The basking shark (Cetorhinus maximus) is the largest fish in the Mediterranean Sea (the second largest in the world, behind only the whale shark). Adults can reach sizes of 10 meters and weigh about 7 tons. Although it may appear to be a large predator (it belongs to the order Lamniformes, like the great white shark), its teeth are tiny, practically vestigial, as this species feeds passively, filtering zooplankton and small fish through its gills, so we usually see them swimming slowly on the surface. Occasionally sightings of several individuals together can be made. 

The basking shark is also known for its long transoceanic and trans-equatorial migrations in certain seasons of the year. Studies conducted in the late 20th and early 21st centuries suggested that these seasonal movements appear to be closely related to the occupation of highly productive habitats (when plankton abundance is high), both on and off the continental shelf. However, it is true that they could also be due to a preference for warmer waters, movement to mating areas or to philopatry, the tendency of many animal species to remain in the same territory where they were born, or to return to it to reproduce.

In the Mediterranean, the higher volume of basking shark sightings during the spring months and until the end of summer could be related to this increase in phytoplankton concentration, which is higher during the spring.

The distinctive features that allow the identification of this species are:

• Body of chestnut color.
• Conical snout, rounded at the tip in the case of adults (and sometimes with a white tip).
• 5 large gill slits, the first ones almost enveloping the head.
• Large dorsal fin, triangular in shape but with the apical edge tending to be blunt, which may “flop” sideways as the individual swims.
• Considerably large anal fin, similar in size to the second dorsal fin.
• On the surface, when feeding, it can sometimes be identified because 3 parts of the individual protrude out of the water. The tip of the snout, the dorsal fin and the upper lobe of the caudal fin.

Despite its great weight and size, the basking shark can assume considerably fast speeds in short time intervals and propel its entire body out of the water, known as “breaching”. In the recent study by Ruud et al. (2020), not only were a large number of single breaches recorded, but double, triple and even quadruple breaches of the same individual were also recorded, leaving the water at an average speed of 3.9 m/s (similar to that of the white shark).

It is a protected species in multiple countries, included in CITES Appendix II, Appendices I and II of the Convention for the Conservation of Migratory Species, Annex II of the Barcelona Convention and protected by the ‘General Fisheries Commission for the Mediterranean’ GFCM/42/2018/2 and the European Union also prohibits its fishing. It is listed as a Vulnerable species by the IUCN, as it is considered that before being internationally protected its populations declined by more than 50% due to the effect of fisheries directed to the exploitation of its meat and liver oil, in addition to the accidental impact of other fisheries not directed to its capture. Even so, between 2007-2016, 700 kg of meat from Norway was traded to the Chinese market (Source: CITES Trade Database).

In the last few days the first sightings of basking sharks have already been recorded in our waters, so there is a small chance that you too might encounter one if you go out to sea! In that case, we invite you to register your sighting and post it in our Facebook group “Sharks and Rays of the Western Mediterranean” where you can also enjoy many other sightings of different species of elasmobranchs in the Spanish Mediterranean. All the sightings posted in the group, together with their complementary information, will be part of the database of the MECO project, a regional citizen science project in which we collaborate with several other Mediterranean countries and that helps us to have a better and current knowledge of the species that inhabit it. Thank you very much!

The MECO project