Unveiling the Limitations of Scat Surveys to Monitor Social Species: A Case Study on River Otters

1. What they did

Otters are hard to see, so they are often monitored with scat surveys. A common method checks set stretches of shore for droppings and records each site as otters present or absent, assuming that more otters will mean more sites with droppings. Otters, however, tend to leave their droppings again and again at the same places, called latrines. The authors asked whether the number of sites with droppings actually rises with the number of otters.

In Kouchibouguac National Park, New Brunswick, Canada, and the area around it, they surveyed the frozen shores of nine rivers and streams in four winters (2000, 2001, 2003 and 2004). At least 12 hours after each snowfall, two people on light snowmobiles (or on snowshoes) rode slowly along the banks, stopping to look at every otter sign: snow tracks, droppings, burrows, water access holes and sightings. They counted latrines (any site with one or more droppings, lone scats included) and worked out how many otters were present by following the tracks in the snow, counting fresh tracks heading the same way and comparing their widths. Positions and times were recorded with a Global Positioning System (GPS) unit.

They then compared the number of latrines per km per day with the number of otters in each transect, and looked at whether latrines and the distance over which otters left signs grew with time since snowfall.

2. What they found

• They ran 92 transects covering 1,557 km of shoreline. Seven (done in May) were dropped as outliers and 3 because the number of otters could not be worked out from the tracks, leaving 82.
• No otters were detected in 17 transects, 1 otter in 16, 2 in 14, 3 in 14, 4 in 18 and 6 in 3.
• In 16 transects there were otter tracks but no droppings: 9 with one otter, 6 with two, 1 with three.
• Latrine numbers did not rise in a straight line with otter numbers. The relationship was best described by a curve that levelled off from groups of 3 otters upward.
• Latrines per km per otter did not keep rising after about 3 days since snowfall, so otters kept using the same sites.
• The distance over which individual otters or groups left signs did not increase with time since snowfall, suggesting they stayed in their winter home ranges.
• Otters could be followed for several hundred metres to over a kilometre; the average distance over which a group left signs was 1.69 km.
• No otters were found along developed stretches of river.

3. What they concluded

• More otters on a stretch of river do not produce ever more sites with droppings, so the chance of finding otter droppings at a search site does not rise much with otter numbers.
• This is explained by otter social life: family groups and other groups travel together and use the same latrines, and droppings, urine and anal secretions are used to communicate, including by otters passing through others’ home ranges.
• Otters can be present without leaving droppings on the bank, so presence-absence scat surveys can give false negatives, especially when otters are few. Snow may also cover latrines, and some droppings may be left in the water.
• Presence-absence scat surveys are inadequate as an index of relative population size for river otters. The authors advise great caution with such surveys for any species whose droppings or other signs are used in communication and are clumped on the landscape.
• They note that they may sometimes have mistaken two neighbouring groups for one, but argue that such errors would have worked against the pattern they found.

4. How tracking was or was not used

Tracking was central. The team searched riverbanks after fresh snowfall and identified otter snow tracks, droppings, burrows and water access holes. They counted otters by following groups’ trails along the shore, counting fresh tracks heading the same way and comparing track widths. Where two groups’ tracks overlapped or the groups had met, they could often tell neighbouring groups apart. They note that following otters can be hard because otters often switch between travelling on the snow and under the ice.

5. Tracker or observer certification

Not reported. The surveys were done by the authors’ team and named assistants, with no description of their tracking experience or any test of their skill, although the otter counts that the whole analysis rests on come from reading snow tracks.

6. Other technologies, skills or innovations

• Light snowmobiles (Bombardier Tundra models) and snowshoes for continuous shoreline transects.
• Garmin 12 XL GPS unit to record the start and end of each transect and every otter sign.

7. Implications for conservation (from the paper)

• Scat surveys based on presence or absence at search sites may poorly reflect otter population size and changes in it over time.
• Researchers using scat surveys for freshwater otters should acknowledge sources of bias such as false negatives, snow cover and droppings left in water.