Long-term harvesting and male migration in a New Zealand population of Himalayan tahr Hemitragus jemlahicus

Authors


* Present address and correspondence: Dr David M. Forsyth, Centre for Biodiversity Research, Department of Zoology, University of British Columbia, 6270 University Boulevard, Vancouver, BC V6T 1Z4, Canada. Fax: (604) 822 2416. E-mail: forsyth@zoology.ubc.ca.

Abstract

1. Sexual differences in behaviour and ecology have important implications for the management of harvested ungulate populations.

2. This study investigated the long-term effects of harvesting on the abundance and distribution of sexes within an introduced population of Himalayan tahr in the Two Thumb and Sibbald Ranges, New Zealand. Annual counts of tahr were made from 1984 to 1996 in two catchments subject to different harvest regimes.

3. Although populations in the two catchments increased at similar rates over the study period, their adult sex ratios diverged significantly in the late 1980s. In Carneys Creek, which was subject to unrestricted harvest, the adult sex ratio was initially equivalent to the estimated population sex ratio, but from 1991 it was significantly male-biased. In North Branch, 10 km from Carneys Creek but subject only to adult (trophy) male harvest, the population was significantly female-biased in all years.

4. There was a large and male-biased harvest in Carneys Creek in 1993, suggesting that immigration of males may have been responsible for the long-term changes in sex ratio observed there. In contrast, the harvest of males in North Branch was insufficient to explain its female-biased population.

5. Monthly counts of male and female tahr in Carneys Creek between December 1993 and February 1996 revealed a consistent seasonal change in the abundance of sub-adult males, and perhaps adult males, but not females. Sub-adult males immigrated into Carneys Creek during spring, increasing to a summer maximum before emigrating during autumn; few sub-adult males were present during winter.

6. Counts at six additional sites during summer and autumn 1996 indicated that the probable source of these males was an adjacent hunting reserve. This hunting reserve, which included North Branch, contained a female-biased population with moderate female density.

7. The observed changes appear to have been a consequence of sexual differences in habitat selection and mobility. In particular, outside the rut the larger-bodied males avoided habitats containing high densities of females.

8. Harvesting has the potential to affect the spatial distribution of age–sex classes for sexually dimorphic ungulates. Managers need to consider these effects when designing and interpreting harvest and monitoring programmes.

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