Abstract
Research and conservation of wild crocodiles and husbandry of captive crocodiles requires the reliable identification of individuals. We present a method using the individual colour markings on the first 10 single-crest scutes on the tails of Nile crocodiles (Crocodylus niloticus). The scutes are scored by number for colour, with a prefix for left or right providing a binary 11-digit identification number (identification numbers [IDs]; e.g. 12232232242 and 22333233232) per crocodile. A survey of 359 captive Nile crocodiles showed no duplication. However, 42% had asymmetrical scute markings requiring a binary approach. There does not seem to be a change in patterns with age, except that the number of missing scutes increased. A small trial showed that this method can be applied in the field, although more work is needed to determine observer bias and establish parameters for observability in the field. It is unlikely that both left and right IDs would be obtainable for each individual, but other distinctive markings such as scute shape and damage can be used to register the two IDs to one individual. Having two independent IDs for each crocodile provides the possibility of two independent population estimates for equal effort without having to link left and right IDs to individuals. Our proposed method would be useful in conservation, individual tracking and husbandry.Conservation implications: A non-invasive marking and recapture method for Nile crocodile is presented whereby the first 10 single-crest scutes are scored for colour, allowing conservation practitioners to count and monitor crocodile populations and individuals. This method provides two equal-effort estimations of population size, as left and right hand sides are scored independently.Keywords: Crocodylus niloticus; identification; mark - recapture; mark - resight
Highlights
The reasons for accurate counts of crocodiles in any system can vary, but most likely mainly involves research and conservation
We describe and test a binary 11-digit, identification system for individuals based on the colour and presence or absence of the first 10 proximal scutes of the single crest
The chance of two Nile crocodiles in a population of 500 having the same identification number (ID) is less than 0.0001%, assuming that the colouration patterns are expressed randomly
Summary
The reasons for accurate counts of crocodiles in any system can vary, but most likely mainly involves research and conservation. Elsewhere in southern Africa, crocodile numbers have declined in Loskop Dam (Oberholster et al 2010) and Flag Boshielo Dam (Dabrowski, Oberholster & Dabrowski 2014), Ndumu Game Reserve (Calverly & Downs 2014), Lake Sibaya (Combrink et al 2011), and the panhandle of the Okavango Delta (Bourquin & Leslie 2011), or remained constant or increased slightly in the Gonarezhou National Park (Zisadza-Gandiwa et al 2013) and some rivers in the KNP (Ferreira & Pienaar 2011). Correction factors or estimates can be applied to any counting technique to derive population estimates (Ferreira & Pienaar 2011; Combrink et al 2011; Calverley & Downs 2014; Bayliss et al 1986)
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