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Kaimanawa Wild Horses Plan - 7. Strategies for Controlling the Population Growth Rate

7. Strategies for Controlling the Population Growth Rate

Various approaches for controlling the population growth rate are possible, including altering the environment so that it can support fewer horses, increasing the mortality of the horses, or decreasing the fertility of the horses.

The most effective and humane approach is to decrease the fertility, (lower the birth rate). It has been found in trials conducted in the U.S.A. by Dr Kirkpatrick et al, that sterilising dominant stallions does not reduce the birth rate, as mares mate elsewhere, particularly when they have a long breeding season as in the Kaimanawas. In order to have a significant impact on fertility almost all the stallions would need to be sterilised. In the current horse range, this would be a difficult criterion to guarantee. The alternative is to target the mares.

Since mares between 6 and 10 yrs of age have greater reproductive success than mares outside that age group, it could be possible to target these mares. It is unclear at this stage what effect targeting only the most fertile mares would have on the population. A "compensatory effect" (Kirkpatrick 1990) might appear in the remaining mares and the end result might be no change in overall herd fertility. Furthermore, fertility control does not allow for natural selection caused by natural reproductive behaviour.

Kirkpatrick has run immunocontraception trials and successfully prevented pregnancy in mares on Assateague Island, (USA). This population of 150 feral horses is designated a cultural resource as the horses are descendants of horses left there in 1630 by Virginia colonists, (Kirkpatrick, 1991). The immunocontraception used is porcine zona pellucida (PZP), developed by Dr Liu, from the protein in pig ova. "When PZP was injected into the mare, their blood developed antibodies against that protein. The antibodies became attached to sperm receptor sites on the mares' eggs," (Kirkpatrick 1991). These are the sites that the sperm has to recognise and occupy at fertilization. Immunocontraception is reversible, a mare must be inoculated each year to prevent pregnancy. Kirkpatrick is currently working to extend the period of effectiveness, of the PZP vaccine, to longer than a single year and also to improve remote delivery systems for feral populations.

Problems of interference with natural reproduction (and reproductive behaviour) of the herd arise whether sterilizing the stallions or controlling the fertility of the mares. Decisions would need to be made about which and how many mares to immunise, for how many years, the number of foals (if any) that each mare would be allowed to have? etc. The Department has contracted Massey University to apply and research immunocontraception in the Kaimanawa wild horses. The trial was instigated in 1994 and will proceed until 1998.

Any population control method should have minimal effect on the social organisation or behaviour of the treated horses otherwise population dynamics are changed in unpredictable and possibly damaging ways. The impacts of fertility control on social behaviour must be collected during early trials to determine any adverse effects.

"Immunocontraception is a feasible management option, given an absence of any differences in fecundity between ecological zones and age groups", Claire Veltman (1994 Seminar). Current research shows fertility (foal-to-mare) variation between the different zones. It also shows foal mortality variation. Fecundity however, is not thought to be different between the zones, but does vary between age groups.

Immunocontraception may be better implemented in an area which is more uniform so that the selection of mares to target is more predictable.

If it is not possible to control the reproductive success of the horses, it will be necessary to manage the herd by culling, in which case the options considered by the working party for maintaining a specific herd size would be the same as those for reducing the herd size.

Figure 5 (GIF, 3K), shows the possible choices for managing the wild horse population. Three main alternatives arise for controlling the population size: live mustering, shooting and immunocontraception.

Table 6 (page 66) presents a summary of the relative advantages and disadvantages of the three main options. As considerable debate arises as to whether ground shooting or helicopter shooting is the more humane, the shooting option has been further subdivided.

Mustering

Advantages

  • Live capture(relocation/sale possible)
  • Public approval
  • Trials show low stress on horses
  • Few injuries
  • Public able to see horses
  • Research opportunities needing close proximity enhanced
  • Bands taken in entirely
  • Able to recover some costs
  • Trials show technically feasible

Disadvantages

  • May need several yards over horse range
  • Yards need vehicle access
  • Helicopter & horses riders need experience and skill
  • Relatively costly ($150/horse)
  • Escapees hard to deal with
  • Risk to participants
  • Limited market for live animals
  • Transportation from capture point to abattoir or to holding site (possible stress)
  • Dependent on weather

Shooting from helicopters

Advantages

  • Access to most sites possible
  • Australasian Vets Assoc. say best system
  • Fast follow-up to ensure killing is completed quickly
  • More able to ensure local eradication
  • Helicopter able to remove carcass
  • Relatively quick operation time

Disadvantages

  • Some public opposition
  • May be more stressful than ground shoot
  • Outright kill may be more difficult from moving platform at a moving target
  • Relatively costly compared to ground shoot
  • Risk to operators
  • Dependent on weather
  • Skill shooters/markspeople required

Shooting from ground

Advantages

  • Relatively low cost
  • Able to be selective
  • No need for permanent structures (yards, traps, roads)
  • May be less stressful than helicopter shooting

Disadvantages

  • Difficult to follow-up if not killed outright
  • Difficult to guarantee eradication in area
  • Diminishing returns
  • Carcass disposal problems
  • Time consuming
  • Skill shooters/markspeople required

Immunocontraception

Advantages

  • Humane
  • Non-lethal
  • Acts on birth rate not death rate
  • Flexibility
  • Wide public approval
  • Possibility of low social distruption to horses

Disadvantages

  • Unknown behavioural responses in band
  • Possible compensatory reproduction
  • Must target the most fecund mares to be effective
  • on-going vaccinations are needed to ensure effectiveness
  • Could require muster ($150/horse) and holding for 3 weeks in order to guarantee booster dose.
  • Could prove an expensive option (+$25/dose of vaccine).
  • Delivery systems in remote areas untried in N.Z.
  • Any additions (births) need also be captured, and dosed.

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