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Why has Chugach State Park's Dall sheep population declined?

Rick Sinnott
Alaska Department of Fish and Game photo

Beginning this week the Alaska Department of Fish and Game will start capturing Dall sheep in Chugach State Park for a two-year study on population demographics. The research will examine body condition, pregnancy rates, recruitment, and causes of mortality.

The wildlife biologist conducting the research, Tom Lohuis, plans to capture up to 35 ewes, 25 sub-adult rams, and 30 lambs in the park. Sheep will be captured in the more remote corners of the park, including the headwaters of Ship and Peters creeks and Eklutna River. Ewes and sub-adult rams will be captured in March and April, lambs shortly after birth in late May and early June.  All will be fitted with GPS radio collars.

Lohuis is halfway through a similar project in the northern Chugach Mountains, west of Tazlina Lake. His experience with capturing and handling sheep will come in handy in the park, where he’s likely to come under more scrutiny by outdoor recreationists.

Every wildlife biologist nourishes an inner cowboy. Only a few are lucky enough to feed and exercise it regularly. Lohuis wears the lanky, loose-limbed look of a cowboy -- growing up in Wyoming didn’t hurt, either. He’s also got the “aw, shucks” mannerisms down pat. He talks fast, especially when he’s trying to make a point, but if you ask him a direct question, he’ll look you in the eye and give you a direct answer.

Direct answers are what hunters seek. Dall sheep are a popular game animal in Alaska. Nonresidents and some resident hunters are willing to pay as much as $22,000 for a guided hunt. Wildlife viewers also value Dall sheep. Chugach State Park is one of few places where the sheep are readily visible close to a road, in this case along the Seward Highway between Anchorage and Bird.

Compared to annual funds spent on moose, caribou, bear, and wolf projects, Dall sheep research has languished. There’s only so much money to go around and, because the state has managed sheep conservatively, funding for research has been allocated to the most intensively managed species. Recent declines in sheep populations, however, have raised concerns.

Alaska is the only state with Dall sheep, the all-white relatives of the more familiar bighorn sheep found in other western states and Canadian provinces. Alaska’s Dall sheep populations have declined recently, as much as 50 percent in some areas. More than 2,400 sheep were counted in Chugach State Park in the 1980s and 1990s; however, the population declined to about 1,100 sheep in 2010. Last summer’s aerial survey found an estimated 1,100 to 1,200 sheep in the park.

The population declines are not related to hunting. In most of Alaska, only rams with full-curl horns are legal game, and these older sheep comprise a very small percentage of the population. The main causes of mortality are thought to be those that Dall sheep have battled for millennia. Sheep are vulnerable to deep winter snows and icing that covers food and makes it difficult to move around and escape predators. Recent severe winters have been tough on Alaska sheep, especially in coastal areas like the Chugach Mountains. Predators are a significant source of mortality on adult sheep in the Alaska and Brooks ranges, but little is known of predation rates on Chugach Mountain sheep.

Lohuis’ research will answer questions debated for decades with little data to support either side.

Preliminary information has found much lower pregnancy rates in the northern Chugach study area compared to sheep populations in the Alaska and Brooks ranges. Unlike sheep in the northern half of the state, which still carry subcutaneous fat and are relatively well-muscled this time of year, sheep in the northern Chugach Mountains have no subcutaneous fat and have lost a significant amount of muscle by late winter. Lohuis said they look and feel skinny, which is never a good sign in a wild animal. Low pregnancy rates and poor body condition suggest population declines are due to nutritional or mineral deficiencies.

Disease is another serious concern. Lohuis is taking blood, fecal, oral and nasal samples from every sheep to test for diseases that might affect mortality rates. He’s also retrieving dead sheep soon after their collars switch to mortality mode and taking the carcasses to a veterinary pathologist for necropsies. His preliminary work found most of the sheep in the northern Chugach study area tested positive for bacteria associated with respiratory disease in bighorn sheep, although the bacterial strain wasn’t the most virulent.

After analyzing data collected the past two winters, Lohuis has found avalanches and pneumonia contribute to slightly more than half of adult sheep mortality in his study area. Wolverines and wolves have killed only three of the 11 collared ewes that have died so far.  Nevertheless, the survival rate for adult sheep is high. Lambs are much more vulnerable. In his northern Chugach study area about 42 percent of the lambs collared in 2009 survived their first year. In 2010 only 10 percent of the lambs survived. Lohuis attributed about half of the lamb mortality to bears, wolves, wolverines, and golden eagles. Avalanches, accidental falls and drownings, starvation, and pneumonia killed the remainder of the collared lambs that died in their first year.

Lohuis uses different techniques for capturing adults and lambs. His techniques are akin to those that a cowboy would choose, given certain constraints. Dall sheep country is not conducive to riding and roping. Instead, Lohuis’ pilot maneuvers his Robinson R44 helicopter like a cutting horse to work an adult sheep into favorable terrain.  Then Lohuis “lassos” the animal with a shoulder-fired net gun. The gun, which kicks like a mule, throws a 10-by-10-foot-wide net that envelops and immobilizes the sheep. Lohuis and an assistant land nearby and run to the sheep, pinning it down and covering its eyes with a blindfold. Drugs are not used because sheep are relatively easy to hold down and drugs have serious aftereffects for an animal that inhabits steep snowy slopes and rocky cliffs. Lohuis has imposed a 20-minute limit from the time the sheep is approached by the helicopter until all samples are taken and the animal is collared and released. Similarly, he doesn’t chase sheep more than three minutes, to avoid overheating. If a sheep seeks refuge on a cliff or steep slope, Lohuis looks for another one.

Lambs are caught by landing nearby and running them down. In late May the lambs are tiny, about the size of a human infant. Ewes hover close by and gather up their offspring as soon as the helicopter departs. The radio collars used on lambs are expandable, and the monofilament stitching deteriorates, allowing the collars to fall off in about a year.

Having already captured 142 Dall sheep, Lohuis has perfected his rodeo routine. He can capture as many as 14 adults or 7 lambs a day. After initial captures, he’ll return with the helicopter to investigate causes of death and to pick up the radio collars. The Robinson R44 is lighter and not as noisy as larger, turbine-powered helicopters.

The GPS collars are programmed to record a sheep’s location every four hours. This data will, for the first time, give insight into the seasonal movements of Dall sheep in Chugach State Park.

Lohuis gets a twinkle in his eyes when he describes how he captures sheep and what the data has shown so far. He’s obviously enjoying himself. He’s doing important work.

I didn’t have the heart to tell him that most cowboys grow out of mutton busting by the time they are 6 years old.

Rick Sinnott is a former Alaska Department of Fish and Game wildlife biologist. The views expressed here are the writer's own and are not necessarily endorsed by Alaska Dispatch. Contact him at rickjsinnott@gmail.com