Fisher Predation on Canada Lynx in the Northeastern United States

1. What they did

Canada lynx are protected as a threatened species in the United States, but little was known about what limits them at the southern edge of their range. From 1999 to 2011, the Maine Department of Inland Fisheries and Wildlife ran a radio-tracking study of lynx in northern Maine. The authors expected lynx there to die of the same causes as in Canada’s boreal forests. This short paper (a “Note”) reports a cause of death that had never been documented before: lynx being killed by fishers.

• Collaring lynx. They captured 187 lynx and fitted 85 with very high frequency (VHF) or Global Positioning System (GPS) collars. Each collar had a mortality sensor that changed its signal when the animal had not moved for 4 hours. They checked the collars from a small plane about twice a week, weather permitting.
• Investigating deaths. When a collar gave a mortality signal, they found the dead lynx, photographed the site, and collected remains and data.
• Necropsies. Lynx without visible injuries went to a pathologist to find the cause of death (for example, starvation). Those with injuries were examined by the team, and thigh bone marrow was checked for body condition.
• Identifying the predator from bite marks. They measured the distance between canine tooth punctures on the carcasses and compared them with published skull measurements of fishers, lynx and coyotes.
• Identifying the predator from tracks and signs. In snow, they counted a death as predation when predator tracks led to a resting lynx, or when the tracks of a lynx being chased ended at a kill site. They identified the predator from track features, measurements and trail pattern.
• Habitat. They described the forest stand at each kill site from landowners’ stand maps and their own notes at the site.

2. What they found

Causes of death

• Of 65 dead lynx, 18 were killed by predators, 17 starved, 17 died of unknown causes, 12 died from human causes (legal harvest in Canada, illegal killing and vehicle collisions), and 1 from disease.
• Fishers killed 14 lynx (9 females, 5 males) and probably killed 2 more. The predator of the last 2 could not be identified.
• Thirteen of the 14 lynx killed by fishers were adults with established home ranges. The other was an 8-month-old kitten, killed and cached at the same site as its mother. Most were healthy or in fair condition.

What the tracks and signs showed

• Most deaths happened in winter, when tracks and signs in the snow let the team reconstruct what had happened.
• Ten lynx had been resting in a bed when a fisher ambushed and killed them. Two had been chased by a fisher and killed.
• The only predator tracks at these sites were fishers’. Other predator tracks and signs were absent.
• Fishers often dragged the lynx (drag marks from 1 m to 201 m long, often with bright red drops of blood along them) and cached them in tree cavities, root masses, hollow logs, under fallen trees or dense conifers, or buried in the snow.
• There were usually many fisher scats and a network of fisher trails to and from the carcass. Once, a fisher was found in a cavity with the remains of a lynx. Twice, dark brown guard hairs were found inside a dead lynx’s mouth.

Bite marks

• On 6 lynx, 19 of 20 canine puncture measurements fell within the range for fishers and outside the ranges for coyotes and lynx, confirming what the team had seen in the field.
• Measuring was often hard: tissue stretches, attacks involved repeated biting, and many fishers have broken or missing canines.

Where and when

• Twelve of the 14 lynx killed by fishers (86%) died in mature conifer forest, although this made up only 12% of the study area. Most of these stands were dominated by northern white cedar or black spruce.
• Twelve of the 14 (86%) were killed from January to March, when snow was deepest, and often during a snow event.
• Fishers killed more lynx after 2006, when snowshoe hare density in regenerating clear-cuts dropped from about 2 to about 1 hare per hectare. While hares were more plentiful, only female lynx were killed.
• The kills fell within the territories of several radio-collared fishers over 12 years.

3. What they concluded

• This is the first study to document fishers killing lynx.
• Fishers seem to be opportunistic predators of lynx, not a few individuals with a learned habit and not only targeting weak animals.
• Snow probably helps fishers approach resting lynx unseen and ambush them; the limited signs of struggle at kill sites support this. Mature conifer forest gives cover and resting places to both species, which may bring them together, and snow may be shallower there.
• In winter, especially when hares are scarce, a lynx may be the most profitable meal available to a fisher. Fishers average 4.6 kg, about half the weight of an adult lynx, but are aggressive predators.
• The absence of other lynx predators (wolverines, mountain lions, wolves) and the habitat conditions may have created a unique opportunity for fishers in northern Maine.
• There was no evidence that fishers were limiting lynx numbers or range, or excluding them from habitats.

4. How tracking was or was not used

Tracks and signs in snow were central to working out how each lynx died. The team identified predation when predator tracks led to a resting lynx’s bed, or when a chase trail ended at a kill site, a patch of disturbed snow with tracks, scratch marks, broken branches, blood and tufts of lynx hair. They identified the predator from track features, measurements and trail pattern, and recorded drag marks, caches, scats, fisher trails to and from carcasses, and hair in the lynx’s mouth. Where fresh snow or bare ground hid the tracks, the predator was identified from any signs present, from bite measurements, or recorded as unknown. Sites were visited 1 to 13 days after the estimated time of death.

5. Tracker or observer certification

Not reported. The paper does not say who read the tracks and signs at the kill sites or what experience they had, although the identification of fishers as the predator rests largely on this. Many field technicians and volunteers are thanked.

6. Other technologies, skills or innovations

• VHF and GPS collars with 4-hour mortality sensors, monitored from fixed-wing aircraft.
• Measuring the distance between canine punctures on carcasses and matching it to published skull measurements of possible predators.
• Necropsies and bone marrow condition to separate predation from starvation and to judge the health of killed lynx.
• Geographic Information System (GIS) stand maps to describe habitat at kill sites.

7. Implications for conservation (from the paper)

• Maine did not increase fisher trapping to help lynx, because Maine’s lynx population was growing with abundant habitat and prey.
• Fisher trapping rules have instead become stricter, to reduce the accidental catching of lynx. This has reduced fisher harvest, so predation may come to have a greater effect on lynx.
• Where lynx and fishers live together, the authors recommend monitoring both species, and say managers could consider changing fisher harvest if predation is holding back lynx population growth.