Illegal Offtake

Levels of illegal hunting could affect the survival of roan, sable and tsessebe on both State Land and Conservancies. Martin used a population model to explore the maximum illegal harvest which a population of roan, sable or tsessebe could sustain. It is assumed that mortality would affect both sexes and all ages equally. The calculation of the number of years a population would take to reach 2,500 animals is based on an assumed starting population of 250 animals (which is not far off the present estimates for the populations of all three species in their "natural range").

• The generic population model is suitable for all three species
• The model applies only in a situation where rainfall is above 400mm and is not in any major long term rainfall deficit mode. Without any illegal hunting the population grows at slightly under 14% per annum and it can sustain a maximum offtake of about 12%. The higher the proportional offtake, the lower is the growth rate of the population and the effects become severe above a 6% offtake.

Illegal offtake exceeding 12%

To examine rates of population decline when the illegal harvest exceeds 12%, it is not useful to examine percentage offtakes because these result in a lower and lower number of animals being killed as the population declines so that the population tends to stabilise at some low level. A more realistic examination of rates of decline for unsustainable harvests has been done with a fixed number being removed from the population each year which inevitably results in extinction. In the table below, the number of years to extinction is shown for various fixed offtakes from a starting population of 1,000 animals.

Sport Hunting

Martin used a population model to explore the effects of hunting quotas on roan, sable and tsessebe populations.

Findings: Sport hunting quotas for roan, sable and tsessebe should never exceed 2%. (Figure 23)

Again, the assumption is that one model suits all. Hunting selectivity is centred on the 8-9 year old males, with 40% of trophies coming from this age group, but the rest spread fairly evenly over the range of age classes. The available quota of trophy animals is taken from the various age classes in the proportions set by the selectivity profile (i.e. if the quota was 100 animals it would take 5 animals from the 5 year old age-class, 10 animals from the 6 year-old age class, 15 animals from the 7 year-old age class . . . and so on). However, if there are insufficient animals in any age class to meet the quota demand, the animals are then sought in the age class immediately below it as would happen in practice. The percentage quotas apply to the total population and it is assumed that males under 5 years old would not be hunted.

As the quota is increases from zero, the older age classes are 'cleaned out' very quickly (Figure 23). The annual recruitment to the part of the age pyramid from which males are hunted is very low - the proportion of males recruited annually to the 5 year age class is about 3% of the population and, to the 8 year-old age group, it is about 2% of the population. Thus a hunting quota of 2% will claim every male 8 years old and over. The sex ratio in the population in the absence of hunting is 1male:2females and, as hunting quotas increase, this shifts in favour of females. With the quota at 2%, the sex ratio becomes 1male:3.3females.

By monitoring the age of huntin trophies an adaptive quota setting system can be developed, which can be reversed to estimate population sizes based on hunting trophies.