Sustainable hunting regulations take the speed of trophy growth into account

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Schindler, S., Festa-Bianchet, M., Hogg, J., & Pelletier, F. (2017). Hunting, age structure, and horn size distribution in bighorn sheep The Journal of Wildlife Management DOI: 10.1002/jwmg.21259

Hunting regulations aim to keep trophy hunting sustainable. Yet most regulations fall short of this aim and trophy size is becoming shorter over time in most hunted populations, such as Bighorn sheep, Impala, Mouflon, and Sable antelope. This might be due to ignoring the speed of trophy growth when deciding on hunting regulations.

Bighorn trophy. Collection of the Life Sciences Building of the University of Bristol. (c) Susanne Schindler

The speed of trophy growth varies surprisingly between different herds, because the local landscape influences the micro-climate such as wind shelter, rain shelter or supply, and sun exposure. These conditions determine how much food is available, and sufficient food supply determines the growth rate of expensive trophies. Despite the marked differences in trophy growth speed, hunting regulations do not take them into account. This oversight can mean that unwanted hunting effects are bigger than anticipated.

Our international team of researchers from the UK, Canada, and the US investigated whether hunting effects differ between two herds with different trophy growth rates. We created a realistic computer model of two artificial Bighorn populations: one with slowly growing trophies and one with fast growing trophies. We subjected these populations to different hunting regulations, that is, different trophy size thresholds from when an animal is allowed to be hunted. We also subjected the two populations to different hunting intensities by varying the fraction of animals above the size threshold which are allowed to be hunted in a season.

Our computer simulations show that the faster trophies grow in a population, the more vulnerable this population is to negative hunting effects. For example, populations with fast growing trophies show a crossly distorted age structure with an over-abundance of young animals. This is because as the trophy growth rate increases, more animals fulfil the threshold of the hunting regulation at a younger age and risk being shot earlier in life. Therefore, strong hunting rates have a more drastic effect in populations with rapid horn growth rate.

In summary, how much hunting reduces average trophy size or distorts the age structure very much depends on how fast trophies grow. For hunting managers, this means that data available should be used to estimate the local trophy growth rate to arrive at more realistic estimates of the likely impacts of different hunting regulations and hunting rates. This is necessary to limit the distortion of age structure in the population, limit the reduction of average trophy size, reduce the risk of artificial selection, and ultimately come closer to make hunting sustainable.