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Predation not likely cause of Dall sheep crash in Alaska's Chugach Mountains

Rick Sinnott
Graduate student Brianne Boan handles a Dall sheep in Peters Creek during a research field trip. Alaska Department of Fish and Game photo

Watching a small band of Dall sheep ewes with a single lamb, Tommy Levanger recalled seeing a small black speck in his peripheral vision. The speck was a golden eagle in a 50-to-60-degree dive towards the lamb. When Levanger first noticed the bird, it was still about 200 feet above him. With its wings folded against its body, Levanger said, “it looked like a missile.”

Flying a Robinson R44 helicopter near the headwaters of Eagle River, Levanger was hovering about 200 feet above the lamb. He had just dropped off Brianne Boan and wildlife technician Corey Stantorf, who were stalking the sheep over unstable shale, hoping to capture the one-to-two-day-old lamb. The ewes, aware that trouble was approaching on foot, were contouring across the mountain slope, away from the biologists. No one but Levanger was focused on the 15-pound golden eagle, moving at approximately 150 mph, a feather-covered battering ram. 

At that speed, things happen fast. The lamb, which weighed less than the eagle, was lagging some 30 feet behind the ewes. The last ewe in line, almost certainly the lamb’s mother, spotted the eagle perhaps 30 feet before impact. She spun around and bounded between the predator and its intended victim. The eagle, flaring its seven-foot wingspan -- Levanger called it “applying its airbrakes” – and swinging its talons forward, hit the ewe broadside “like hitting a brick wall.” The bird ricocheted off, crashed into the ground, then immediately launched itself off the mountain. There are easier animals to prey on in the vicinity. Perhaps an arctic ground squirrel would suffice.

According to Levanger, the ewe wasn’t knocked off her feet and didn’t appear to be injured. She and the lamb soon caught up with the other sheep. Dall sheep, after all, are tough critters.

Why are Dall sheep populations declining?

The biologists were trying to capture the lamb to put a collar on it. Tom Lohuis, a wildlife biologist with the Alaska Department of Fish and Game, is the lead investigator in a five-year project investigating the causes of recent declines in sheep populations. Boan, a graduate student from the University of Nevada-Reno, is assisting.

Their two study sites are in the northern Chugach Mountains, between the Matanuska and Tazlina glaciers, and in Chugach State Park. Dall sheep populations in both areas, like other areas in Alaska, are about half what they were 25 years ago; in Chugach State Park alone, numbers have declined from about 2,400 animals to 1,200.

Lohuis is still collecting data and has no final analysis; hence, it’s too early for any firm conclusions. However, it is noteworthy that neither hunters nor predators appear to be limiting the size of the sheep population in either study area. Instead, sheep are dying for reasons that, in some circles, might be deemed acts of God.

If not hunters, then what?

Sheep may be affected by human-caused factors or factors that humans can control. On the other hand, there may be nothing we can do to avoid or even minimize the declines. Fish and Game’s research aims to find out which factors are curbing Dall sheep numbers in Southcentral Alaska.

It’s unlikely hunting is affecting Dall sheep populations because the number of hunters in both areas is limited by permit and, with some exceptions, only rams with full-curl horns may be taken. Very few sheep in any population become full-curl rams. It typically takes about eight years for a ram’s horns to achieve a full-curl and, even in an unhunted population, few rams live as long as 11 years.

Lohuis is looking into several factors that may be limiting population growth: weather, disease, predation, nutrition, and range condition. Most of these factors cannot be measured remotely; they require an intensive, hands-on approach. To answer these questions, Lohuis and his assistants have captured 200 adult sheep and 150 lambs over the past five years: ewes in March and April, before lambing, and lambs from May 15 to June 15, shortly after they were born.

Sheep are fitted with radio collars so they can be closely monitored. The radio collars emit a different signal when an animal hasn’t moved for hours, which is a sure sign the sheep isn’t going to move again. This allows Lohuis to find the carcass, land nearby in a helicopter, and determine the cause of death, often within hours and usually before other predators or scavengers visit the carcass and muddle up the “crime scene.”

When he collars the sheep, Lohuis collects blood, mucus, and fecal pellets for a variety of tests. With the blood he can determine whether a ewe is pregnant, detect the presence and prevalence of diseases, and profile its health. Swabbing mucus from the nasal and pharyngeal passages facilitates the analysis of some diseases and genetic characteristics. Examining the droppings can identify the presence of some parasites. Finally, he qualitatively rates each animal’s body condition based on the prominence of certain bones, such as ribs and pelvic girdle, which indicate the absence of fat and muscle mass.

When all the data are collected and analyzed, Lohuis will be able to pinpoint one or more causes for the low productivity. He’ll also be able to compare the vital statistics of Dall sheep in the Chugach Mountains with those living in the Alaska and Brooks ranges.

Northern Chugach Mountains study area

Lohuis began studying sheep in his northern Chugach study area in 2009. He’s captured and recaptured about 40 ewes annually. Their body condition appeared extremely poor, even for late winter. The sheep had no subcutaneous fat, which means by late winter they had exhausted the energy stored the previous summer to help them survive winter. Lohuis, a no-nonsense researcher who typically relishes using the appropriate scientific terms, kept calling them “skinny” sheep.

Pregnancy rates were also relatively low. In the first three years of his research only 62 to 88 percent of the adult ewes were pregnant. Pregnancy rates as high as 100 percent have been reported for other Dall and Stone sheep populations in Alaska and Canada, so Lohuis’ findings are cause for concern. Last summer, only 21 percent of his collared ewes were pregnant.

Getting pregnant and bringing a lamb to term is the easy part. Once the lamb hits the ground, other factors conspire to kill it before the animal becomes part of the breeding population.

Only 21 percent of 66 lambs collared in 2009-2011 were known to survive a year. Predators killed 35 percent of the lambs, while 38 percent succumbed to other causes, like avalanches, rockslides, falls, drowning, disease, and malnutrition. The fate of 6 percent of the lambs couldn’t be determined.

Golden eagles were the leading lamb predator, closely followed by wolverines and brown bears. It’s likely that not all eagles learn how to knock lambs off their feet, killing or fatally injuring them in the subsequent tumble down the mountain. Fortunately for the sheep, golden eagle predation occurs primarily in the first three weeks of a lamb’s life. Wolves, black bears, and coyotes each accounted for 1 of 52 lamb mortalities, with two additional lamb carcasses visited by several predator species.

Some hunters, who witness predators chasing sheep or read that as many as a third of lambs are killed by predators before they are a year old, argue that killing predators is the logical way to grow more sheep. However, before we start singing a rousing chorus of “Kill the Eagles,” let’s try to remember that most lambs are killed by inanimate hazards. And some young lambs were killed by predators because they were too weak to escape or had less attentive mothers. These lambs wouldn’t have been as likely to survive other hazards or grow into the trophy-size rams that many hunters seek.

Surprisingly, Lohuis has found ewe mortality to be relatively low in this study area. In the past four years, adult mortality has averaged nine percent a year, better than that found in the Alaska and Brooks ranges. Of 13 mortalities where the cause of death was known, only three ewes were killed by a wolverine or wolf. Most of the remaining deaths were attributed to avalanches or disease.

Chugach State Park study area

Lohuis began capturing sheep in the park a year ago, the first year of a two-year effort. Last year he captured 35 adult ewes and 19 juvenile rams. The rams will be captured only once, radio-collared, and monitored for movements and survival. This will help department biologists evaluate the “full-curl” hunting strategy.

As in the northern study area, evidence of viral and bacterial diseases was low. Sheep in Chugach State Park also have poor body condition, but it’s marginally better than that found in the other study area. Unfortunately, the pregnancy rate last winter was only 43 percent. Lohuis said that rate and the 21 percent pregnancy rate found in the northern study area in 2012 were “among the lowest ever recorded in an ungulate population.”

No wolf hunting or trapping is permitted in Chugach State Park, so it’s reasonable to assume that wolf predation might pose a serious threat to sheep in the park. The findings so far suggest otherwise. Only one of the 35 radio-collared ewes died between April and December 2012. An avalanche was the cause of death. Since then, two other ewes have died, another in an avalanche and one from an predator of some kind, probably a wolverine or wolf. The usual suspects -- wolves and coyotes -- aren’t even killing many lambs. In the past year six of 26 radio-collared lambs have been killed by golden eagles, one by a coyote, and one drowned.

Because sheep are social creatures, the death of a ewe doesn’t necessarily mean her lamb will die. The ewe killed in February by an unknown predator had a lamb, approximately nine months old. Several weeks later Boan found the collared lamb in a band of other ewes.

If not predators, then what?

It’s noteworthy that, overall, only about one in five ewes and half of the lamb mortalities were attributed to predators. This is a low percentage inflicted by a variety of predators, suggesting that this sheep population may not be limited by predation.

This would be a very different finding compared with results of earlier research by Steve Arthur and Brad Scotton, both Fish and Game researchers who studied sheep in the central Alaska Range. During the first four years of Arthur’s research, all 14 radio-collared ewes that died during the project were killed by predators, most by wolves, although, in the final analysis, he found the annual survival rate of ewes was much higher. Arthur and Scotton also found higher predation rates on lambs than Lohuis has seen so far, with more than 90 percent of the lambs killed by predators.

In another twist, annual pregnancy rates in the Chugach Mountains were lower than those observed in the Alaska Range. Pregnancy rates in the Chugach Mountains appear to be chronically low, suggesting the sheep population is limited not by predation, but by nutrition, range condition, weather, or some combination of these factors.

Lohuis plans to collect his radio collars and wrap up the field research later this year. His analysis will take longer. Because weather – in particular deep snow and persistent ice cover – wreak havoc on Dall sheep survival, Lohuis is monitoring temperature and snow depth in winter sheep range. The temperature data should help identify freeze-thaw events that cover winter ranges in ice, but he also hopes to directly measure snow hardness during winter. He’ll use satellite imagery to determine the extent and type of snow coverage.

Boan is particularly interested in immunity. The function of the immune system affects survival and reproduction. She’ll analyze blood serum for immune system “memory” and transfer of maternal antibodies between ewes and lambs. In the Lower 48 states, wild sheep populations have been decimated by pneumonia and other viruses carried by domestic livestock. And although Dall sheep have contracted the same strains of pneumonia, for example, a deadly “cocktail” of those infectious diseases hasn’t affected Alaska’s Dall sheep. Not yet.

It’s not surprising that different environmental conditions and vulnerability to predation result in very different pressures on sheep populations. But it is interesting that, at least this time, data are showing predators have little effect on Dall sheep in southcentral Alaska.

Lohuis is letting the sheep tell their own tale. If weather or range conditions are the cause of declining populations, then there is nothing wildlife biologists can do about it. Hunters, including golden eagles, will have to learn to live with long-term cycles of sheep abundance.

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