The Elephant shark (Cetorhinus maximus)

MORPHOLOGY: Easily recognized from other sharks by the huge gill openings that almost completely surround the head; presence of highly developed branchiospines; pointed snout; huge, sub terminal mouth with small hooked teeth; caudal peduncle with obvious lateral carina.

SIZE: Hatchlings are probably born with an estimated length of around 1.5-1.7 m, but this is not known for sure; the majority of specimens are around 7-9 m; the largest reported specimen reaches a length of 11.5 m and a weight of 4,500 kg.

DISTRIBUTION: Cosmopolitan. Western Atlantic: Newfoundland, Canada to Florida, USA; southern Brazil to Argentina. East Atlantic: Iceland, Norway, Mediterranean and Senegal; southern Africa. Western Pacific: from Japan to New Zealand. Eastern Pacific: Gulf of Alaska to Chile.

Distributed throughout the Mediterranean basin, but sparsely found in the eastern basin. Absent in the Black Sea.

ECOLOGY: Basking shark is often associated with schools of other fish such as Clupea harengus and Scomber scombrus in the North Atlantic. To filter water, it possesses long branchiospines that facilitate the retention of plankton. The gills are highly developed and almost completely surround the shark’s head; the mouth is also highly developed. It feeds entirely on plankton, especially copepods of the genus Calanus, but also fish eggs, ketognaths and crustacean larvae. The cetorino feeds by swimming slowly at the surface. While swimming at the surface, the wide mouth is held open for about 30-60 seconds. The retention of basking shark specimens lacking functional gills during the fall and winter months has led to the erroneous assumption that during these periods the shark is in a resting state, without feeding.

REPRODUCTION: probably ovoviviparous; gestation period probably over a year; estimated birth length around 1.5 and 2 m; mating period occurs during early summer.

THREATS: fishing, but also collision with boats. Catches in the Mediterranean are incidental; in this area C. maximus has never been subject to a directed fishery. In the Mediterranean, cetorino is caught in trawls, trammel nets, shrimp nets, and nets with small or large meshes.

CONSERVATION: Highly migratory species at high risk of extinction due to its low productivity. The basking shark is listed in Appendix II (Endangered or Threatened Species) of the Barcelona Convention for the Protection of the Mediterranean (1976) Protocol Concerning Specially Protected Areas and Biological Diversity in the Mediterranean, but only in Malta is the species legally protected. The Mediterranean population is also listed in Appendix I of the Bern Convention for Conservation of European Wildlife and Habitats. International trade in cetorino is restricted and controlled by CITES (Appendix II, 5/28/2003).

Biology

Cetorhinus maximus is a widely distributed species in temperate waters, known by the common name basking shark represents the second largest fish in the world after the whale shark (Rhincodon typus). The name comes from the Greek terms kétos, literally “sea monster,” for its size and rhinòs, “nose,” for the conformation of the elongated snout.

It reaches in adult individuals maximum lengths of 9-11 m with an average size of 3,500 to 4,500 kg. Records of the presence of this shark in temperate waters are in most cases due to either chance sightings or incidental catches. Incidental catch is mentioned because Cetorhinus maximus has, at least as far as the Mediterranean is concerned, no commercial interest, although, on the contrary, it has been a target for fishing in colder marine regions and wildly exploited for a long time. Sightings in the open sea are extremely scarce, while its presence is often reported in coastal areas, probably due to increased fishing activity in these areas that increases the number of catches.

The basking shark is, therefore, a pelagic littoral fish, typically sighted at the surface where it moves slowly as it feeds on plankton while advancing with its mouth open, recognizable by its snout, dorsal fin, and caudal fin surfacing on the surface.

Little is known about the life cycle or longevity of this shark, which appears to be ovoviviparous. At the time of reproduction, in the spring season, males inject spermatophores through pterygopods, modified pelvic fins, into the female’s body, and fertilization occurs when the female releases millions of eggs. The first embryos to implant in the uterus feed on the remaining eggs whether fertilized or not, a phenomenon known as intrauterine cannibalism. The only recorded birth produced one offspring, a group of six young that measured 1.5 to 1.7 m in length. Predictions for the gestation period are 1 to 3 years, with uncertain success possibly resulting in a litter consisting of a small number of young.

Little data have been collected on length at maturity, but these indicate that males mature at 4.5 to 6 m, at about 12 to 16 years of age, and females at 8 to 10 m, at about 20 years of age.

Thus, the basking shark has two characteristics that make it extremely vulnerable to exploitation, which are a relatively low reproductive rate and a slow attainment of sexual maturity. Although these characteristics are known, the lack of knowledge about movements and habitat requirements continues to hinder an adequate assessment for the conservation of this species.

Food and migration

Basking sharks feed on plankton (copepods, fish eggs, decapod and baleen larvae) by collecting it through their modified gills, called branchiospines.

During the spring and summer months, basking sharks feed by filtering zooplankton close to shore from boreal to warm-temperate waters; for the rest of the year, their whereabouts remain a mystery.

It has long been thought that in winter these sharks migrate to the deep waters apparently losing their filtering apparatus, and to compensate for the loss of energy due to the absence of remunerative feeding, it has been proposed that they hibernate on the seafloor until spring, a time when zooplankton increases in density. However, recent studies have shown that basking sharks feed profitably at much lower zooplankton density thresholds than previously imagined, thus suggesting that hibernation was not necessary. In addition, studies conducted showed that most basking sharks trawled off South Island, New Zealand, were caught on or near the bottom, suggesting that they were unlikely to hibernate floating in neutral buoyancy in midwater, given the enormous energy costs involved. These observations led to the conclusion that these sharks overwinter in deep water on the continental slope. Hypothesis endorsed by the recent observation of five satellite-tagged sharks in which no individuals appeared to hibernate during the winter, instead they undertook extensive horizontal (up to 3400 km) and vertical movements (> of 750 m depth) probably looking for temporary “hot-spots” on the continental slope, never showing prolonged movements in the open ocean regions away from shelf waters. Thus, sharks are able to exploit both shelf- and slope-connected zooplankton communities in both mesopelagic (200 – 1000 m) and epipelagic (0 – 200 m) environments. Annual variations in sightings and catches can therefore be strongly influenced by water condition, temperature, and cyclical fluctuations in zooplankton distribution and abundance.

Presence and distribution in the Mediterranean.

In the Mediterranean, the presence of the basking shark, Cetorhinus maximus, was studied by analyzing data collected during the ” Mediterranean Large Elasmobranchs Monitoring (MED_LEM) Program” and from data from the literature.

The MED_LEM represents an examination of the presence of large elasmobranchs that began in 1985 in Italian waters and has since expanded to other Mediterranean countries using a common protocol for data collection. Information was collected with anecdotes going back as far as 1795, but an important contribution came from collaboration with military authorities, professional and recreational fishermen, and various research institutions.

To obtain a biogeographical analysis, the Mediterranean is divided into four regions corresponding to the same Mediterranean sub-basins: Balearic (B), Tyrrhenian (T), Adriatic (A), and Eastern Basin (E).