*The individual populations of roan on commercial farms are all very small numbering from less than 10 to 60 animals.

Data Quality and Interpretation

The data available for roan, sable and tsessebe are not very valuable for comparative purposes or for detecting trends:

• the same areas have not been surveyed consistently from year to year (this effect is particularly prevalent in the Caprivi) and
• different survey techniques have been used on different surveys. The surveys which have been carried out include
  • waterhole counts,
  • total counts from fixed wing aircraft,
  • total counts from helicopters and
  • sample surveys based on line transects with calibrated strip widths.

At present there is no acceptable alternative to the standard transect survey method or the random block count method.

Tthe population estimates for all areas (with the exception of roan in the Waterberg from 1975-1990) are too erratic to justify any attempts to fit modelled data to observed numbers. It is necessary to be conscious at all times that many of the observed swings in population numbers may be no more than artifices arising from irregular and incomplete surveys.

• the accumulated rainfall surplus reached a peak in the five years between 1975 and 1980. It was also during this time that the populations of all three species appeared to be booming.
• After 1980 the surplus began to decrease although it did not change to a deficit in all areas simultaneously.12 After 1994 all areas went into a rainfall deficit mode13 and this appears, on most of the figures, to coincide with population 'slumps' for all species populations.
• Therelationship with the cumulative rainfall surpluses and deficits becomes increasingly critical as the lower rainfall limits of the species range are approached. In the Eastern Caprivi, where annual rainfall is normally well above 500mm, the effects appear less striking. In Kruger National Park, the range preferred by roan and tsessebe is in a marginal rainfall area and the rainfall deficit/surplus relationship is strong. The decline of roan and tsessebe since the 1970s in north-western Zimbabwe may be due to the same phenomenon.
• There is little to be gleaned from the late dry season rainfall data, mainly because the small ups and downs in population numbers from year to year which this factor would influence are obscured by the poor population estimates.