Factors Affecting Detectability of River Otters During Sign Surveys

1. What they did

Sign surveys, looking for an animal’s tracks, scats and other signs, are a cheap and popular way to study where animals live. River otter surveys have usually been short, single visits, so an otter that is present but leaves no sign the surveyor finds goes down as “absent”. The authors wanted to know what affects the chance of finding otter sign when otters are there (the detection probability).

They tested five factors: the ground surface (substrate), the type of sign (scat or tracks), the observer, the length of the survey, and whether sign is concentrated near access points such as bridges, where many otter surveys start.

• Sites. 110 randomly chosen sites along stream and reservoir shorelines in eastern Kansas, each 1,200 to 3,600 m long and 5 m wide, starting at a bridge, low-water crossing or boat launch. Surveys ran in late winter and early spring 2008 and 2009, which is the otters’ breeding season, when scent marking is expected to peak and vegetation is low. No site was surveyed within 2 days of rain.
• Surveys. Observers walked one side of the shoreline and recorded every otter track and scat, with a description. Only sign they rated 75 to 100% sure was otter was used. Every 400 m they stopped and noted the main substrate (vegetation, mud, rock, leaf litter or snow). Searching took on average 18 minutes per 400 m.
• Observers. All observers had one day of field training in sign identification. At 19 sites, two or three observers searched the same stretch independently on the same day: two “experienced” observers (49 to 81 sites each over the study) and two “novice” observers (7 to 20 sites).
• Analysis. Each 400 m stretch was treated as a repeat survey of the site, and occupancy models estimated both how many sites had otters and the chance of finding their sign. Shorter and longer stretches (200 to 1,000 m) were made by pooling the records.

2. What they found

Overall

• Otter sign was found at 35 of 110 sites. After allowing for missed sign, the estimated share of sites with otters was 0.33.
• The chance of finding sign in one 400 m survey, when otters were there, was 0.34: about one time in three.
• If only the first 400 m of each site had been surveyed, just 10 sites would have counted as occupied, giving an occupancy estimate of 0.09.

Substrate

• Sign was easiest to find on mud (0.60) and hardest on snow (0.18) and leaf litter (0.27). Rock and vegetation fell in between (about 0.32 and 0.37, read from the figure).
• No tracks were found on snow, which was the main substrate in only 2.1% of surveys.

Sign type

• Scat was found far more readily (0.53) than tracks (0.18), almost three times as often.

Observers

• The two experienced observers had the highest detection (0.78 and 0.71).
• One novice was a little lower (0.55); the other, with the same training, was much lower (0.15).

Survey length

• Detection rose almost in a straight line with survey length, from 0.23 for 200 m to 0.61 for 1,000 m, about 0.05 more for every extra 100 m.

Access points

• Otter sign was no more or less common near bridges and other access points than farther along the shore.

3. What they concluded

• Missed sign (perception bias, from observer differences) and hidden or unavailable sign (availability bias, from substrate, sign type and survey length) both affected whether otters were detected.
• Short, single-visit sign surveys can greatly underestimate how many sites animals occupy.
• Track surveys found otters much less often than scat surveys, which is a concern because track surveys are common for many species, and good tracking surfaces such as snow and mud are not always available. Recording both scat and tracks takes little extra effort, so the authors suggest recording both and using models that allow for substrate.
• Observers should gain field experience, and at least some sites should be surveyed by more than one observer to correct for observer differences. They also suggest observers be thoroughly trained and tested on scat and track identification, pointing to a standardized tracker evaluation used by Evans et al. (2009), and that genetic testing or scat detection dogs could help confirm scat.
• An earlier European otter study found sign within the first 200 m at most occupied sites; here, a 200 m survey found otter sign only about a quarter of the time. Surveys of 200 to 1,000 m gave reasonable detection rates (0.2 to 0.8).
• Starting surveys at or near bridges probably does not bias results.
• Factors not tested, such as type of water body and population density, may also matter.

4. How tracking was or was not used

The whole study is a sign survey: observers walked shorelines looking for otter footprints (one footprint or more counted) and scats, identified them, rated their confidence, and recorded each one’s location, type and size. No trailing was reported. The study measures how often trackers find sign that is there, not how accurately they identify it: the analysis assumed no sign was ever wrongly identified, and only sign rated 75 to 100% certain was used. The authors note that misidentification was still possible, and that other researchers have found observers confuse raccoon and otter scat.

5. Tracker or observer certification

Reported but not validated. All observers had one day of field training in sign identification and were asked to use measurements and field guides. “Experienced” and “novice” were defined only by how many sites each person surveyed during the study. Their ability to find sign was compared in the study, but their identification skill was not tested and no certification is reported. Field technicians M. Jeffress and K. Blecha are thanked in the acknowledgments.

6. Other technologies, skills or innovations

• The first study to report using spatial replication (treating each 400 m stretch as a repeat survey) to measure detection in river otter sign surveys.
• Multi-method occupancy models, which estimate detection separately for scat and tracks.
• Independent observers on the same stretch on the same day, used as repeat surveys to compare observers.

7. Implications for conservation (from the paper)

• Many past sign surveys did not account for imperfect detection; better survey and analysis methods should give less biased estimates of where a species occurs.
• Future studies should run longer surveys with repeats in space or time, account for substrate and observer differences, and record both scat and tracks.
• The results could help build a standardized river otter survey protocol, making results easier to compare, and apply to other areas and other species surveyed by sign. Surveys could also record several species at little extra cost.