 Reference Base  Mortality factors and breeding performance of translocate... |
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Brett, R.A., 1998. Mortality factors and breeding performance of translocated black rhinos in Kenya: 1984-1995. Pachyderm 26: 69-82, figs. 1-8, tables 1-2
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| Location: |
Africa - Eastern Africa - Kenya |
| Subject: |
Translocation - Methods |
| Species: |
Black Rhino |
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Analysis of 121 black rhino, Kenya, 1984-1995. Causes and timing of mortality
Intraspecific fighting with resident rhinos was the major cause of mortality among translocated rhinos, accounting for 12 out of 23 deaths, and more than half of the 19 ?natural' deaths (those not directly caused by humans). 'Ihe next most common cause of death was falling over cliffs, accounting for three rhinos in the Lewa Downs reserve. Other 'natural' deaths included one rhino that fell over a fallen treetrunk, one kflled by an elephant, and another death from old age. Three other deaths were from unknown causes; in two of these cases, fighting was suspected. Most 'natural' deaths occurred in the first five years after introduction, particularly in the case of fighting mortality, which was restricted to the first two years. Closer examination of the timing of fighting mortality after release revealed an initial peak of deaths within the first three months (25%) related to the translocation process, and another peak between six and 12 months in residence. In all cases where combatants were identified (8 out of 12), resident adult males killed translocated rhinos.
There was a slight bias of all deaths in the translocated sample towards adult males and immature females. A similar, but not significant bias was found with deaths from fighting. The sex and age composition of groups of rhinos released in new reserves, and the incidence of mortality with group size show that most deaths occurred to rhinos released in small groups (four rhinos or less). Fighting deaths were only found in small groups, particularly with rhinos released singly. Although it appears that there was a sex and age bias in mortality of small groups (eg. all adult males killed in fights were released singly), this was due to a significant bias in the sex/age composition of small groups translocated towards adult males and immature females (eg. singles and pairs of rhinos).
Mortality factors
Intraspecific fighting of translocated rhinos in the first two years after introduction to new reserves in Kenya was the major mortality factor affecting all sex and age groups. High levels of fighting mortality in translocated populations have also been recorded in southern African populations since 1989, accounting for 41 % of natural deaths. Overall mortality of translocated rhinos has been similar (24% of 148 translocated rhinos, vs.. 21% for Kenya: 1984-95). Intraspecific fighting is clearly a major problem facing rhino managers in translocating animals to form new populations, and in particular, in introducing rhinos to established populations. This factor clearly needs to be reduced in order to promote the rapid growth of translocated populations.
In past rhino translocation programmes in Africa, mortality related to disease, dispersal, capture and stress was more prevalent, and these have now been much reduced through improved capture methodology and translocation management. However, accidental death, the other significant mortality factor identified in this study, may be an inevitable toll on rhinos, translocated or otherwise. The black rhino's vulnerability to falling over cars and getting stuck in wateroles was always a feature of former, very large wild populations. For exarnple, after poaching, these were the major mortality factors in over 700 rhino mortalities recorded by Tsavo East NP wardens D.L.W Sheldnck and FW Woodley between 1949 and 1974 (KNP, 1949-74). A further two cliff deaths occurred on Lewa Downs in 1995, involving the calf and grand-calf of one of the first adult females translocated from Solio in 1984. Published accounts of predation by lions (Panthera leo) and spotted hyaenas (Crocuta crocuta) on black rhino and their calves have been little more than anecdotal; no cases of predation have been reported on this translocated sample, or their calves.
Mortality of rhinos during capture and translocation in Kenya during 1984-95 was relatively low (<10% of translocations), but difficult to compare with recent figures for other range states, where fighting mortality within the first three months after introduction ('post-release fighting') is judged to be 'translocation-related', and subsequent fighting mortality recorded as 'natural'. Although a quarter of all fighting deaths to translocated rhinos in Kenya were recorded within the first three months of residence, fighting mortality occurred through the first 16 months of residence, indicating that it may take some time before the repercussions of the introduction of a strange rhino are felt in terms of the probability of a potentially lethal encounter with a resident.
A recent statistical analysis of this data set examined the effects of 30 different explanatory variables on the probability of individual survival and fighting mortality after translocation, using generalised linear modelling. The explanatory variables tested included attributes of the rhino translocated (eg. sex), of the recipient rhino population (eg. rhino density), of the recipient reserve (eg. estimated Ecological Carrying Capacity (eECC)), of time (eg. years since release) and of the translocation procedure (eg. release group size). Although individual survival was found to be primarily related to attributes of the recipient reserve (eg. reserve eECC) with some effect of the age group of the introduced rhino), the best model of the probability of fighting mortality was related to one factor: the size of the group of rhinos introduced together. It was expected that rhino density, in absolute terms and as a proportion of eECC, would have a strong effect on post-translocadon mortality, perhaps associated with overstocking in some recipient areas. These attributes, however, were found to be of secondary importance. Clear recommendations for managers were derived from this analysis: to improve individual survival after translocation; move rhinos to reserves with excellent habitat (high EECC density) at a relatively low rhino density, and in large groups; and select adults for translocation, and adult females if available.
In Kenya and elsewhere in Africa, sub-adult rhinos (four to seven years old) have often been selected preferential for translocation and younger animals (two to three year old) still with their mothers have also been included. 42 % of the translocated sample studied here were of the sub-adult category; these rhinos are more convenient to move (since they have no dependants), would normally be dispersing in order to establish themselves in a new range in the natal area, and have their whole breeding life ahead of them. These results suggest that immature rhinos (particularly females) are least likely to survive after translocation. This may be because they are least able to look after themselves after release into an unfamiliar area and population, often seeking out the company of other rhinos, which can get them into dangerous situations if they approach an adult male. The poor survival of immature females m Kenya and southern Africa (where one to four year olds were at highest risk) may also be linked to lethal encounters with resident males who try to mate with them.
Another clear finding of this study was the preponderance of deaths (including all fighting deaths) suffered by animals released singly, or in small groups (four or less). The consensus of rhino managers in eastern and southern Africa is that large umbers of rhinos should be moved into a vacant reserve area within a relatively small time, giving little opportunity for any resident animals to assert themselves, or to become aggressive to newcomers Due to financial and other constraints, it is rarely possible to stock a new reserve with more than 20 rhinos in one operation, and thus other variables (eg. the age and sex of translocated rhinos, the density and eECC of recipient reserves) may assume increased importance in translocation decisions.
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