# Comparing direct and indirect methods to estimate detection rates and site use of a cryptic semi-aquatic carnivore

> Do otter droppings tell the whole story? At latrines along Utah’s Provo River, researchers set cameras beside their monthly scat counts. The cameras caught otters 1.5 times as often, and in 20 site-months otters visited without leaving any scat that was found, yet the more scat a site held, the more otter visits the cameras recorded.

Summary by Kersey Lawrence, Original Wisdom. Updated 9 November 2026. Page: https://www.originalwisdom.com/tracking-literature/2016-day-westover-comparing-direct-indirect-methods/

## Citation and access

- **Authors:** Casey C. Day, Matthew D. Westover, Lucas K. Hall, Randy T. Larsen and Brock R. McMillan
- **Year published:** 2016
- **Journal or source:** Ecological Indicators
- **Full citation:** Day, C. C., Westover, M. D., Hall, L. K., Larsen, R. T. and McMillan, B. R. 2016. Comparing direct and indirect methods to estimate detection rates and site use of a cryptic semi-aquatic carnivore. Ecological Indicators 66: 230–234.
- **DOI:** 10.1016/j.ecolind.2016.01.039
- **Access status:** Publisher link only
- **Where to find it:** https://doi.org/10.1016/j.ecolind.2016.01.039

## Tags

- **Species:** Mammals > North American river otter
- **Region:** North America > United States
- **Tracking:** Scat; Sign surveys; Tracking; Tracks and signs identification
- **Behaviour and ecology:** Behaviour
- **Methods and technology:** Camera traps; Chi-square test; Detectability; Global Positioning System (GPS); Kendall’s tau rank correlation; Radio telemetry
- **Data type:** New field data
- **Observer qualification:** Not reported
- **Discipline:** Behaviour and ecology; Wildlife tracking
- **Year published:** 2016

## Detailed keywords

- **Species:** Studied: northern river otter (Lontra canadensis), also called the North American river otter. Discussed from other studies: Eurasian otter (Lutra lutra) and giant otters. The cameras recorded other species, but the paper does not name them.
- **Location and habitat:** 64 km of the Provo River and its tributaries along the Wasatch Range of the Rocky Mountains, Heber Valley near Heber City, north-central Utah, United States. The area holds two large reservoirs, and the river winds through several towns and farming areas. Annual average temperature 8.1 °C (summer 19.2 °C, winter −3.3 °C); annual precipitation 41.2 cm, mostly snow from late fall to early spring. Otter latrine sites on riverbanks.
- **Topics:** comparing direct (camera traps) and indirect (scat count) monitoring methods; detection rates; false negatives (false absences); site use and visitation at latrines; scat identification; latrine surveys; scat counts as an index of activity; high-end versus low-end camera traps; snow burying scats; choosing the interval between sampling visits; translocated otters.
- **Statistics or models:** monthly encounter histories (12 months by 10 sites, 120 site-months); detection rate as detections divided by encounter occasions; z-test for proportions; chi-square test on the number of discrepancies between methods; Kendall’s Tau-b rank correlation (chosen because the data were not normally distributed and had many zero ties); visits defined by 30-minute gaps, rechecked with 1-hour gaps; analyses in Program R, alpha = 0.05. Small inconsistency: the paper gives a mean of 1.86 scats per site per month, but 228 scats over 120 site-months works out at 1.90.
- **Data type:** New field data. Latrine searches and monthly scat monitoring from February 2010 to February 2012 (radio-tracking of 23 translocated otters); camera traps at 10 latrines from March 2011 to February 2012 (3310 trap nights).

## Synopsis

### 1. What they did

Otters are hard to see, so most of what is known about northern river otters comes from indirect methods: searching for their signs, especially latrines (places where they repeatedly leave scat) and tracks. Earlier studies that compared scat surveys with cameras often found the cameras unreliable, but those studies mostly used early or “low-end” cameras. The authors asked whether modern, high-end cameras and scat counts give the same picture of how often otters are detected and how much they use a site.

- Finding latrines. They walked the riverbanks of 64 km of the Provo River and its tributaries in Utah twice. From February 2010 to February 2012 they also found latrines by radio-tracking 23 translocated otters.

- Counting scats. At each latrine they counted fresh otter scats (wet, soft and pungent), told apart from other species’ scat by size, shape, smell, contents and mucous. They recorded a Global Positioning System (GPS) location and removed all scat, so old scats would not be counted again. Latrines were checked monthly.

- Adding cameras. After one year they chose 10 latrines at random and set an infrared (no-flash) Reconyx PC900 camera at each, about 3 m from the latrine, from March 2011 to February 2012 (3310 trap nights). Scats were still counted on the monthly camera visits.

- Comparing the two. For each site and month (120 site-months) they noted whether otters were detected by scat (one or more scats) or by camera (one or more images), and how many otter visits the cameras recorded. Photos separated by at least 30 minutes without otters counted as separate visits.

### 2. What they found

What each method recorded

- The cameras took 3956 images of otters, making up 758 visits (an average of 6.32 visits per site per month, range 0 to 102). They worked all year with no apparent malfunctions.

- Over the same period, 228 scats were collected (an average of 1.86 scats per site per month, range 0 to 43).

Detection

- Cameras detected otters in 43% of site-months, against 28% for scat counts: about 1.5 times as often, a significant difference.

- In 20 site-months the cameras recorded otters but no scat was found (1 to 149 otter images in those months). In only 2 site-months was scat found (a single scat each time) with no otter images. This difference was significant.

- In 17 of the 20 months where scat missed otters, the cameras had recorded 5 visits or fewer.

- The two largest mismatches were probably caused by heavy snow burying the scats that month.

Site use

- The number of scats at a site was clearly linked to the number of otter visits the cameras recorded (Kendall’s Tau-b = 0.675). More scat meant more visits.

- When visits were defined by 1-hour gaps instead of 30 minutes, the link was weaker but still significant (Tau-b = 0.555).

### 3. What they concluded

- Modern, high-end cameras detected otters better than scat surveys at latrines and gave fewer false negatives.

- Even so, scat counts tracked how heavily otters used a latrine. So, depending on what is being measured, scat counts may be an acceptable stand-in for direct methods for otters and other cryptic species: they indicate use, but are not reliable for detection at latrine sites.

- Rare or elusive species may be more prone to false negatives from scat counts than river otters, which are social and leave large, conspicuous latrines. Scat counts are also more likely to be accurate in high-density populations.

- Weather can shorten the effective sampling period, so the interval between scat collections should be chosen with the local climate in mind.

- Their correlation was stronger than one found in a daily study of Eurasian otters, suggesting indirect methods may do better over longer sampling intervals.

- Without data comparing methods, the authors advise preferring direct methods such as camera trapping when drawing conclusions about distribution, abundance or habitat use.

### 4. How tracking was or was not used

The indirect method was a sign survey. The researchers walked riverbanks to find otter latrines, identified otter scat by its size, shape, smell, contents and the presence of mucous, judged freshness (wet, soft, pungent) and counted fresh scats each month. Radio-tracking of translocated otters was used to find more latrines and to confirm that all sites were occupied. No trailing was reported.

### 5. Tracker or observer certification

Not reported. The paper does not say who identified and counted the scats or how experienced they were; the acknowledgements thank “many technicians and volunteers who analyzed camera pictures and counted and collected otter scat.” Scat identification was not checked against the camera images or by any other test.

### 6. Other technologies, skills or innovations

- High-end infrared (no-flash) Reconyx PC900 camera traps, set at 0.5 to 1 m above the ground on “high” sensitivity, taking two images per trigger 1 second apart with a 15-second pause between events.

- Visits counted from date and time stamps, using the largest number of otters seen in one picture during a visit, so groups counted for more than single otters.

- Radio telemetry of 23 translocated otters, and GPS locations for latrines.

### 7. Implications for conservation (from the paper)

- Much of what managers know about otter distribution, abundance and habitat use comes from latrine and scat surveys, with little evidence of how well they reflect actual use. This study shows that scat counts do reflect intensity of use but miss otters that are present.

- Modern, high-end cameras may give more accurate and reliable indices of animal presence and site use than indirect methods.

### 8. Implications for tracking (Original Wisdom's observations)

_These are Original Wisdom's observations, not the authors'._

- Scat at a latrine is good evidence that otters were there, but no scat is weak evidence that they were not: in 20 of 120 site-months otters visited without leaving any scat that was found.

- Snow and other weather can erase signs between visits, a reminder that the time since the last check matters when reading an absence of signs.

- The study tested scat counts only, and only at latrines. Tracks, slides and other otter signs at the same sites were not recorded, so it does not show how a full tracks and signs survey would compare with cameras.

## Cite this summary

Lawrence, K. 2026. Summary of: Comparing direct and indirect methods to estimate detection rates and site use of a cryptic semi-aquatic carnivore (Day and Westover et al., 2016). Original Wisdom, Tracking Literature. https://www.originalwisdom.com/tracking-literature/2016-day-westover-comparing-direct-indirect-methods/