The state of White Shark populations in Australia

The white shark (Carcharodon carcharias) is the most iconic predatory fish on Earth. They are found in temperate and sub-tropical coastal waters throughout the world. In Australia, the debate over protection of white sharks has been heating up. Incidents of attacks on both the West and East coast of Australia and reports of increased sightings of white sharks in recent years is fuelling concerns that this might be linked to an increase in the number of white sharks. This has lead to premature calls by some political figures for the delisting of white sharks and removal of their protection in Australian waters (see here), pending (or not) the results of the report (see here and [3]). In Australia the white shark is currently listed as vulnerable under the Federal Environment Protection and Biodiversity Conservation Act 1999 and protected internationally under Appendix II of the Convention on International Trade in Endangered Species (CITES) and the International Union for Conservation of Nature (IUCN, Category VU A1cd+2cd).

Some would argue that the protection of White Sharks was not based on evidence (see here). However it could be equally argued that conjecture, anecdotes and unconfirmed accounts are not evidence for increasing numbers of sharks. Studying animals underwater is extremely difficult. White sharks are difficult to find, catch and handle so we know significantly less about many marine animals in comparison to their terrestrial counterparts. Despite this, many rigorous scientific studies have contributed to our understanding of white sharks in Australian waters. Most recently,  independent studies have identified two distinct populations in the east and west separated by the bass straight [1,2].  The first is an eastern population ranging along the east coast from Tasmania to central Queensland; the second is a western population ranging from western Victoria to north-western Western Australia. These populations are genetically distinct [1] and tracking studies of their movement support this two-population model [2].

When it comes to the question, how many white sharks are there, research by the CSIRO through the National Environmental Science Program’s Marine Biodiversity Hub [see 3] have used close-kin-mark recapture (a DNA based method using the recapture of closely-related kin) to estimate current adult abundance for the eastern Australian population as 750(CI 470 to 1,030). They estimate the southern-western adult population is estimated as 1,460(CI 760 to 2,250). Including the available information about juvenile White Sharks, estimates of total size for the eastern population in 2017 was 5,460, with an uncertainty range of 2,909 to 12,802 [3].

However, the core of this debate – whether white sharks could have increased in abundance to a level where they no longer need our protection– needs to be addressed with long term monitoring to determine trends. Likewise, population models on which to base management decisions need more detailed population trends and estimates (past and present) as well as consideration of biological and ecological factors.

Project GenoJaws will examine evidence for changes in the size of the Australian white shark population based on genetic comparisons of historical and contemporary samples. We will use historical DNA from sharks jaws from around the world to understand genetic diversity in the past and compare this information to contemporary DNA from sharks living today. This information will be further used to look at adaptation and genetic selection in white sharks as well as other important demographic and genetic factors.  The outcomes of this study will bring evidence to support future management and policy decisions.

Written by Danielle Davenport; edited by Danielle Davenport on 02/2018
** Disclaimer ** The opinions expressed in this article are those of the author, and they do not reflect in any way those of the collaborations or institutions to which they are affiliated.

References

[1] Blower, D.C., Pandolfi, J.M., Bruce, B.D., Gomez-Cabrera, M.D.C. and Ovenden, J.R., 2012. Population genetics of Australian white sharks reveals fine-scale spatial structure, transoceanic dispersal events and low effective population sizes. Marine Ecology Progress Series, 455, pp.229-244.

[2] Bruce, B., 2015. white shark population and abundance trends from: http://nerpmarinebiodiversity2015.report/white-shark-population-and-abundance-trends/

[3] Hillary, R. M., Bravington, M. V., Patterson, T. A., Grewe, P., Bradford, R., Feutry, P., … & Duffy, C. A. J. (2018). Genetic relatedness reveals total population size of white sharks in eastern Australia and New Zealand. Scientific reports, 8(1), 2661.

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