Managing Disease Dynamics through Ewe/Lamb Hunts in Wyoming

Facts from the Wyoming Game and Fish Department about the 2023 ewe/lamb hunt for the Jackson Bighorn Sheep herd (Hunt Area 7)

By Aly Courtemanch, Wyoming Game and Fish Department Wildlife Biologist in Jackson, WY

Current Situation

• The Jackson bighorn sheep herd has been prone to pneumonia die-offs when it reaches a population of between 400-500 animals. This likely happens because the population surpasses what the available habitat can support over the long term; in other words, exceeding the habitat’s carrying capacity.
• The Jackson herd has been growing since 2013 and has surpassed its herd objective of 400 sheep +/- 20% (320- 480 sheep).
• 505 bighorn sheep were counted during the February 2022 helicopter survey. The February 2023 survey was not as reliable due to unusual bighorn sheep spatial distribution.

• Despite the tougher winter conditions this year, ewe mortality has only been slightly above normal. Most ewes are expected to survive this winter, which means the population will remain above objective.

• There have been two significant pneumonia die-offs in this herd in the last 20 years and both coincided with high population sizes (500 sheep in 2001 and 417 sheep in 2012) (Figure 1).

• Pneumonia die-offs result in decreased ram hunting opportunity through decreased licenses, oftentimes for many years. For example, the 2001 die-off resulted in a 70% reduction in ram licenses over 8 years (a loss of over 100 ram licenses from 2002-2009 compared to pre-die off license levels).
• Recent, repeated testing in this herd shows that pneumonia-causing pathogens are currently present and circulating amongst sheep. It’s not a question of if, but when another die-off will happen under the right conditions.
• Higher population size leads to more competition for food, poorer nutritional condition of individuals, reduced immune system function, and potentially more frequent disease transmission between sheep, which could all lead to another pneumonia die-off.

• Collared bighorn ewes entered the winter in December in the poorest body condition measured in the last 7 years, despite good moisture and forage production last summer and fall. This indicates there are too many sheep on the landscape competing for food.

• Collared ewes captured in March 2023 also had the lowest body condition and pregnancy rates we’ve seen in the past 7 years. This is partly due to a tough winter, but mostly due to entering the winter without much fat due to too many sheep on the landscape. This ongoing research is being conducted in cooperation with the Monteith Shop at the University of Wyoming.

Benefit of Ewe Hunting for Population Management

• Reducing and stabilizing the Jackson Herd to its population objective is expected to benefit the herd and hunters.
  • Individual sheep will have more access to food so that each will be in better condition.
  • Sheep will have more energy to use for immune function, which may result in avoiding a pneumonia die-off.
  • Ewes that are in better nutritional condition are better moms.
    • They have higher pregnancy rates, more lambs being born, and their offspring are more likely to survive and be stronger.
    • Males born to healthy mothers often have bigger horn growth throughout their lifetime.
  • Preventing another pneumonia die-off will benefit ram hunters by allowing for sustained numbers of ram licenses over time. Ewe/lamb licenses create additional hunter opportunity and the chance to go on a sheep hunt without preference points.

• Wyoming Game and Fish is proposing 30 ewe/lamb licenses for 2023. Based on past hunter success rates, we anticipate approximately 12-15 ewes will be harvested on these licenses. Ewe hunter success was 40% last year, which is in line with other western states.

• Many western states have been using ewe hunting as a management tool for the past decade.

• There is no guarantee that this herd will bounce back from a pneumonia die-off. Most herds west-wide have chronically low lamb survival for years or decades after die-offs, like the Whiskey Mountain Herd in the Wind River Range south of Dubois, WY.

• By reducing the population to the desired population objective over the next few years, managers believe they can proactively prevent another significant pneumonia die-off instead of just waiting for it to happen.