Aspects of monitoring wild and captive Nile crocodile (Crocodylus niloticus) populations in southern Africa.

Abstract

As biodiversity across the globe declines because of anthropogenic activities, the need for conservation efforts increases. For conservation efforts to be successful, it is imperative that detailed information about species and their populations; size and status within and outside of protected areas be collected. In freshwater systems, crocodile population demographics can provide an integrated view of ecosystem state, but the habitat and cryptic nature of crocodilians confound the derivation of population demographics for the taxa. Crocodile populations were historically monitored by fixed-wing aircraft, helicopter or limited spotlight surveys in those areas that are navigable by boat. These techniques are costly and labour-intensive; require specialised personnel and equipment, and are subject to observer bias and low accuracy in size class estimations. Furthermore, they produce population demographic data that is not verifiable as they rely on decisions and opinions of observers in the moment of surveying, often from fast-moving platforms. Lately, unmanned aerial vehicle (UAV) techniques have been shown to accurately and effectively count crocodiles, but they still require costly software and hardware packages. In this study, low-cost, open-source UAV techniques were developed as an alternative method to monitor and survey crocodilians, particularly Nile crocodiles (Crocodylus niloticus), both in captivity and in the wild. In South Africa, Nile crocodiles occur in open bedrock systems with relatively little riparian vegetation, an ideal scenario for the application of UAVs. The possibility of improved population demographic data for wild Nile crocodile populations by converting size data derived from UAVs to age data was explored by radiocarbon dating Nile crocodile claws. Morphometric correction factors applicable to UAV census are derived, and a fixed-wing survey is compared with a commercial-grade UAV survey of wild Nile crocodile populations in the Kruger National Park. The limitations and applicability of these approaches for crocodilian and other ecological studies were assessed. Their future applications in ecology and management are proposed.