtedly faces significant challenges to its survival. Conservation efforts are absolutely critical to saving this majestic species from extinction, for at the end of the day, their fate lies in our hands.