By Elisa Botting
With the looming anthropogenic driven sixth mass extinction, the extinction of many organisms is becoming increasingly probable. Organisms that require specialised habitats and are strongly affected by the slightest environmental changes are arguably the first victims of the extinction period. Due to its specialised habitat of subtropical to cool temperate waters and its reliance on a highly sensitive ecosystem, the grey nurse shark is a key example of an at-risk species.1
Why has the population steadily declined?
The grey nurse shark has its populations distributed around the Australian coast and its migration is minimal from the north and the south coast. This has left the eastern and western grey nurse shark populations to become geographically isolated populations. However, as found by a study conducted by the NSW Fisheries from 1998 to 2001, almost all of these populations have been declining rapidly and the populations in New South Wales in particular have fallen to as low as 300 sharks in 2001.1
The population decline can be attributed to several factors that are largely anthropogenically driven. The indirect methods of line fishing and commercial shark fishing are argued to be the largest contributors to the decline of the nurse shark1. This is because they occur and cause deaths on such a large scale that the grey nurse shark with its late maturation (of three months) and low breeding success (of one offspring every two years) is unable to mitigate.1 Deaths caused by fishing are exacerbated by the small dispersal distance of the grey nurse sharks, which limit them in their escape from commercial fishing areas.1
Additionally, the decline of the grey nurse shark population can be attributed to shark control measures such as beach meshing programmes and drum lines.2 These measures are only found in New South Wales and Queensland – habitats in which the nurse shark resides and also passes through when it migrates from the North to the South of Australia.2 Whilst the object of such programmes is to reduce shark attacks close to recreational beaches, they also target specific species for euthanization.2 The grey nurse shark was counted as a target when the project started in 1937 but ceased to be one when they were listed as vulnerable in 1984.3 Nevertheless, the euthanizations of the grey nurse shark prior to 1984 contributed heavily to the decline in the population.
Conservation efforts
Whilst grey nurse sharks have been protected since the 1980s and some conservation efforts have been made, the effect on population size recovery has been minimal, as the population has continued to decline. One conservation effort aimed to reduce deaths by catch through implementing tunnel excluders that allow non-target species, like the grey nurse shark, to escape and survive.4 Some tunnel excluders have been shown to reduce non-target catch by 40-100%.4
Additionally, habitat protection in New South Wales has been deployed to recover the population. The aim was to allow an increase in sexually mature grey nurse sharks that would then allow for an increase in reproductive numbers. However, such efforts were later undermined by the New South Wales government allowing commercial fishing to recommence in Bateman’s Bay Marine Park on the South Coast.5
Current status of the population and future efforts
Due to Australia’s large commercial fishing industry, grey nurse shark deaths by fishing is arguably unavoidable. However, the deaths have the potential to be reduced by using new taggingCSIRO technology to identify common breeding grounds and migration routes that can then be protected6. Further reduction in the grey nurse shark deaths could also be achieved by changing the size of fishing nets. Using smaller nets may maintain the fishing yield, whilst also potentially reducing the number of mature sharks caught. Nevertheless, these methods provide a compromise between the protection of the endangered species and the fishing industry and complete protection of all the habitats of the grey nurse shark. However, the overall future of the grey nurse shark relies on the actions of the government and the public being co-operative but also timely, as delayed action could result in the complete extinction of the species.
References:
1. Australian Government Department of Agriculture, Water and the Environment. Carcharias taurus, Grey Nurse Shark (East Coast population). n.d; Available at: https://www.awe.gov.au/environment/biodiversity/threatened/conservation-advices/carcharias-taurus. Accessed 25 January, 2022.
2. Australian Government Department of Agriculture, Water and the Environment. Death or injury to marine species following capture in beach meshing (nets) and drum lines used in Shark Control Programs. Available at: https://www.awe.gov.au/environment/biodiversity/threatened/nominations/ineligible-ktp/death-or-injury-to-marine-species. Accessed 25 January, 2022.
3. Australian Government Department of Agriculture, Water and the Environment. Grey Nurse Shark. n.d.; Available at: https://www.awe.gov.au/environment/marine/marine-species/sharks/greynurse. Accessed 25 January, 2022.
4. Zeeberg J, Corten A, de Graaf E. Bycatch and release of pelagic megafauna in industrial trawler fisheries off Northwest Africa. Fisheries Research 2006 May 1,;78(2):186-195.
5. Australian Marine Conservation Society. Critically Endangered Grey Nurse Shark Habitat Protection Slashed. 2019; Available at: https://www.marineconservation.org.au/critically-endangered-grey-nurse-shark-habitat-protection-slashed/. Accessed 25 January, 2022.
6. Evans K, Bradford R, Hobday A, Lansdell M. CSIRO code of practice for tagging marine animals (second edition). CSIRO Research Publications Repository 2015 -04-28.
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