Federico Betti – Reef Check Italia Associate Program Manager
On June 11, along the coast of Ancona (Italy) a young male of basking shark (Cetorhinus maximus) was found dead, caught in a fishing net; it was 3,65 meters long, from the snout to the caudal peduncle, while adults of this species may reach a maximum length of more than 10 meters.
The news aroused people curiosity, but the presence of this species in the Adriatic waters is not occasional: every year, at the beginning of summer, many sightings are made in the area. Let see why.
Cetorhinus maximus is, like other sharks, a cartilaginous fish, and among them
is behind the only whale shark Rinchodon typus for maximum size. As this one, the basking shark feeds on plankton, that is small organisms which flow with the current in the
water column; to feed, this teeth-lacking shark swim slowly close to the surface, usually upstream, with its mouth open wide.
North-Adriatic waters are rich in plankton, in particular between spring and summer when, thanks to nutrients imported by the river Po, zooplankton can find a lot of phytoplankton to feed on, and its abundance quickly increase.
When zooplankton begin to be abundant, predators appear, and among them the basking shark, which seems to be able to filter 2000 m of water per hour (it swims at about two knots, and its mouth can reach a diameter of one meter).Water circulation in the Adriatic Sea proceed clockwise, going up along Slavic coasts and going down along Italian coasts: that’s why schools of basking sharks travel upstream from Otranto to Trieste, and some of them, unfortunately, may be trapped in fishing nets during their journey.
Also in other areas of the Mediterranean Sea some accidental catch are registered during the period of maximum abundance of the zooplankton.
The presence of basking sharks represent a good indicator of biological coastal cycles.