Scalloped hammerhead
Introduction to the scalloped hammerhead and why they are important
Scalloped hammerhead (Sphyrna lewini) are found globally in tropical and sub-tropical waters. In Australia they extend across northern Australia and throughout the Torres Strait, the Great Barrier Reef, the Coral Sea and south through Sandy Strait into NSW. The Queensland east coast shark fishery operates almost exclusively in shallow nearshore coastal waters. Based on catches in this fishery, there appears to be strong sex and size segregation, with predominantly males and juveniles only in the catches (Harry et al., 2011a; Harry et al., 2011b). The vast majority of east coast scalloped hammerhead are caught in the gillnet fishery, however a small number are also taken in the trawl fishery, although this has been reduced by the introduction of bycatch reduction devices, and also in the shark control program (Harry and Tobin, 2014). Although historical catch reporting by species has been poor, this is improving and the recent introduction of a total allowable catch for sharks has resulted in fewer scalloped hammerheads being caught (Harry and Tobin, 2014).
Globally, scalloped hammerheads are listed as Endangered by the IUCN Red List of Threatened Species. In 2014 they were added to Appendix II of the Convention on International Trade in Endangered Species (CITES), requiring countries exporting products derived from this species to certify that they were legally sourced and that their capture will not have a detrimental effect on the population. The species was also listed on Appendix II of the Convention on Migratory Species (CMS) in 2015, which under Australian law would require it to be protected, however the Australian Government took a reservation against this listing and so protection was not required. Recently, scalloped hammerhead was assessed for threatened species listing under the EPBC Act and determined to be ‘conservation dependent’ (Anon, 2018).
Key concepts that relate to connectivity
Although genetic studies show a single homogeneous east coast population also shared with Indonesia, vertebral chemistry analyses indicate several separate stocks comprising mainly of juveniles and adult males (Ovenden et al. 2011; Schroeder et al. 2011). The spatial scale of stock separation was in 100’s of km’s with differences among the Far North/Cairns regions, Cairns/Townsville, Mackay/Brisbane and Brisbane/northern NSW (Schroeder et al. 2011). The study sample locations do not allow discrimination of stocks among jurisdictions north and south of the GBRMP, however they do provide an indication of the possible spatial scales of population connectivity.
Importantly, the stock structuring shown by Schroeder et al. (2011) was comprised of males and juveniles. Chin et al. (2017) found that, from extensive fisheries observer and monitoring data collected around much of northern Australia, no adult female scalloped hammerheads were recorded on the east coast. They do acknowledge that fisheries on the east coast are nearshore and that, based on observations elsewhere, adult females appear to prefer more offshore continental ridges and edges. Also, Noriega et al. (2011) report some pregnant female scalloped hammerheads recorded in the Queensland Shark Control Program on the east coast, although Chin et al. (2017) express concerns over the accuracy of species identification from this dataset. Numerous adult females however are recorded from adjacent waters of Indonesia and PNG, as well as north-western Australia, suggesting that females may migrate to Australian nearshore waters to give birth. It is likely that distant populations of female scalloped hammerheads are critical to the replenishment of Australian populations (Ovenden et al. 2011; Welch et al. 2011). It is also likely that Australian scalloped hammerheads are under threat from high levels of fishing in Indonesia and Papua New Guinea that predominantly capture females (Anon, 2018; Lack and Sant, 2009; White et al., 2008; White and Kyne, 2010).
Multiple traditional tagging studies are consistent with these results and show scalloped hammerheads sometimes make long distance migrations of up to 3,000 km (Holland et al. 1993; Ketchum et al. 2014; Kohler and Turner 2001). These movements make the management of this species more difficult when fishing pressures differ between jurisdictions sharing stocks (Chin et al. 2017).
References
Anon (2018) Listing Advice: Scalloped hammerhead, Sphyrna lewini. Threatened Species Scientific Committee, Established under the Environment Protection and Biodiversity Conservation Act 1999, Canberra, Australia. https://www.environment.gov.au/biodiversity/threatened/species/pubs/85267-listing-advice-15032018.pdf
Chin, A., Simpfendorfer, C.A., White, W.T., Johnson, G.J., McAuley, R.B. & Heupel, M.R. (2017) Crossing lines: a multidisciplinary framework for assessing connectivity of hammerhead sharks across jurisdictional boundaries. Scientific Reports 7,46061.
Harry, A. V., Macbeth, W. G., Gutteridge, A. N. & Simpfendorfer, C. A. (2011a). The life histories of endangered hammerhead sharks (Carcharhiniformes, Sphyrnidae) from the east coast of Australia. Journal of Fish Biology 78, 2026-2051.
Harry, A. V., Tobin, A. J., Simpfendorfer, C. A., Welch, D. J., Mapleston, A., White, J., Williams, A. J. & Stapley, J. (2011b). Evaluating catch and mitigating risk in a multi-species, tropical, inshore shark fishery within the Great Barrier Reef World Heritage Area. Marine and Freshwater Research 62, 710-721. https://dx.doi.org/10.1071/MF10155
Harry, A and Tobin, A. (2014) Scalloped hammerhead, Sphyrna lewini. In: Welch, D.J., J. Robins, and T. Saunders (editors) (2014) Implications of climate change impacts on fisheries resources of northern Australia. Part 2: Species profiles. Final report to Department of Agriculture, Fisheries and Forestry, and the Fisheries Research and Development Corporation, Canberra, Australia. https://docs.wixstatic.com/ugd/9d521f_3bdc7908b0b84f25a8a17caf74903dac.pdf
Holland, K., Wetherbee, B., Peterson, J., & Lowe, C. (1993). Movements and Distribution of Hammerhead Shark Pups on Their Natal Grounds. Copeia, 1993(2), 495-502. doi:10.2307/1447150.
Ketchum, J.T., Hearn, A., Klimley, A.P., Peñaherrera, C., Espinoza, E., Bessudo, S., Soler, G. and Arauz, A. (2014) Inter-island movements of scalloped hammerhead sharks (Sphyrna lewini) and seasonal connectivity in a marine protected area of the eastern tropical Pacific. Marine Biology (2014) 161(4): 939-951. https://doi.org/10.1007/s00227-014-2393-y
Kohler, N.E. and Turner, P.A. (2001) Shark tagging: a review of conventional methods and studies. In: Tricas, T.C. and Gruber, S.H. (eds) The behavior and sensory biology of elasmobranch fishes: an anthology in memory of Donald Richard Nelson. Developments in environmental biology of fishes, vol 20. Springer, Dordrecht.
Lack, M. & Sant, G. (2009) Trends in Global Shark Catch and Recent Developments in Management. TRAFFIC International Cambridge.
Noriega, R., Werry, J. M., Sumpton, W., Mayer, D. & Lee, S. Y. (2011). Trends in annual CPUE and evidence of sex and size segregation of Sphyrna lewini: Management implications in coastal waters of northeastern Australia. Fisheries Research 110, 472-477. DOI 10.1016/j.fishres.2011.06.005
Ovenden, J.R., Morgan, J.A.T., Street, R., Tobin, A., Simpfendorfer, C., Macbeth, W. & Welch, D.J. (2011) Negligible evidence for regional genetic population structure for two shark species Rhizoprionodon acutus (Ruppell, 1837) and Sphyrna lewini (Griffith & Smith, 1834) with contrasting biology. Marine Biology 158,1497-1509.
Schroeder, R., Simpfendorfer, C. and Welch, D.J. (2011) Population structure of two inshore shark species (Sphyrna lewini and Rhizoprionodon acutus) using laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) along the east coast of Queensland, Australia, In: Welch, D. J., Ovenden, J., Simpfendorfer, C., Tobin, A., Morgan, J. A. T., Street, R., White, J., Harry, A. H., Schroeder, R. & Macbeth, W. G. (2011). Stock structure of exploited shark species in north eastern Australia. Report to the Fisheries Research & Development Corporation, Project 2007/035. Fishing & Fisheries Research Centre Technical Report No. 12,. p. 130. Townsville, Australia: James Cook University.
Welch, D. J., Ovenden, J., Simpfendorfer, C., Tobin, A., Morgan, J. A. T., Street, R., White, J., Harry, A. H., Schroeder, R. & Macbeth, W. G. (2011). Stock structure of exploited shark species in north eastern Australia. Report to the Fisheries Research & Development Corporation, Project 2007/035. Fishing & Fisheries Research Centre Technical Report No. 12,. p. 130. Townsville, Australia: James Cook University.
White, W.T., Bartron, C. & Potter, I.C. (2008) Catch composition and reproductive biology of Sphyrna lewini (Griffith & Smith) (Carcharhiniformes, Sphyrnidae) in Indonesian waters. Journal of Fish Biology 72,1675-1689.
White, W.T. & Kyne, P.M. (2010) The status of chondrichthyan conservation in the Indo- Australasian region. Journal of Fish Biology 73,2090-2117.