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Topic: Basking shark

A basking shark swimming in the surface, a dorsal fin breaking the surface

Basking shark is the second largest fish species in the world. 

Photo: Institute of Marine Research

The basking shark is a highly migratory shark species that lives in temperate waters in both the southern and northern hemisphere. The basking shark grows slowly but can be more than 12 meters long and weigh up to 19 tons! It is thus the second largest fish species on the planet, after the whale shark, which is also a planktivorous cartilaginous fish.

The Latin name Cetorhinus maximus refers to both the characteristic shape (rhinos=nose) and size (maximus=largest). Ceto (Keto) was an early goddess of Greek mythology and is also a term for sea monsters. 

Although the basking shark is related to sharks, rays, and other cartilaginous fish, it does not have the fearsome dentition like many other sharks have. It feeds on plankton that it filters from the water. As basking sharks can appear in groups near the surface, perhaps snouts, dorsal fins and tails breaking the surface from such aggregations have given rise to the notion of the sea serpent. There is room to suggest that the basking shark has been a model for some of the "sea monsters" that often appeared on early nautical charts.   

The basking shark does not have solid fat stored under the skin like marine mammals. It is stored in the sharks’ liver which can make up to 25% of its body weight. The oil helps the basking shark to stay neutrally buoyant as is does not have an air-filled swim bladder like most bony fish. The commercially valuable oils in the liver in addition to their dorsal fins, caused the basking shark to be hunted and repeatedly over-exploited in the past. Indeed, the earliest directed fisheries for pelagic shark were probably for this species. In Norway, written sources talk about basking shark fishing in Hordaland (Western Norway) from the 1770s, and in the Oslo fjord and in Southern Norway in the 1960s–1970s. Today, the basking shark is considered endangered in Europe and globally, and no commercial fishing is carried out any longer. 

Despite the commercial interest, surprisingly little is known about key aspects of basking shark’s biology and ecology, such as their realized global distributions, seasonal migrations, population sizes, or their reproductive biology.

As far as their reproduction is concerned, it is evident that basking sharks become sexually mature relatively late, around an age of 18, and reproduce slowly. It is assumed that embryos hatch in the uterus from their eggshell and are born fully developed with a length of around 1.5 to 2 m (ovoviviparity) after a gestation period estimated to last 1 to 3 years. In the uterus the embryos seem to not only use their own yolk-sac but also other unfertilized eggs for nourishment (oophagy), which might explain their elongated snout tip as juveniles.

The typical gills slits, which almost encircle the head, allow the basking shark both to breath as well as to feed. On each of the gill arches there are gill lamellae that enable respiration and gill rakers which are comb-like structures allowing the shark to filter zooplankton out of the seawater. The basking shark performs so-called “ram-filter feeding”, which means that it must ensure a continuous flow of seawater by active forward swimming to feed. Therefore, you will always find the basking shark swimming with a widely open mouth when feeding. 

Despite their size they are completely harmless to humans. If you're lucky enough to encounter a basking shark, just enjoy the sight of this giant's slow elegance and register your sighting on the citizen science platform “dugnad for havet”.

Other shark species in Norwegian waters are spurdog, velvet belly lanternshark, sleeper shark (håkjerring) and blue shark (håbrann).