Tana Delta Irrigation Project, Kenya: An Environmental Assessment

Rehabilitation of the Tana Delta Irrigation Project, Kenya: An Environmental Assessment.
SUMMARY
A census was carried out to clarify the conservation status of the Tana River red colobus and
Tana River crested mangabey to assess the current situation within a spatial and temporal
framework. Intermittent data is available for the 23 forests in the lower Tana River delta area
beginning in 1972 with the last census carried out in 2001. The 2001 data shows that
approximately 20 groups of red colobus in eleven forests (45.8% of the forests surveyed) and
14 groups of mangabey in nine forests (37.5% of forests surveyed) remained.
This study observed/heard 14 groups of colobus in six forests and in one previously
unlabelled forest patch. In addition, a single colobus individual remains in an eighth forest.
Total forest patches having colobus is 34.8%. Twelve groups of mangabeys were seen in
seven (31.8%) forest patches. The results show a decrease from the 2001 group numbers of
six and two groups, for red colobus and mangabeys respectively. The numbers of colobus
individuals have reduced significantly from the 1994 census, decreasing from 260 to 127
whereas mangabey individuals have stayed stable (144 in 1994, >149 in 2005).
Colobus distribution has continued with a fairly regular pattern over the census years of two
populations, a very small one in the northern part of the TARDA managed area and a larger
population centred on forest #58. Colobus were observed in varying forest sizes while
mangabeys appear to have favoured the larger forests, as they were found only in the six
largest forest blocks censused and two small isolated patches.
GIS maps in this report illustrate forest area surveyed, troop locations for colobus,
mangabeys, sykes and baboons. In addition, changes in distribution over time are presented
for colobus and mangabeys.
INTRODUCTION
A census of the Tana River red colobus and Tana River crested mangabey in the Tana and
Athi River Development Authority (TARDA) area of the lower Tana River delta was carried out
between 3 rd January and 11 th January 2005. The census was carried out to assess the
conservation status and distribution of these endemic and endangered species. The
assessment is part of an overall project to develop habitat corridors included in the TDIP
project.
METHODOLOGY
A primate census was carried out in forests in the TARDA area of the lower Tana River,
Kenya. Using the Marsh/Decker forest designation system, 22 patches plus one unnumbered
patch were censused: #46 to #69 inclusive, excluding #51 and #52. See Forests Surveyed
map.
The field map used for this census was a 19xx TARDA irrigation scheme technical drawing
which included delineations of forest boundaries. Forest boundaries in the field were
estimated based on change of vegetation generally from forest and isolated trees and bush to
bushland or grassland.
Census data was gathered between January 3 rd and 11 th 2005. Quadratic transect data
collection methods (based on Struhsaker, 1981) were used with transect spacing set at 50
metre intervals with field errors ranging ±20 metres. Initially five teams were employed which
increased to eight teams on 7 th January once the local field technician team from Wenje and
the Colobus Trust team had spent time censusing together. During this time, the Colobus
Trust team familiarized themselves with vocalisations and behaviours of the colobus and
mangabeys. The Colobus Trust team was already competent in the survey techniques. Field
teams normally consisted of one field technician responsible for setting the bearing and data
collection and one field worker to assist in cutting the transect and observing for monkeys.
Two of the eight teams were accompanied by a guard for security purposes.
Morning transects were generally walked between 06:00 and 10:30 while afternoon transects
began at 15:00. Only in the largest forest #64 did the teams continue censusing through
midday in order to finish the survey before stopping. All forests were censused once.
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