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Hippos of the Black Volta River Print E-mail

Tables 3.1 and 3.2 give locations, numbers and age classes of hippos seen during the study.
 
TABLE 3.1. Number and location of hippo groups seen from the river

In total 31 hippo sightings were made 23 from the river and eight from the riverbank. Hippos were most commonly seen in groups of 1-9 animals (mean 5, standard deviation 2.7, median 5, interquartile range 3-7.8). These groups included adult males, adult females, juveniles and babies. All groups of four or more hippos included babies. Aggregations were most common around the outlets of three tributary steams (Lubia, Coco and Japoli), but we could find no evidence that hippos ever enter these tributaries. In the case of the Coco and Lubia streams, dense vegetation make it unlikely that they are frequented by hippos. Outlets of tributaries therefore provide good vantage points from which to estimate hippo numbers during riverbank counts.
 
TABLE 3.2. Number and location of hippo groups seen from the riverbank.

Because few, if any, individuals could be recognised it was not possible to calculate the total number of animals present in this portion of the river. The minimum number present is derived from riverbank sightings on 24 July, when 15 individuals were seen at the same time. However, this is likely to be an underestimate of total numbers, because the hippos seen on this occasion probably represent two groups from the northern sectors of the study area. Another group of at least nine animals occupies the rocky sectors A & B and occasionally stray towards Batoo Village.

Observations suggest the following age structure for the hippo groups in this part of the Black Volta:

TABLE 3.3 Population structure of hippos

A male seen on 19, 21, 28 & 29th July was the only individual we could identify with certainty, because of his large size and dark colouration. This animal was associated with at least one group (Group 3) most of the time, but left the group towards evening, presumably to forage on his own.

Length of river sections used for survey (estimated from the map) averaged 2.3km  (standard deviation 0.47, range 1.8-3.0). The mean number of hippos seen per section of the river (Table 3.3) suggests a  mean density of  2.11 (+/-0.17) hippos per km of river. Extrapolating this for the whole park suggests a total population of  140-164 animals. This number is likely to be an underestimate for three main reasons.

1.  Length of the river in the wet season is underestimated by the map.
2.  "Maximum counts" were based on the largest number of heads seen at any one time. Because large sections of the river were inaccessible to canoes, hippo groups could have been missed. Fishermen’s’ estimates of hippo group sizes were usually higher than ours by one or two animals. They estimated that the group seen on 24th July included over 20 animals. They did not agree with any of our attempts to identify individuals (except the male mentioned above) and did not agree with our conclusion that the river sector contained three groups of animals. According to fishermen, the hippos are very mobile and will travel large distances along the river in a few hours. The hippos seen in the study site could therefore have come from anywhere.
3.  The density of hippopotamus probably increase further into the park. Casual observations further upstream (see below) suggest that there many hippos around Abru Bhunu. The many thickly forested islands in this area of the park could only be surveyed in dry season conditions.
 

AREA     A B C D E
LENGTH (KM) 1.8 3.0 2.0 2.3 2.5
MEAN NO. HIPPOS 4 6.5 4 5 4.7
STANDARD DEVIATION 4.24 1.52 1.41 2.62 4.03
HIPPOS PER KM 2.29 2.17 2.0 2.22 1.88
TABLE 3.4. Distribution of hippos in study area.
 

FIGURE 3.1. Group size for hippos seen from the river


 
 

 

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