To understand the abundance, distribution, and population trends of the common hippopotamus (Hippopotamus amphibius) in the Gonarezhou National Park (GNP), Zimbabwe, we (i) carried out an aerial survey along sections of major perennial rivers, namely Runde, Save and Mwenezi and other inland water pans, and (ii) analysed the long-term aerial survey data (1965–2008). For the aerial survey, a Piper PA-18 Super Cub aircraft was flown once along each of the three major rivers and associated pans in November 2008. We recorded a total of 187 hippos and 19 groups in the GNP. All sightings were in Runde River, Tambohata Pan and Massasanya Dam (density = 2.4 hippos/km). We found three trends in hippo populations in the GNP for the period 1965–2008. First, the period 1965–1982 was characterized by a significant increase in the hippo population (simple linear regression: loge[hippo population estimate] = 0.05[year] - 89.01; F1,13 = 56.26, P < 0.001, R2 = 0.81; average annual exponential rate of increase [r] = 0.05, 95% confidence interval [CI] for r = 0.03–0.06); second, the period 1983–1997 was characterized by a significant hippo population decline (simple linear regression: loge[hippo population estimate] = 355.87–0.18[year]; F1,10 = 10.44, P < 0.001, R2 = 0.51; r = -0.18, 95% CI for r = -0.30 to -0.05) and thirdly, r between two survey counts in 1997 and 2008 was 0.06. We attributed the overall decline in hippo abundance, the disappearance of the hippos in the Mwenezi River, and their huge decline in the Save River in the GNP primarily to past droughts, siltation and persecution in adjacent communal areas. We recommend collaborative efforts by the relevant authorities to ensure a continuous flow of water in the Mwenezi River. This will allow for the maintenance of hippo habitats downstream of the Manyuchi Dam, thus creating conducive environment for the re-establishment of the hippos in the Mwenezi River section in GNP. We also recommend that GNP's management should continue to monitor the hippo population and distribution in the park's major rivers and natural pans.
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Vol. 40 • No. 2