Territory Stories

Management program for the saltwater crocodile in the Northern Territory of Australia 2009-2013

Details:

Title

Management program for the saltwater crocodile in the Northern Territory of Australia 2009-2013

Creator

Fukuda, Yusuke; Delaney, Robyn; Leach, Gregory J

Issued by

Northern Territory. Department of Natural Resources, Environment, The Arts and Sport

Collection

E-Publications; E-Books; PublicationNT

Date

2009-04

Description

The draft program is open for public comment to Friday 29 May 2009. Includes Summary document.

Notes

Date:2009-04; Made available via the Publications (Legal Deposit) Act 2004 (NT).

Language

English

Subject

Crocodylus porosus -- Northern Territory; Crocodiles -- Conservation -- Northern Territory; Crocodiles -- Control -- Northern Territory; Crocodiles -- Government Policy -- Northern Territory

Publisher name

Northern Territory Government

Place of publication

Darwin

Edition

Draft.

Format

60 pages : illustration, maps ; 30 cm.

File type

application/pdf

ISBN

9781921519260

Use

Attribution International 4.0 (CC BY 4.0)

Copyright owner

Northern Territory Government

License

https://creativecommons.org/licenses/by/4.0/

Related links

http://hdl.handle.net/10070/214159[Final Edition]

Parent handle

https://hdl.handle.net/10070/212633

Citation address

https://hdl.handle.net/10070/716134

Related items

https://hdl.handle.net/10070/716136

Page content

Draft Management Program for the Saltwater Crocodile in the Northern Territory 46 APPENDIX 3: SALTWATER CROCODILE DENSITIES IN THE RIVERS MONITORED IN THE NORTHERN TERRITORY Spotlight survey data are used to estimate the density of crocodiles on individual rivers as an index of the total crocodile population in the catchment. The density is calculated in terms of the number (sighting) and biomass (kg) of crocodiles per kilometre of a river surveyed. Three candidate regression models (linear, exponential and logistic) are then fitted to approximate the pattern of population growth for each river. For each river the fit of each model is compared to determine the model that best describes the population growth pattern using model selection parameters of Akaikes information criterion corrected (AICc) (or AICc modified by quasi-likelihood (QAICc) depending on data dispersion), difference in AICc (i), and Akaike weight (wi). The smaller the AICc a model shows, the more support the model is considered to have. If a model has small i (<2), it is generally considered highly supported. The strength of evidence that a model is the best in a set is measured by wi (%). The following graphs plot non-hatchling crocodile density for each river with the model lines fitted. The model selected as best is highlighted in bold. The expected asymptote is also shown on the graphs where the logistic model is selected. Details of the model selection parameters are provided in the table following the graphs. Note that the South Alligator River and West Alligator River in Kakadu National Park are control rivers where no harvesting has occurred since protection in 1971. The sighting density of crocodiles for six of the twelve rivers, including those with intensive harvesting (e.g. Adelaide River), most strongly support the logistic model (Figure 1, Table 1). This suggests that the populations have been stabilising in recent years and are approaching carrying capacity. Other rivers are still increasing either linearly or exponentially due to large variation in natural habitat quality (e.g. availability of nesting sites). The biomass density continues to increase in all the rivers (Figure 2, Table 2). This is as expected for a healthy, recovering population with individuals continuing to grow to a bigger size. Again, the large variation in density between rivers is attributable to varying habitat quality (e.g. the Mary River is known to have an unusually high density with large-sized animals) rather than an impact of harvesting as they show the same trends as the control rivers.