Grey Nurse shark conservation
From Wikipedia, the free encyclopedia
Sharks are found throughout the world but their populations are declining every day. This has led to protection of some species. One of the first shark species to be protected was the Grey nurse shark (Carcharias taurus). The biology, distribution and conservation of this species are dealt with in the following paragraphs with a main focus on Australia as it was here it first became protected.
Contents |
[edit] Biology
The Grey Nurse shark (Carcharias taurus), also called the Sand Tiger shark or Ragged-tooth shark10, is an elasmobranch and belongs to the Odontaspidae family. It can easily be recognized by its characteristic conical snout and underhung jaw. Both jaws are filled with sharp, long and pointed teeth. The head is flattened and it has a large and stout body which ranges up to 3.2m and may weight up to 300kg. The body is grey to grey-brown dorsally and off-white on the belly. The juveniles (young sharks) usually have dark spots on the upper two thirds of the body. The first and second dorsal fins are of similar size and the caudal fin is asymmetric. Once believed to be a man-eater it is now known that this shark rarely attacks humans and if it does it is only in defence. Sharks are the top predators in our oceans, and as such they are important for the marine ecosystems as important regulators of other species. They eat the weak, the old and the dead animals. The Grey Nurse sharks eat mainly lobsters, crabs, smaller sharks, fish, rays and squid.
[edit] Distribution
The Grey Nurse sharks live near the coast in sub-tropical to cool-temperate waters near most continental land masses (not found in the eastern Pacific Ocean off North and South America). They have a preference for some places resulting in an uneven distribution. For example there are few Grey Nurse sharks found in north Australia while they are relatively abundant in the southern part of the eastern and western Australian waters.
They are usually found swimming slowly, just above the seafloor, in sandy-bottomed gutters or in rocky underwater caves near inshore rocky reefs and islands. They can be found at depths ranging from 10m (near the coastline) to 200m (on the continental shelf)10. They are generally solitary but at times small schools of Grey Nurses are found swimming and feeding together.
[edit] Conservation status in Australia
The Grey Nurse shark is one of the most critically endangered shark species and believed to be the first protected shark in the world10. It was declared ‘vulnerable’ in the waters of New South Wales (Australia) in1984 and later throughout the world. In 1996 the species was listed globally ‘vulnerable’ by the International Union for the Conservation of Nature (IUCN) and declared ‘vulnerable’ in Commonwealth waters of Australia. According to the Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act 1999) it is believed that there are two separate populations of Grey Nurse shark in Australia. The population in the east coast is listed as ‘critically endangered’ whereas the west coast population is listed as ‘vulnerable’ under the EPBC Act (1999)3. The Grey Nurse sharks are also protected under the Fisheries Legislation in New South Wales, Western Australia, Victoria, Tasmania and Queensland. In the Northern Territory the species is classified as ‘data deficient’ by the Territory Parks and Wildlife Conservation Act (2000)NTG.
[edit] Current status
The population has declined dramatically in recent decades, especially in the 1960s and 1970s10. After 20 years of protection the population is still declining and there are approximately 400-500 Grey Nurses left in Eastern Australia. This has led to increased public awareness and conservation of these sharks is now greatly supported.
Some scientists, fishers and divers and peak organisations such as the Australian Underwater Federation believe that the methodology used to estimate numbers of approximately 400-500 is limited and that grey nurse shark numbers have increased since protection in 1984 and that the east coast population numbers have been quoted as up to 1000, 2000 and 6000.
[edit] Causes for decline
Every year, millions of sharks are killed by drift-nets, bycatch8, revenge actions, beach protective shark meshing, commercial-, recreational- and spear-fishing2,3. An alternative view is that grey nurse are totally protected and selective methods of fishing such as spearfishing have NO impact on this species. The main risk is from bycatch of indirect methods of line fishing and particularly bottom-set commercial fishing lines targeting wobbegong sharks. The Grey Nurse sharks are particularly vulnerable to these threats due to their late maturation and low breeding success8. They reach sexual maturity at the age of 6-8 years and give birth to 1 or 2 young every second year, thus the population grows very slowly. Furthermore their limited distribution and specific habitats8 make it difficult for them to migrate to other areas. Beach nets, unfortunately, cause the death of hundreds of shark that are caught in the nets and cannot escape5. This also occurs in trawls and fishing nets. Another threat for the sharks is finning. Finning involves cutting the fins of sharks (used for fin-soup) and often it is pregnant female sharks that are caught. The sharks are often thrown back into the ocean, alive without fins. This kind of finning, without using the rest of the shark, has been prohibited by most countries and shark-fishing boats are now strictly controlled by regulatory authorities.
[edit] Conservation efforts
The increased public awareness has led to the development of methods that reduce the unintentional killing of elasmobranch (sharks and rays), turtles and marine mammals. An inexpensive method to reduce bycatch is by using tunnel excluders. These enable sharks, turtles and rays to escape and survive. A prototype used by the Dutch achieved a 40-100% reduction of the bycatch of the most vulnerable species12.
The size and texture of nets are also of great importance for the survival of larger vulnerable species. Often smaller nets have been shown to catch the same amount of target fish and reduce the bycatch greatly, especially of the mature sharks11.
Foundation of protected marine areas is particularly valuable for protecting sharks4 and a new method, taggingCSIRO, can reveal their preferred forage and breeding areas. A good example of managed marine areas is to be found in Jervis Bay, NSW7. Jervis Bay has been divided into zones, some for fishing and some for diving, and the strategy has been approved by both fishermen and divers. However, “Divers that regularly dive at places like South West Rocks in New South Wales will tell you that up to 70 per cent of the sharks there are trailing hooks from line fishing.” says Nicola Beynon from The Human Society International. Another widely used method for preserving sharks is ecotourism such as scuba diving6, cage-diving and feeding of sharks. However, it is crucial that this is strictly managed1,3, and that the sharks’ behaviour is monitored. Feeding and touching of marine animals should be strictly discouraged since it can alter their behaviour, and result in long lasting and severe stress to the animals. Divers have noticed sharks altering their behaviour due to increased hierarchy behaviour around feeding areas. The lowest in the hierarchy become stressed and exhibit unpredictable behaviour, which could result in attacks on humans.
[edit] References
1 Barker, N. H. L. and Roberts, C.M. (2004) "Scuba diver behaviour and the management of diving impacts on coral reefs." Biological Conservation 120(4): 481-489.
2 Clarke, S. C., McAllister, A.K., Milner-Gulland, E.J, Kirkwood, G.P., Michielsens, C.G.J., Agnew, D.J., Pikitch, E.K. Nakano, H. and Shivji, M.S. (2006) "Global estimates of shark catches using trade records from commercial markets." Ecology Letters 9(10): 1115-1126.
3 Commonwealth of Australia (2002) “Recovery Plan for the Grey Nurse Shark (Carcharias Taurus) in Australia.” Environment Australia.
4 Garla, R.C., Chapman, D.D, Wetherbee, B.M. and Shivji, M. (2006) “Movement pattern of young Caribbean reef sharks, Carcharhinus perezi, at Fernando de Noronha Archipelago, Brazil: the potential of marine protected areas for conservation of nursery ground.” Marine Biology 149, pp. 189-199.
5 Krogh, M. and Reid, D. (1996) “Bycatch in the protective shark meshing programme off South-Eastern New South Wales, Australia.” Biological Conservation 77, pp. 219-226.
6 Libosada, C. M. (1998) Ecotourism in the Philippines. Makati City, Bookmark.
7 Lynch, T.P., Melling, L., Hamilton, R., Macready, A. and Feary, S. (2004) “Conflict and Impacts of Divers and Anglers in a Marine Park.” Environmental Management Vol. 33, No. 2, pp. 196-211.
8 Otway, N.M., Bradshaw, C.J.A. and Harcourt, R.G. (2003) “Estimating the rate of quasi-extinction of the Australian grey nurse shark (Carcharias taurus) population using deterministic age- and stage-classified models.” Biological Conservation 119 pp.341-350.
9 Otway, N.M, Burke, A.L., Morrison, N.S. and Parker, P.C. (2003 ) “Monitoring and identification of NSW Critical Habitat Sites for conservation of Grey Nurse Sharks.” NSW Fisheries Final Report Series, No. 47.
10 Pollard, D. A. and Smith, M.P.L.(1999) "Threatened fishes of the world: Carcharias taurus (Rafinesque, 1810)(Odontaspididae)." Evironmental Biology of Fishes 56, pp. 365
11 Tudela, S., Kai, A.K., Maynou, F., Andalossi, M.E. and Guglielmi, P. (2005) “Driftnet fishing and biodiversity conservation: the case study of the large-scale Moroccan driftnet fleet operating in the Alboran Sea (SW Mediterranean).” Biological Conservation 121, pp. 65-78.
12 Zeeberg, J.J., Corten, A. and Graaf, E. (2006) “Bycatch and release of pelagic megafauna in industrial trawler fisheries off Northwest Africa.” Fisheries Research 78, pp. 186-195.
Australian Government, Department of the Environment and Heritage, Marine Species Conservation. [ http://www.deh.gov.au/coasts/species/sharks/greynurse/index.html]
Northern Territory Government, Department of Natural Resources, Environment and the Arts, [1] [2]
CSIRO marine research: [3]
