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).
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).
From 1795 to 2002, about 600 reports of cetorino catches or sightings were collected in the Mediterranean (Fig. 1). Many of these records are deficient in some information such as size, sex, weight, etc. In the Mediterranean, this species seems to be restricted to the Western and Central basin (about 1.3 million km2). The preferred habitat of the species appears to be the coastal area characterized by a narrower continental shelf, as in the Liguro-Provençal area (northernmost sector of the western Mediterranean basin). This fact may be related to the upwelling phenomenon known in this area.
The highest relative abundance of this species in Italian waters coincides with International Cetacean Sanctuary (Law 11/10/2001 No. 391) of the Mediterranean about 96000 km2.
The frequency of incidental catches and sightings of this shark by year was analyzed on 522 specimens and shows 3 peaks probably due to an increase in scientific interest that has occurred especially since 1990. Of 446 records we have information about the month of capture. Cetorino catches and sightings are greatest in spring and winter, with a maximum in March (25 percent of the total) in Liguria and the northern Tyrrhenian Sea, while in the Balearic Islands it occurs during winter. From these data, the spatial distribution by season was also obtained. The peak relative to presences in the Adriatic is recorded in spring, this is related to the higher abundance of zooplankton in this period. Recent studies support this hypothesis; in fact, a positive relationship has been found between the presence of cetorino and the abundance of zooplankton. As in the case of the southern area of England, the presence of basking shark in the Northeast Adriatic is closely related to the abundance of copepods, especially Calanus helgolandicus. Similar is first the presence of adult specimens between March and May, and then later the arrival of immature juveniles.
The LFD (Length Frequency Distribution) of the individuals whose total length (n=390) is available shows a maximum peak for the 6 m total length TL. Only for 151 specimens is sex specified; of these, 55% (n=83) are males and 45% (n=68) are females, with a sex ratio of 0.45 (number of females/total). Among them, for 138 specimens there is information of both sex and length.
Only 386 records contain information on size and location of capture or sighting. Many records (n=155) were recorded in Tyrrhenian Sea, and all size classes are represented. For regions A, B and T, there is a predominant presence of subadult and adult specimens (67%, 80% and 51%, respectively) with a TL between 4 and 9 meters. In region E, on the contrary, there is a higher precentage of juveniles (65%) (TL < 4 m) and 35% are adult specimens.
Currently, the only possible information on C. maximus in the Mediterranean comes from chance sightings (42%) and incidental catches. The gear most responsible for catching this species is the trammel net with 15 percent of the 323 records analyzed. For the most part, the fishing technique by which the shark was caught is not specified. Of these 323 records, only for 122 is the total length specified. The LFD was analyzed only for specimens caught with the trammel net, and it appears that mainly specimens with a total length around 6 m are caught with this gear.
The LFD analyzed for 292 records during the 4 seasons shows a maximum of young individuals (< 350 cm TL) in autumn (50%) and a peak relative to adults (about 700 m TL) in winter. The spatial distribution of cetorino in the Mediterranean is correlated with mean surface water temperature, closely correlated with wind speed, surface current and chlorophyll concentration. Basking shark seems to be concentrated in coastal areas where water temperature is between 18.2 and 19.4 °C (OCEAN Project, 2000). In the same area, during the two peak cetorino seasons in the Mediterranean (spring and winter), we can observe a high concentration of chlorophyll with values of 10-1.3 mg/m3. In particular, the highest concentration of chlorophyll (10-6 mg/m3) is located in the northern Adriatic, along the Liguro-Provençal and French coasts (OCEAN Project, 2000), where there is a maximum presence of this large shark. The distribution of chlorophyll in winter in the Mediterranean is the highest of all seasons, with small and rare areas of low concentration (0.12-0.05 mg/m3) (OCEAN Project, 2000). This situation suggests a subsequent explosion in the same area of zooplanktondove previously recorded chlorophyll concentration. This fact may explain the high presence of cetorino in the Mediterranean in spring. It is generally assumed that the return of the basking shark along the coasts during spring is associated with the period of maximum biological production, when the concentration of its prey is abundant.
The lack of data on the occurrence of cetorino in the eastern Mediterranean basin may depend on the biological and chemical and physical characteristics of this area. In fact, the southern Mediterranean coasts show a high water temperature value (20.6-24 °C) and a very low chlorophyll concentration value (0.3-0.05 mg/m3) in almost all seasons. The few reports of sharks recorded for this area correspond to small coastal areas where chlorophyll concentrations are slightly higher (Israel, Turkey, Tunisia).
Presence and distribution in Sardinia
From information gathered during informal interviews with fishermen and from data in the literature, it appears that Sardinia, particularly in the northern area, there have been several sightings of basking sharks. Forty-three reports were collected, from 1910 to 2006: 28 sightings and 15 catches (length between 250 and 800 cm).
Since 2005 and 2006, sightings have intensified, sightings of aggregation of several animals, up to 11 together, are reported, but unfortunately as many as 6 animals have also been captured.
Figure 2 shows the 3 points of highest concentration of sightings: Northwest (Porto Torres); Northeast Maddalena Archipelago; Orosei.
In subsequent years, sightings were very limited, but as a constant there was the fact that following a report of an animal swimming near the coast, one was accidentally caught by gill nets.
Basking Shark can be considered the species that most requires protection measures. In addition to the few observations made in the open sea, the presence of this shark, is unfortunately evidenced by the many incidental catches made by gillnets or other artisanal fishing systems frequently used in the coastal waters of many Mediterranean countries. For this reason, the Cetorino has been listed in Appendix II (Threatened and Endangered Species) of the Barcelona Convention, Appendix II (Strictly Protected Species) of the Bern Convention, and very recently (December 2002), it was also finally listed in Appendix II of CITES.
While in the Atlantic areas the movements of basking sharks are very regular, in the Mediterranean they are not, making it very difficult to undertake effective conservation measures that take into account both the needs of the species and those of fishermen, who unwittingly are the main cause of their decline. The seasonal aggregations found in recent years in Sardinia provide an excellent opportunity to gather more information about these animals and, more importantly, create an opportunity to talk about sharks, animals as feared as they are unknown. The good relations that are being established between CRD staff and fishermen, along with the cooperation of Park staff and law enforcement, may serve to open a channel of dialogue on the subject and find together the best solutions to protect this harmless giant of our seas.