Estimating the Abundance of Saltwater Crocodiles, Crocodylus-Porosus Schneider, in Tidal Wetlands of the Northern Territory - a Mark-Recapture Experiment to Correct Spotlight Counts to Absolute Numbers, and the Calibration of Helicopter and Spotlight Coun

A mark-recapture experiment was used to estimate the total population of crocodiles in four 10-km sections of the Adelaide River; a downstream and upstream section of the mainstream, and two sections of tidal side creeks. The Petersen Estimate yielded 135 ± 5 (SE) crocodiles in the downstream river section, 54 ± 6 in the upstream section, and 44 ± 2 in the two side creeks combined (approximately 22 crocodiles per 10 km of side creek). For the two mainstream sections and the combined creek sections, respectively, 66%, 59%, and 35% of crocodiles present were sighted in a typical spotlight survey. The probability of sighting a crocodile decreased with increasing density of bank vegetation, with decreasing stream width, and with increasing stream sinuosity. Detection of crocodiles in the downstream river section was also affected by size-related wariness: the probability of detecting hatchlings was large (P = 0.69), and for crocodiles longer than 3.0 m, small (P = 0.15). Diurnal counts of crocodiles from a helicopter were calibrated to spotlight counts from a boat at low tide in a 40-km mainstream section and four tidal side creeks of the Adelaide River system. In river habitats with large exposed mudbanks, aerial observers missed 20-33% of sightable crocodiles in their field of view; and in side creeks lined with dense mangroves, 33-75%. When corrected for this observer bias, helicopter surveys in the mainstream at low spring tide produced similar counts to spotlight surveys. In the mainstream, helicopter counts at low neap tide were 37% lower than spotlight counts at low spring tide, but in side creeks were 45% higher. In comparison with spotlight counts, only 18% and 29% of crocodiles were seen . from a helicopter in two densely vegetated freshwater billabongs. A feasibility survey showed that in remote areas helicopter surveys produced counts of crocodiles similar to those by spotlight surveys from boats, and cost one-quarter as much ($7 km-1 compared with $28 km-1 of habitat surveyed).