Harvesting predators: simulation of population recovery and controlled harvest of saltwater crocodiles Crocodylus porosus
Yusuke Fukuda
A D , Grahame Webb B , Glenn Edwards A , Keith Saalfeld A and Peter Whitehead C
+ Author Affiliations
- Author Affiliations
A Northern Territory Department of Environment Parks and Water Security, PO Box 496, Palmerston, Darwin, NT 0831, Australia.
B Wildlife Management International Pty Limited, PO Box 530, Karama, NT 0812, Australia.
C Research Institute for Environment and Livelihoods, Charles Darwin University, Darwin, NT 0909, Australia.
D Corresponding author. Email: yusuke.fukuda@nt.gov.au
Wildlife Research 48(3) 252-263 https://doi.org/10.1071/WR20033
Submitted: 21 February 2020 Accepted: 4 September 2020 Published: 28 October 2020
Abstract
Context: The population of saltwater crocodiles, Crocodylus porosus, in the Northern Territory, Australia, has been recovering from a period of intensive, unregulated harvest (1945–1971) since protection in 1971. Consequently, the management goal is shifting from restoring a seriously depleted population to managing an abundant population through controlled harvests for both commercial purposes and public safety.
Aims: We conducted this study to (1) examine whether the controlled harvest of eggs and adults since protection has had an adverse effect on population size and structure, and (2) explore the effect of future harvest scenarios on population size and structure by adjusting harvest levels of both eggs and adults.
Methods: On the basis of 40 years of population monitoring data and knowledge of population attributes from previous research, we developed density-dependent, structured matrix population models to explore our aims.
Key results: The models supported that the depleted population recovered rapidly under protection and that the harvest rates since protection were benign. The model estimated the 2017 harvested population, 46 years after protection, to be ~102 000 non-hatchlings (>0.6 m crocodiles), of which 42.2% are large (>2.1 m total length) individuals. This is similar to the estimated population before the period of intensive, unregulated harvest. Like other crocodilians, the harvest simulations showed that the viability of the population is highly sensitive to adult survival rates. The estimated population should be able to sustain an annual harvest of up to 135 500 eggs if the harvest of large crocodiles remains small (<500 per year).
Conclusions: Although egg harvest has little impact on population size and structure, population size is sensitive to adult harvest.
Implications: Crocodile populations are highly sensitive to adult survival, which needs to be taken into account when considering future harvest scenarios.
Keywords: carnivore, density dependence, human–wildlife conflict, matrix population model, stage-based, sustainable.
References
Bradshaw, C. J. A., Fukuda, Y., Letnic, M., and Brook, B. W. (2006). Incorporating known sources of uncertainty to determine precautionary harvests of saltwater crocodiles. Ecological Applications: A Publication of the Ecological Society of America 16, 1436–1448.| Incorporating known sources of uncertainty to determine precautionary harvests of saltwater crocodiles.Crossref | GoogleScholarGoogle Scholar |
Briggs‐Gonzalez, V., Bonenfant, C., Basille, M., Cherkiss, M., Beauchamp, J., and Mazzotti, F. (2017). Life histories and conservation of long‐lived reptiles, an illustration with the American crocodile (Crocodylus acutus). Journal of Animal Ecology 86, 1102–1113.
| Life histories and conservation of long‐lived reptiles, an illustration with the American crocodile (Crocodylus acutus).Crossref | GoogleScholarGoogle Scholar | 28657652PubMed |
Bureau of Meteorology (2020). ‘Climate Data Online.’ Available at http://www.bom.gov.au/climate/data/index.shtml [verified 20 February 2020].
Campbell, H. A., Watts, M. E., Sullivan, S., Read, M. A., Choukroun, S., Irwin, S. R., and Franklin, C. E. (2010). Estuarine crocodiles ride surface currents to facilitate long-distance travel. Journal of Animal Ecology 79, 955–964.
| Estuarine crocodiles ride surface currents to facilitate long-distance travel.Crossref | GoogleScholarGoogle Scholar | 20546063PubMed |
Campbell, H. A., Dwyer, R. G., Irwin, T. R., and Franklin, C. E. (2013). Home range utilisation and long-range movement of estuarine crocodiles during the breeding and nesting season. PLoS One 8, e62127.
| Home range utilisation and long-range movement of estuarine crocodiles during the breeding and nesting season.Crossref | GoogleScholarGoogle Scholar | 23650510PubMed |
Carroll, C., and Miquelle, D. G. (2006). Spatial viability analysis of Amur tiger Panthera tigris altaica in the Russian Far East: the role of protected areas and landscape matrix in population persistence. Journal of Applied Ecology 43, 1056–1068.
| Spatial viability analysis of Amur tiger Panthera tigris altaica in the Russian Far East: the role of protected areas and landscape matrix in population persistence.Crossref | GoogleScholarGoogle Scholar |
Caswell, H. (2001). ‘Matrix Population Models: Construction, Analysis, and Interpretation.’ (Sinauer Associates: Sunderland, MA, USA.)
Dunham, K., Dinkelacker, S., and Miller, J. (2014). A stage-based population model for American alligators in northern latitudes. The Journal of Wildlife Management 78, .
| A stage-based population model for American alligators in northern latitudes.Crossref | GoogleScholarGoogle Scholar |
Elsey, R. M., and Woodward, A. R. (2010). American alligator Alligator mississippiensis. In ‘Crocodiles. Status Survey and Conservation Action Plan’. (Eds S. C. Manolis and C. Stevenson.) pp. 1–4. (Crocodile Specialist Group: Darwin, NT, Australia.)
Fergusson, R. A. (2010). Nile crocodile Crocodylus niloticus. In ‘Crocodiles. Status Survey and Conservation Action Plan’. (Eds S. C. Manolis and C. Stevenson.) pp. 84-89. (IUCN Crocodile Specialist Group: Darwin, NT, Australia.)
Fukuda, Y., and Cuff, N. (2013). Vegetation communities as nesting habitat for the saltwater crocodiles in the Northern Territory of Australia. Herpetological Conservation and Biology 8, 641–651.
Fukuda, Y., and Saalfeld, K. (2014). Abundance of saltwater crocodile hatchlings is related to rainfall in the preceding wet season in northern Australia. Herpetologica 70, 439–448.
| Abundance of saltwater crocodile hatchlings is related to rainfall in the preceding wet season in northern Australia.Crossref | GoogleScholarGoogle Scholar |
Fukuda, Y., Whitehead, P., and Boggs, G. (2007). Broad-scale environmental influences on the abundance of saltwater crocodiles (Crocodylus porosus) in Australia. Wildlife Research 34, 167–176.
| Broad-scale environmental influences on the abundance of saltwater crocodiles (Crocodylus porosus) in Australia.Crossref | GoogleScholarGoogle Scholar |
Fukuda, Y., Webb, G., Manolis, C., Delaney, R., Letnic, M., Lindner, G., and Whitehead, P. (2011). Recovery of saltwater crocodiles following unregulated hunting in tidal rivers of the Northern Territory, Australia. The Journal of Wildlife Management 75, 1253–1266.
| Recovery of saltwater crocodiles following unregulated hunting in tidal rivers of the Northern Territory, Australia.Crossref | GoogleScholarGoogle Scholar |
Fukuda, Y., Saalfeld, K., Webb, G., Manolis, C., and Risk, R. (2013). Standardised method of spotlight surveys for crocodiles in the tidal rivers of the Northern Territory, Australia. Northern Territory Naturalist 24, 14–32.
Fukuda, Y., Manolis, C., and Appel, K. (2014). Management of human–crocodile conflict in the Northern Territory, Australia: review of crocodile attacks and removal of problem crocodiles. The Journal of Wildlife Management 78, 1239–1249.
| Management of human–crocodile conflict in the Northern Territory, Australia: review of crocodile attacks and removal of problem crocodiles.Crossref | GoogleScholarGoogle Scholar |
Fukuda, Y., Manolis, C., Saalfeld, K., and Zuur, A. (2015). Dead or alive? Factors affecting the survival of victims during attacks by saltwater crocodiles (Crocodylus porosus) in Australia. PLoS One 10, e0126778.
| Dead or alive? Factors affecting the survival of victims during attacks by saltwater crocodiles (Crocodylus porosus) in Australia.Crossref | GoogleScholarGoogle Scholar | 26020948PubMed |
Grant, A., and Benton, T. G. (2000). Elasticity analysis for density-dependent populations in stochastic environments. Ecology 81, 680–693.
| Elasticity analysis for density-dependent populations in stochastic environments.Crossref | GoogleScholarGoogle Scholar |
Hebblewhite, M., Percy, M., and Serrouya, R. (2003). Black bear (Ursus americanus) survival and demography in the Bow Valley of Banff National Park, Alberta. Biological Conservation 112, 415–425.
| Black bear (Ursus americanus) survival and demography in the Bow Valley of Banff National Park, Alberta.Crossref | GoogleScholarGoogle Scholar |
Hill, R., and Webb, G. (1982). Floating grass mats of the Northern Territory floodplains: an endangered habitat? Wetlands Australia 2, 45–50.
| Floating grass mats of the Northern Territory floodplains: an endangered habitat?Crossref | GoogleScholarGoogle Scholar |
Hines, T. C., and Abercrombie, C. L. (1987). The management of alligators in Florida, USA. In ‘Wildlife Management: Crocodiles and Alligators’. (Eds G. J. W. Webb, S. C. Manolis, and P. J. Whitehead.) pp. 43–47. (Surrey Beatty: Sydney, NSW, Australia; Conservation Commission of the Northern Territory: Darwin, NT, Australia.)
IUCN (2014). ‘The IUCN Red List of Threatened Species. Version 2014.3.’ (IUCN Species Survival Commission: Gland, Switzerland.) Available at www.iucnredlist.org [verified 16 September 2020].
Kendall, B. E., Fujiwara, M., Diaz-Lopez, J., Schneider, S., Voigt, J., and Wiesner, S. (2019). Persistent problems in the construction of matrix population models. Ecological Modelling 406, 33–43.
| Persistent problems in the construction of matrix population models.Crossref | GoogleScholarGoogle Scholar |
Kéry, M., and Schaub, M. (2011). ‘Bayesian Population Analysis using WinBUGS: A Hierarchical Perspective.’ (Academic Press: Burlington, MA, USA.)
Lanhupuy, W. (1987). Australian aboriginal attitude to crocodile management. In ‘Wildlife Management: Crocodiles and Alligators’. (Eds G. J. W. Webb, S. C. Manolis, and P. J. Whitehead.) pp. 145–147. (Surrey Beatty: Sydney, NSW, Australia; the Conservation Commission of the Northern Territory: Darwin, NT, Australia.)
Leach, G., Delaney, R., and Fukuda, Y. (2009). ‘Management Program for the Saltwater Crocodile in the Northern Territory of Australia, 2009–2014’. (Northern Territory Department of Natural Resources, Environment, the Arts and Sport: Darwin, NT, Australia.)
Messel, H., Vorlicek, G. V., Wells, G. A., and Green, W. J. (1981). ‘Monograph 1. Surveys of the Tidal Systems in the Northern Territory of Australia and their Crocodile Populations. The Blyth–Cadell River Systems Study and the Status of Crocodylus porosus Populations in the Tidal Waterways of Northern Australia’. (Pergamon Press: Sydney, NSW, Australia.)
Mills, L. S. (2012). ‘Conservation of Wildlife Populations: Demography, Genetics, and Management.’ (John Wiley & Sons: Chichester, West Sussex, UK.)
Morris, W. F., and Doak, D. F. (2002). ‘Quantitative Conservation Biology: Theory and Practice of Population Viability Analysis.’ (Sinauer Associates: Sunderland, MA, USA.)
Natural Resource Management Ministerial Council (NRMMC) (2009). ‘Code of Practice for the Humane Treatment of Wild and Farmed Australian Crocodiles.’ Available at http://www.environment.gov.au/resource/code-practice-humane-treatment-wild-and-farmed-australian-crocodiles [verified 3 May 2019].
Nichols, J. D. (1987). Population models and crocodile management. In ‘Wildlife Management: Crocodiles and Alligators’. (Eds G. J. W. Webb, S. C. Manolis, and P. J. Whitehead.) pp. 177–187. (Surrey Beatty: Sydney, NSW, Australia; the Conservation Commission of the Northern Territory: Darwin, NT, Australia.)
Nichols, T., and Letnic, M. (2008). Problem crocodiles: reducing the risk of attacks by Crocodylus porosus in Darwin Harbour, Northern Territory, Australia. In ‘Urban Herpetology. Herpetological Conservation’. (Eds J. C. Mitchell, R. E. Jung Brown and B. Bartholomew.) pp. 503–511. (Society for the Study of Amphibians and Reptiles: Salt Lake City, UT, USA.)
Northern Territory of Australia (2013). ‘Animal Welfare Act.’ Available at http://notes.nt.gov.au/dcm/legislat/legislat.nsf/d989974724db65b1482561cf0017cbd2/28ae66acac5f957569257bd7000a75f2?OpenDocument [verified 17 January 2014].
Pople, A. (2004). Population monitoring for kangaroo management. Australian Mammalogy 26, 37–44.
| Population monitoring for kangaroo management.Crossref | GoogleScholarGoogle Scholar |
Read, M. A., Grigg, G. C., Irwin, S. R., Shanahan, D., and Franklin, C. E. (2007). Satellite tracking reveals long distance coastal travel and homing by translocated estuarine crocodiles, Crocodylus porosus. PLoS One 2, e949.
| Satellite tracking reveals long distance coastal travel and homing by translocated estuarine crocodiles, Crocodylus porosus.Crossref | GoogleScholarGoogle Scholar | 17895990PubMed |
Richardson, K. C., Webb, G. J. W., and Manolis, S. C. (2002). ‘Crocodiles: Inside Out. A Guide to the Crocodilians and their Functional Morphology.’ (Surrey Beatty & Sons: Sydney, NSW, Australia.)
Ryan, C. (1998). Saltwater crocodiles as tourist attractions. Journal of Sustainable Tourism 6, 314–327.
| Saltwater crocodiles as tourist attractions.Crossref | GoogleScholarGoogle Scholar |
Saalfeld, K., Fukuda, Y., Duldig, T., and Fisher, A. (2015). ‘Wildlife Trade Management Plan for the Saltwater Crocodile (Crocodylus porosus) in the Northern Territory of Australia, 2016–2020.’ (Northern Territory Department of Land Resource Management: Darwin, NT, Australia.)
Saalfeld, K., Fukuda, Y., Duldig, T., and Fisher, A. (2016). ‘Management Program for the Saltwater Crocodile in the Northern Territory of Australia, 2016–2020’. (Northern Territory Department of Environment and Natural Resources: Darwin, NT, Australia.) Available at https://nt.gov.au/__data/assets/pdf_file/0007/443581/crocodile-management-program.pdf [verified 16 September 2020].
Treves, A., and Karanth, K. U. (2003). Human–carnivore conflict and perspectives on carnivore management worldwide. Conservation Biology 17, 1491–1499.
| Human–carnivore conflict and perspectives on carnivore management worldwide.Crossref | GoogleScholarGoogle Scholar |
Treves, A., Wallace, R. B., Naughton-Treves, L., and Morales, A. (2006). Co-managing human–wildlife conflicts: a review. Human Dimensions of Wildlife 11, 383–396.
| Co-managing human–wildlife conflicts: a review.Crossref | GoogleScholarGoogle Scholar |
Tucker, A. D. (2001). Sensitivity analysis of stage-structured demographic models for freshwater crocodiles. In ‘Crocodilian Biology and Evolution’. (Eds G. C. Grigg, F. Seebacher, and C. E. Franklin.) pp. 349–363. (Surrey Beatty: Sydney, NSW, Australia.)
Wallace, K., Leslie, A., and Coulson, T. (2013). Re-evaluating the effect of harvesting regimes on Nile crocodiles using an integral projection model. Journal of Animal Ecology 82, 155–165.
| Re-evaluating the effect of harvesting regimes on Nile crocodiles using an integral projection model.Crossref | GoogleScholarGoogle Scholar | 22963590PubMed |
Webb, G. J. W. (1977). Chapter 14. The natural history of Crocodylus porosus habitat and nesting. In ‘Australian Animals and their Environment’. (Eds H. Messel and S. Butler.) pp. 239–284. (Shakespeare Head Press: Sydney, NSW, Australia.)
Webb, G. J. W. (1991). The influence of season on Australian crocodiles. In ‘Monsoonal Australia: Landscape, Ecology and Man in the Northern Lowlands’. (Eds M. G. Ridpath, C. D. Haynes, and M. J. D. Williams.) pp. 125–131. (A.A. Balkema: Rotterdam, The Netherlands.)
Webb, G., and Manolis, S. C. (1989). ‘Crocodiles of Australia.’ (Reed Books: Sydney, NSW, Australia.)
Webb, G. J. W., and Manolis, S. C. (1992). Monitoring saltwater crocodiles (Crocodylus porosus) in the Northern Territory of Australia. In ‘Wildlife 2001: Populations’. (Eds D. R. McCullough and H. D. Barrett.) pp. 404–418. (Elsevier Applied Science: New York, NY, USA.)
Webb, G. J. W., and Manolis, S. C. (1993). Conserving Australia’s crocodiles through commercial incentives. In ‘Herpetology in Australia, a Diverse Discipline’. (Eds L. Lunney and D. Ayers.) pp. 250–256. (Surrey Beatty: Sydney, NSW, Australia.)
Webb, G. J. W., and Messel, H. (1978a). Morphometric analysis of Crocodylus porosus from the north coast of Arnhem Land, northern Australia. Australian Journal of Zoology 26, 1–27.
| Morphometric analysis of Crocodylus porosus from the north coast of Arnhem Land, northern Australia.Crossref | GoogleScholarGoogle Scholar |
Webb, G., and Messel, H. (1978b). Movement and dispersal patterns of Crocodylus porosus in some rivers of Arnhem Land, northern Australia. Wildlife Research 5, 263–283.
| Movement and dispersal patterns of Crocodylus porosus in some rivers of Arnhem Land, northern Australia.Crossref | GoogleScholarGoogle Scholar |
Webb, G. J. W., and Messel, H. (1979). Wariness in Crocodylus porosus (Reptilia: Crocodilidae). Wildlife Research 6, 227–234.
| Wariness in Crocodylus porosus (Reptilia: Crocodilidae).Crossref | GoogleScholarGoogle Scholar |
Webb, G. J. W., Messel, H., and Magnusson, W. E. (1977). The nesting biology of Crocodylus porosus in Arnhem Land, northern Australia. Copeia 1977, 238–249.
| The nesting biology of Crocodylus porosus in Arnhem Land, northern Australia.Crossref | GoogleScholarGoogle Scholar |
Webb, G. J. W., Messel, H., Crawford, J., and Yerbury, M. J. (1978). Growth rates of Crocodylus porosus (Reptilia: Crocodilia) from Arnhem Land, northern Australia. Wildlife Research 5, 385–399.
| Growth rates of Crocodylus porosus (Reptilia: Crocodilia) from Arnhem Land, northern Australia.Crossref | GoogleScholarGoogle Scholar |
Webb, G. J. W., Manolis, S. C., Buckworth, R., and Sack, G. C. (1983). An examination of Crocodylus porosus nests in two northern Australian freshwater swamps, with an analysis of embryo mortality. Wildlife Research 10, 571–605.
| An examination of Crocodylus porosus nests in two northern Australian freshwater swamps, with an analysis of embryo mortality.Crossref | GoogleScholarGoogle Scholar |
Webb, G., Manolis, S., Whitehead, P., and Letts, G. (1984). ‘A Proposal for the Transfer of the Australian Population of Crocodylus porosus Schneider (1801), from Appendix I to Appendix II of CITES.’ (Conservation Commission of the Northern Territory: Darwin, NT, Australia.)
Webb, G. J. W., Bayliss, P. G., and Manolis, S. C. (1989). Population research on crocodiles in the Northern Territory, 1984–86. In ‘Crocodiles. Proceedings of the 8th Working Meeting of the IUCN–SSC Crocodile Specialist Group’. pp. 22–59. (IUCN: Gland, Switzerland.)
Webb, G. J. W., Britton, A. R. C., Manolis, S. C., Ottley, B., and Stirrat, S. (2000). The recovery of Crocodylus porosus in the Northern Territory of Australia: 1971–1998. In ‘Crocodiles. Proceedings of the 15th Working Meeting of the IUCN–SSC Crocodile Specialist Group’. pp. 195–234. (IUCN: Gland, Switzerland.)
Webb, G. J. W., Manolis, S. C., and Brien, M. L. (2010). Saltwater crocodile Crocodylus porosus. In ‘Crocodiles Status Survey and Conservation Action Plan’. (Eds S. C. Manolis, and C. Stevenson.) pp. 99–113. (Crocodile Specialist Group: Darwin, NT, Australia.)
Weber, W., and Rabinowitz, A. (1996). A global perspective on large carnivore conservation. Conservation Biology 10, 1046–1054.
| A global perspective on large carnivore conservation.Crossref | GoogleScholarGoogle Scholar |
Wildlife Management International Pty Limited (WMI) (2007). Monitoring Saltwater Crocodile (Crocodylus porosus) Nests in the Northern Territory: Trends in Melacca Swamp over 28 years (1980–2007).
