- the difficulty with aerial surveys - Carrying Capacity

Monitoring of Roan, Sable and Tsessebe Numbers

To increase the numbers of roan, sable and tsessebe in Namibia, the management strategy relies on an aggressive process of monitoring the performance of existing populations and starting new populations whenever opportunities are presented. An annual review process should be built into the Management Plan whereby a committee (which may include persons outside government) examines the monitoring data objectively at the end of each year and recommends management decisions to the Ministry for the following year.

Progress towards realising the ecological objectives for roan, sable and tsessebe depends on knowledge of the trends in population numbers. If considerable effort is to be put into increasing numbers of these species, then it would be desirable to measure the effects of this investment.

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Aerial Surveys

Present air survey techniques are not suited to precise or accurate estimates of roan, sable and tsessebe populations. Most aerial surveys tend to focus on elephants and buffalo and, as a result, roan, sable and tsessebe are usually under-counted. Sable are more visible from the air than roan and tsessebe so that the estimates for this species may be better than the other two. In deriving estimates for these species a dual problem may occur. If the animals are not present in large groups or large numbers, they tend not to be seen and the resultant estimates suffer from large confidence intervals because too few animals make up the sample for the estimate. If all the animals are being seen but the population is sparse, the estimates will still have large confidence intervals for the same reason. Typical confidence limits for estimates of the species are seldom better than + 50% of the value of the estimate even when the numbers are fairly high, and caution needs to be exercised in pronouncing apparent upward or downward trends in populations.

However, there appears to be little in the way of alternatives which will produce superior results. Population estimates would be greatly improved in the areas where roan, sable and tsessebe populations are being established if annual surveys using the standard methodology of Craig (2000) were done at a fairly high sampling intensity (10-20%) using the same strata on every survey and, where possible, the same observers. The required operating costs for state protected areas contain a provision for such a survey for each area every year. It is recommended that Botswana and Namibia collaborate on annual air surveys to achieve this.

In addition to annual aerial surveys, there is sufficient justification to merit experimenting with alternative methods, including the development of local community monitoring systems, adaptive quota setting and the method of inference from monitoring the age of hunting trophies. The critical requirements for monitoring systems are

• self-diagnosis and self-correction of management
• the system must be capable of being expanded depending on the priorities which emerge for monitoring.

Carrying capacity

It is difficult to provide general criteria for what might constitute carrying capacity for these species and the situation is likely to be area-specific. By definition, a population is approaching carrying capacity when its rate of population increase is beginning to slow down. It is recommended that this criterion is treated fairly liberally so that whenever there is a local abundance of any of the three species, whether or not there is data to suggest the population growth rate is decreasing, the opportunity should be taken to begin another population.

The monitoring of roan, sable and tsessebe population trends has a significance for decision-taking under this Management Plan. It is recommended that any population of roan, sable and tsessebe which is not increasing at a growth rate greater than 5% (i.e. the population is effectively stationary), should be translocated in its entirety to an area where conditions are more favourable for its increase. In making this assessment, the current status of the cumulative rainfall deviations above and below the annual mean rainfall needs to be taken into account. If the rainfall regime is in a deficit mode it is unlikely that populations will increase and the best that can be hoped for is that they can be maintained with minimum loss until the rainfall regime enters a surplus mode.

In general, most monitoring activities should be applied within an adaptive management framework. This includes keeping the levels of research and monitoring affordable and sustainable. An important purpose of monitoring is to assess whether the objective of increasing roan, sable and tsessebe populations is being achieved. If any single limiting factor or a combination of factors is exerting an influence which is keeping a particular population of roan, sable or tsessebe depressed and management actions are not ameliorating the situation, a decision as to whether to relocate the population should be taken.

One criterion on which to base such a decision is simply the population growth rate, as stated in the previous subsection. However, there may be qualitative factors to be taken into account which cannot be specified with rigid criteria in this Plan and which will rely on the judgement of senior officials in the Ministry of Environment and Tourism.

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