A.). J. Ecol. 1984, Volume 22, pages 143-147

Notes and Records

Census of the Mara River hippopotamus (Hippopotamus amphibius), southwest Kenya, 1980-1982

Five counts of hippopotami inhabiting a 124-km stretch of the Mara River were conducted during a 2-year ecological investigation in southwest Kenya. Three sur- veys (including a ground count) were made during 1980, with two additional aerial counts in 1982. All counts were conducted along 125 km of river from the first bridge near the Tanzanian border to the bridge southeast of Emarti (Fig. 1).

Ground counts (10-23 August 1980) covered the 124-km aerial census section with the exception of a 3 1 -km stretch (sector B) of thick riverine forest that was considered unsafe to enter. Use of a mirror to reflect sunlight from a position downwind of hippopotami was effective in evoking curiosity, bringing most of the animals to the surface for counting. Repeated ground counts over specific sections of river during a 2-year period indicated that ground counts were generally precise.

Flights were made with high wing aircraft at an average altitude of 100 m above ground level, circling sharp bends in the river to provide a clear view. Locations and numbers of individuals within groups were identified by one or two rear-seat observers and recorded on 1 :50,000 topographic maps by the front-seat navigator. Flights were made between mid-morning and mid-afternoon, the period when the maximum number of animals were expected within pools.

On 9 August 1980, the first aerial census was conducted using KREMU's twin- engined Partenavia. The second flight was made on 20 November 1980 using a single-engined Cessna 185 in order to capitalize on slower flying speeds and greater manoeuvrability. The final two aerial counts were made on 17 September and 1 October 1982. Identical procedure was followed and the aircraft were similar, Cessna I80 and 182, with the door adjacent to the observer removed.

The efficiency of aerial census in 1980 was 52% for the Partenavia and 75% for the Cessna, when compared with sectors counted on the ground. The major influence on counting accuracy for both aircraft types was the relative meander of the river. Repeated circling is necessary to follow a convoluted river channel and increases the chances of missing entire groups. Although the efficiency of counting from the Cessna was independent of density, increasing proportions of the population were missed at high densities with the larger, less manoeuvrable Partenavia. The efficiency of the 1980 aerial counts can be approximated by com- parisons with the ground count, though they were conducted 3 months apart, as the results of a 2-year ecological study assured us that major distribution or population changes during this period were unlikely.

The observed increase in numbers within the census section from raw Cessna aerial data between 1980 and 1982 was 18.9%, an intrinsic annual growth rate of 9.0% (Table 1). Most of this increase has occurred outside the Reserve, 2 1.9% annually within sectors D and E.

143

144 Notes and Records

Kissii.

Kilgoris,

I . .

1 STUDY AREA LOCATION MAP 1 .. . .:: , .

PLAINS

Fig. 1. Study area of the census.

Notes and Records 145

Table 1. Number of hippopotami observed for 1980 and 1982 censuses of the Mara River

Sector

A B C D E Total (39 km) (31 km) (4 km) (21 km) (29 km) (124 km)

I980 * Ground 808 94 I89 153 -

Aerial (Part.) 486 327 25 125 I40 I I03 Aerial (Cess.) 677 683 78 I05 50 I593

Aerial (Cess.) 855 595 42 222 180 I894 Aerial (Cess.) 713 595 52 249 I94 1863

I982

*Sector B not ground counted. Part., Partenavia; Cess., Cessna.

The annual rate of increase of hippo within the Masai Mara National Reserve was 16.5O/o from 1959 to 1971 (Darling, 1961; Olivier & Laurie, 1974), 10.3% from 1971 to 1980, and 0 9 % from 1980 to 1982. Historical data for the upper reaches of the Mara River are not available.

Based on I980 Cessna air/ground count efficiencies, we estimated 28 19 hippo- potami within the 124-km census section in 1982, a density of 19.6 hippo km-1 within the Reserve, 8-4 km-I outside. For sector B, which was not ground counted, corrections were based on the overall 1980 Cessna count efficiency.

Hippopotamus densities within the Serengeti National Park were 5.5 animals km-' higher than within the Mara Reserve in 1971 (Olivier & Laurie, 1974). As there is no evidence to indicate a differential rate of increase between the Serengeti and Mara populations, it is assumed that the densities within the Serengeti do not allow for a substantial backflow to the south for the expanding Masai Mara population. The opposite has apparently occurred, with redistribution in a northerly (upstream) wave. The difference in animal numbers inside and outside the Reserve was highly significant between the 1980 and 1982 counts (x2=229, P< 0.00 1).

The movement of animals from areas of high to low density may be behaviour- ally directed, as high densities of hippopotami result in correspondingly high levels of aggression (Attwell, 1963). This is primarily determined through group size, with the territorial behaviour of dominant males resulting in fixed spatial requirements.

Average group sizes for the 1980 and 1982 counts were 10.6 and 8.9, respect- ively. A higher proportion of groups with five to ten individuals as well as large aggregates of forty or more were noted in the 1980 counts (Fig. 2). Group sizes exhibited different modal frequencies between the 1980 and 1982 counts, five to ten and two to five, respectively. Water levels were considerably lower during the counting period in 1980, resulting in fewer groups and a higher proportion of the total population within large groups.

146 Notes and Records

Fig. 2. Frequency distribution of group sizes over the 124-km study section. Data are from 1980 ground count.

An altered rainfall pattern in the Mara-Serengeti area has provided generally wetter dry seasons with resultant increased forage productivity (Sinclair & Norton-Griffiths, 1979). The increase of the Mara hippopotamus population may be in response to this abundance of food. Competition with livestock has not deterred expansion to the present high densities north of the reserve. Livestock biomass along the northern river sectors was low during this study, 7% of the total herbivore biomass compared with KREMU’s average for the Mara Study Area of 42% (Amuyunzu, 198 1). Presently, the riparian ecology associated with the Mara River is supporting the population with little indication of stress.

Acknowledgments We are grateful to the Kenya Range Ecology Monitoring Unit (KREMU) for providing support including aircraft for the two initial aerial counts. Mr J. Ole Naiguran, Senior Warden, assisted in providing permission and a ranger for streamside counts within the Reserve. We appreciate the assistance of the Operation Drake team, headed by Mr I. Gordon. Thanks also go to Mr C. Sandmeier of Buffalo Camp who acted as navigator for the final two aerial counts. The continuous support and encouragement of KREMU’s project manager. Mr D. K. Andere is gratefully acknowledged. Funds were provided by C.I.D.A. and the African Fund for Endangered Wildlife.

References AMUYUNZU, C.L. (I98 1 ) Seasonal populations and distributions of wildlife and livestock within the

ATTWELL, R.I.G. (1963) Surveying Luangwa hippo. Puku 1,29-SO. DARLING, F.F. (1961) An ecological reconnaissance of the Mara plains in Kenya Colony. Wikd.

Mara study area during 1980. KREMU technical report Series No. 3 4 .

Monogr. 5 , 1 - 4 1 ,

Notes and Records 147

Oi.ivim, R.C.D. & LAUKII:., W.A. (1974) Habitat utilization by hippopotamus in the Mara River. E.

SINCLAIK, A.R.E. & NORTON-GRIFFITHS, M. (Eds) ( I 979) Serengeti; Dynamics Q/ an Ecosystem. Ajr . Wild/. J . 12, 249-27 I .

University of Chicago Press, Chicago.

E. L. Karstad and R. J. Hudson, Department of Animal Science, University of Alberta, Edmonton, Alberta T6G 2P5, Canada.

(Received 20 May 1983)