OCCURRENCE OF THE BASKING SHARK, CETORHINUS MAXIMUS IN THE NORTHERN LEVANTINE, THE EASTERN MEDITERRANEAN
Ahmet E. Kideys
Institute of Marine Sciences, P.O. Box 28, Erdemli 33731 TURKEY
Mediterranean Fisheries Congress, 9-11 April 1997, Izmir, Turkey. (In Press).
Abstract
Occurrence of the basking shark, Cetorhinus maximus (Gunnerus, 1765) in the northeastern Mediterranean (and also in Turkish coastal waters) is reported for the first time. The biological and ecological characteristics of this fish are summarized. The occurrence of this plankton-feeding gigantic fish in this region could be an indication of increasing coastal productivity. Besides increasing the biodiversity of Turkish waters, this fish is a valuable species because it converts suplus organic matter into flesh in eutrophic waters and also presents a great potential for eco-tourism. Therefore, the necessary measurements should immediately be taken to ensure the protection of the basking shark in this area.
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Photograph 1: Photo of swimming basking shark (Photo Dr Rupert Lewis) |
Photograph 2: fished basking shark |
INTRODUCTION
The basking shark Cetorhinus maximus is the second largest fish in the world (Photograph 1), exceeded in size only by the whale shark (Rhincodon typus). Like the whale shark this fish is also harmless, feeding on plankton.
Basking sharks have been reported from all the oceans of the world. They also occur in the western Mediterranean but have never before been reported from the Levantine basin (eastern Mediterranean) or from the Black Sea (Fischer et al., 1997). Since it is a new species for Turkish waters, the present work aims to give information to the public on the occurrence and biological characteristics of this large fish. However surprisingly little work has been undertaken on the basking shark elsewhere. The basking shark visits the Isle of Man coasts (Irish Sea) almost every year during late spring and summer. I personally had the chance of swimming together with these large fish in Manx waters. Some of the information on the biological characterics of this fish have been obtained from the Isle of Man (Great Britain) Basking Shark Project.
OBSERVATION
In May 1995 two basking sharks were caught by stationary nets off Erdemli, Mersin Bay, southern Turkey (Photograph 2). One was measured as 4.7 m. No local fishermen recognised the fish and they all stated that they saw them for the first time. During August and September of 1996, several sightings of basking shark were also reported in and around the harbour of the Marine Sciences Institute in Erdemli (personal communication with Brian Lavery, Glasgow University, UK and Prof. Dr Süleyman Tugrul, Institute of Marine Sciences, Erdemli, Turkey, and others).
BIOLOGICAL AND ECOLOGICAL CHARACTERISTICS OF THE BASKING SHARK
Taxonomy
The Norwegian naturalist and cleric Johann Ernst Gunnerus was the first to describe the basking shark in 1765. Thomas Pennant, a few years later, described this fish, known as the sun fish because of its apparent basking habit, as commonly seen in the Irish Sea and off the south and west coasts of Ireland and Scotland. Until the five enormous pairs of gill arches were observed in stranded specimens the sun fish had long been assumed to be a species of whale. In British waters the basking shark is still known to some as the sun fish or, in acknowledgement of its tall dorsal fin, the sail fish.
Springer & Gilbert, 1976 suggested that four species of basking sharks exist, two in the north Atlantic/Mediterranean and others in the south Atlantic and the waters around Australia. These suggestions were made mainly due to differences in body proportion arising naturally during growth and it is generally taken that only one species of basking shark exists.
According to Fischer et al. (1987) Cetorhinus maximus belongs to the family Cetorhinidae.
Morphology and Growth
Whilst the smallest specimen seen by Parker & Stott (1965) was measured as 1.68 m, the largest size attained is more controversial. A maximum size of 15.4 m was reported by Bigelow & Schroeder (1948). According to Holden (1974) they can reach a theoretical maximum length of 13.72 m. Bigelow and Schroeder (1948) recorded the weights of two sharks measuring 8.4 m and 9 m as 2991 kg and 3909 kg, respectively.
If the hand is passed towards the tail the skin is relatively smooth to touch but is highly abrasive if passed towards the head. The dorsal fin of this fish is very slippery due to thick, black, mucus (personal observation).
The twin lobed liver runs the length of the abdominal cavity and may account for up to 25% of the body weight. The liver is rich in a terpenoid hydrocarbon oil, squalene which is possibly used for control of buoyancy. The liver may act as a winter food store.
Feeding
When at or near the surface, the basking shark feeds by swimming with its mouth (of approximately 1 m wide) open with the gill rakers erect. The gill rakers filter particulate matter from the water passing through the gill slits. Assuming a feeding speed of 2 knots, the volume filtered is estimated to be 3-4 thousand tons water per hour. It appears that the basking shark is an indiscriminate planktivore, its main food constituent being the dominant species within the layer through which the shark is feeding at any one time. I observed them filtering seawater with an abundant amount of the ctenophore Pleurobrachia pileus. No evidence is given as to the basking shark seeking out a particular plankton source.
A somewhat surprising observation that several basking sharks caught between October and December had no visible gill rakers but rather immature, under-developed and seemingly non-functional rakers has led to the suggestion of a resting, non feeding, demersal stage in the early winter. It is further suggested that the new gill rakers are fully developed by February.
Fisheries
Until recently basking sharks were hunted mainly for their livers (approx. one third of body weight) which contain the oil squalene, this was used for engine lubrication and cosmetics. The most recent reports suggest that they are now killed solely for their fins which are sold to the Far East to be used in shark fin soup.
Basking shark fisheries presently operate in Norway, China, Japan, California and Scotland. Several fisheries have recently collapsed due to the local disappearance of basking sharks. In several areas they have been hunted to extinction.
Reproduction Like most other sharks, the basking shark is ovoviparous giving birth to live young rather than laying eggs. They can give birth to up to six. The newborn sharks are estimated to be between 1.5m and 2m long. The young commence feeding immediately after birth.
Whilst the age of females at the onset of sexual maturity is unknown, Bigelow & Schroeder (1948) suggested that in males it is reflected by changes in the relative size of the claspers as they age and that maturity is reached at a body length of between 4.6 and 6.1 m. Parker & Stott (1965) calculated this to be in the shark's fourth or fifth year. Matthews (1950) reported that although spermatophores were not present there were signs of testicular activity in a specimen of 6.9m.
DISCUSSION
The occurence of the basking shark in Mersin Bay (Levantine Basin) is important and could be taken as further evidence for the increasing productivity of previously oligotrophic waters. Indeed several other indicator species recently appeared in this region. For example the ctenophore Mnemiopsis sp. which is only competitive in eutrophic waters was reported in Mersin Bay in May 1992 for the first time with a maximum abundance of 1.1 individuals m-3 (Kideys and Niermann, 1993). Later, in the summer of 1995, another plankton feeding large species, the jellyfish Rhopilema nomadica was present in Mersin Bay, causing several injuries to bathers on the beach (Kideys and Gücü, 1995). It is suggested that only a substantial amount of plankton and other organic matter can sustain a high biomass of the aforementioned jellyfish, the ctenophore or repeated appearances of the basking shark. All these species may be assumed as biological indicators for the increasing productivity in the region.
Since large organisms of the seas have difficulty in hiding, and since they reproduce in small numbers after a long period before reaching sexual maturity, they may readily be faced with the danger of extinction due to human activities. This is especially true for the basking shark. This large fish is easily spotted as it swims with its dorsal fin at the sea surface and is easily caught as it swims very slowly (about 2 knots). This explaines why two were caught easily in Mersin Bay. They are already included among the rare species of the sea. Consequently they are protected by conservation laws in most countries such as Ireland, Britain, USA, and Australia. This fish would increase the biodiversity of Turkish waters (Kazanci et al. 1995) and is also a potential ecotourism species. The basking shark also helps eutropic areas by converting surplus organic matter (including gelatinous organisms) into shark flesh. Because of this reason it is recommended as one of the most suitable species for the biological control of the voracious predatory ctenophore Mnemiopsis sp. (Kideys, 1994) which recently caused adverse events in the Black Sea. Therefore this magnificent creature, the basking shark, Cetorhius maximus should be added to the Protected Species List of Turkey immediately.
REFERENCES
Bigelow, H.B. & W.C. Schroeder 1948. Fishes of the Western North Atlantic. Memoir Sears Foundation, for Marine Research, 1 (1), 576 pp.
Fischer W., M. Schneider & M.-L. Bauchot 1987. Mediterranee et Mer Noire - Volume II Vertebres. FAO, Rome. Holden, M.J. 1974. Problems in the rational exploitation of elasmobranch populations and some suggested solutions. Harden Jones, F.R. ed Sea Fisheries Research. Paul Elek, 510 pp.
Kazanci N., I. Kiziroglu & A.E. Kideys 1995. Biological diversity action plan - Report to the World Bank: Wetland, river, marine, lake, island and cave ecosystems. Ministry of Forestry, Ankara, Turkey, 177 pp.
Kideys A.E. 1994. Recent dramatic changes in the Black Sea ecosystem: The reason for the sharp decline in Turkish anchovy fisheries. Journal of Marine Systems 5: 171-181.
Kideys A.E. and A.C. Gucu 1995. Rhopilema nomadica: A poisonous indo-pacific scyphomedusan new to the Mediterranean coast of Turkey. Israel Journal of Zoology 41: 615-617.
Kideys A.E. and U. Niermann 1993. Intrusion of Mnemiopsis mccradyi into the Mediterranean Sea. Senckenbergiana Maritima 23: 43-47.
Matthews, L.H. 1950. Reproduction in the basking shark. Philosophical Transactions of the Royal Society of London Series B, 234, 247-316.
Parker, H.W. & F.C. Stott 1965. Age size and vertebral calcification in the basking shark, Cetorhinus maximus (Gunnerus). Zoologische Mededelingen, 40, 305-319.