Mark-Recapture to Assess Factors Affecting the Proportion of a Nile Crocodile Population Seen During Spotlight Counts at Ngezi, Zimbabwe, and the Use of Spotlight Counts to Monitor Crocodile Abundance

(1) Changes in the absolute abundance of Nile crocodiles (Crocodylus niloticus Laurenti) at Lake Ngezi, Zimbabwe, were followed in 1979-82 by mark-recapture in three size-classes (< 1.2 m, 1.2-2.5 m, > 2.5 m total length). (2) Throughout the study period, there were twenty-eight crocodiles > 2.5 m long and forty crocodiles of 1.2-2.5 m in the population, but the number of animals < 1.2 m long declined from fifty-four in 1979 to eleven in 1982. (3) Over the same period, forty-six spotlight counts, standardized with respect to observer, route, speed and time of night, were conducted. Twelve environmental variables of probable importance to crocodile activity or visibility were measured during each count. These were included in a multiple regression analysis to quantify the effect of the environment on the proportion of the population seen during counts. (4) The proportion of the population counted by spotlight ranged from 0.10 to 0.63 with a mean of 0.36 ± 0.13 (S.D.) over the study period. (5) Two environmental variables, water-level and the difference between water and air temperatures, accounted for 64% of the variation in the proportion of the total population seen during counts. (6) Correction factors to convert spotlight counts to absolute numbers were calculated for the whole population and separately for each size-class. (7) Analysis shows that uncorrected counts undertaken in October/November at the time of lowest water and highest temperatures would have been useful indices of abundance at Ngezi. The use of simple standardized spotlight counts to monitor crocodile abundance is discussed.