# Evaluating bridge survey ability to detect river otter Lontra canadensis presence: a comparative study

> Many otter surveys search the riverbank near bridges because they are easy to reach. Over four winters of reading otter tracks and scats in the snow in New Brunswick, Canada, researchers found that river otters do not avoid bridges, but that the usual 600 m search from a bridge would have found only about half of the otters known to be present.

Summary by Kersey Lawrence, Original Wisdom. Updated 9 November 2026. Page: https://www.originalwisdom.com/tracking-literature/2008-gallant-vasseur-evaluating-bridge-survey-ability/

## Citation and access

- **Authors:** Daniel Gallant, Liette Vasseur and Céline H. Bérubé
- **Year published:** 2008
- **Journal or source:** Wildlife Biology
- **Full citation:** Gallant, D., Vasseur, L. and Bérubé, C. H. 2008. Evaluating bridge survey ability to detect river otter Lontra canadensis presence: a comparative study. Wildlife Biology 14(1): 61–69.
- **DOI:** 10.2981/0909-6396(2008)14[61:EBSATD]2.0.CO;2
- **Access status:** Hosted (freely available elsewhere)
- **PDF:** https://www.originalwisdom.com/wp-content/uploads/2026/11/2008_Gallant_and_Vasseur_etal_evaluating-bridge-survey-ability-to-detect-river-otter-presence-a-comparative-study.pdf

## Tags

- **Species:** Mammals > North American river otter
- **Region:** North America > Canada
- **Tracking:** Burrows; Scat; Sign surveys; Snow tracks; Tracking; Tracks and signs identification
- **Behaviour and ecology:** Behaviour; Human disturbance
- **Methods and technology:** Detectability; Global Positioning System (GPS); Linear regression
- **Data type:** New field data
- **Observer qualification:** Not reported
- **Discipline:** Behaviour and ecology; Wildlife tracking
- **Year published:** 2008

## Detailed keywords

- **Species:** Studied: North American river otter (Lontra canadensis). Compared with published work on the Eurasian otter (Lutra lutra). Habitat plants named: balsam fir (Abies balsamea), birch (Betula spp.), black spruce (Picea mariana) and speckled alder (Alnus rugosa). Beaver (Castor canadensis) is named only in the title of an earlier study by the authors.
- **Location and habitat:** Kouchibouguac National Park of Canada (238.8 km²) and its surroundings, including the Black River Provincially Protected Zone of New Brunswick, New Brunswick, Canada. Flat Maritime Coastal Plains with bogs and swamps; mixed forest of balsam fir and birch, black spruce forest, alder-lined streams; outside the protected areas, land at various stages of succession, light residential areas and farmland. Eight main watercourses (Portage, Carrigan, Fontaine, Black, Rankin, Kouchibouguac, Major and Kouchibouguacis) plus the St-Charles River; about 150 km of rivers and streams surveyed; 18 bridges, six of them in the park. Winter conditions on snow and river ice.
- **Topics:** bridge surveys; sign surveys for otters; random versus bridge search sites; search distance and detection rates; false absences (failing to detect otters that are present); otter response to bridges and roads; marking and defecation (latrine) behaviour; presence-absence data and population trends; statistical power and sample size; differences between North American river otters and Eurasian otters; population monitoring.
- **Statistics or models:** Accumulation (rarefaction) curves A(x) = ax/(b + x), fitted by non-linear regression (Levenberg-Marquardt method, SPSS 8.0); Student’s t-test (upstream versus downstream distances); simple linear regression (distance from bridge against days since last snowfall); random sites chosen with random number tables. Possible inconsistencies: the paper’s own citation line on its first page drops a word from the title (“Evaluating bridge ability”); the text describes the GPS unit as a “Geographic Positioning System”; the regression for days since snowfall is described under Methods with time in hours and days in different places.
- **Data type:** New field data: winter sign surveys from early January to the end of April in 2000, 2001, 2003 and 2004 (92 continuous transects, 1557 km of riverbank).

## Synopsis

### 1. What they did

Otters are hard to see, so they are usually monitored by searching riverbanks for their tracks and signs. Many surveys use bridges as search sites, because they are quick to reach by vehicle. But bridges are not random sites, and if otters avoid them, or mark less near them, bridge surveys could miss otters that are there. The authors tested whether bridge surveys detect North American river otters as well as surveys from randomly chosen sites.

- Winter sign surveys. Over four winters (2000, 2001, 2003 and 2004), from early January to the end of April, a team of two searched both shores of the rivers and streams in and around Kouchibouguac National Park, New Brunswick, Canada. They rode snowmobiles single file at slow speed where the ice allowed, and went on snowshoes along narrower streams. They waited at least 12 hours after snowfall so otters had time to leave signs in the fresh snow.

- Recording signs. They recorded each otter sign with a Global Positioning System (GPS) unit, along with the time since the last snowfall. For each transect that passed a bridge and had otter signs, they measured on maps the straight-line distance from the bridge to the closest otter sign, upstream and downstream.

- Building “what if” surveys. From these distances they worked out what share of the known otter presences a bridge survey would have found if it had searched only a set distance from the bridge. They did the same for a matching set of randomly placed sites on the same rivers, and compared the two with accumulation curves.

- Snowfall check. They tested whether the number of days since snowfall affected how far from bridges the closest signs were found.

### 2. What they found

Survey effort and signs

- 92 continuous transects covered 1557 km of riverbank, by repeatedly surveying a network of about 150 km of rivers and streams. Transects averaged 6.18 km.

- They recorded water access holes, burrows, direct sightings, faeces (scats) and snow tracks. Faeces and snow tracks were the most common signs.

- Of 643 signs, only 32 were outside the protected areas, so the results mostly concern bridges in or near the park.

Bridges versus random sites

- 50 transects included bridges and had otters present, giving 61 distance measurements (26 upstream, 35 downstream). The closest signs were not found at different distances upstream and downstream.

- Days since snowfall had no effect on how far from bridges the closest signs were found.

- The accumulation curves for bridge sites and random sites almost overlapped. For searches of 400 to 1200 m, bridge sites would have found slightly more of the known otter presences; for longer searches, random sites would have found slightly more. Below 2000 m, the largest difference was only 2.4% (at 380 m).

- Searching 600 m of riverbank from bridges, a distance used in many published otter surveys, would have detected only 51% of the known otter presences in this study.

- In earlier summer fieldwork along the park’s rivers, no otter latrines were ever found under bridges (the first author’s own observation).

### 3. What they concluded

- Sign surveys using bridges as search sites can detect river otters as well as surveys using randomly placed sites. River otters in the study area did not actively avoid bridges. The authors describe this as the first demonstration that bridges do not reduce the odds of detecting North American river otters.

- Bridge surveys could be improved by searching farther from each bridge. Because each study area is different, no single search distance can be recommended for everywhere.

- Longer searches mean fewer bridges can be visited with the same effort, so surveys should add time and labour to keep sample sizes large enough to detect trends.

- Unlike Eurasian otters, which often defecate under bridges and leave most signs within 100 to 200 m of them, the river otters in this study did not use bridges as marking sites. The results cannot be applied directly to Eurasian otter surveys.

- Presence-absence results from sign surveys may not track true population size, especially for a social species whose signs are clumped. Such data are best used for mapping distribution.

- Not all places with no signs on ice or snow are free of otters: otters may have passed under the ice near bridges.

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

The whole study rests on tracks and signs identification. The team searched riverbanks on snowmobile and snowshoe and recorded otter snow tracks, faeces, burrows, water access holes in the ice and direct sightings. The authors judged detection to be high in winter because otters leave conspicuous tracks in snow, which link all other signs to them, and because fresh snow gives an even surface for finding signs. They note that otter signs are distinctive and unlikely to be confused with other animals’ signs where only one otter species lives. No trailing is reported.

### 5. Tracker or observer certification

Not reported. The surveys were done by a team of two, and seven people are thanked for help collecting data. Nothing is said about their tracking experience or any test of their skill, although the paper assumes that otter signs were found and identified reliably.

### 6. Other technologies, skills or innovations

- Snowmobiles (Bombardier Tundra models) for long continuous transects on river ice, and snowshoes for narrow streams.

- A handheld Garmin 12 XL GPS for recording the start and end of transects and each otter sign.

- A “what if” approach: using long continuous transects to work out how shorter bridge searches, and searches from random sites, would have performed.

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

- Bridge surveys can be a sound, cost-effective way to monitor river otters, but surveys that search too short a distance may underestimate where otters live. Management decisions based on such underestimates might prompt unnecessary conservation actions.

- Past bridge surveys, which often searched 600 m or less, would probably have needed longer searches to reach high detection rates.

- Survey designers who lengthen searches should increase effort to keep enough sites for the statistical power to detect trends.

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

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

- Survey results depend on where the searcher looks as much as on the animal: a short search from a convenient access point may miss most of the animals present, even when that access point does not put them off.

- The study shows how species can differ in where they leave signs: a habit known in one otter species (marking under bridges) did not hold for a related species in another region.

## Cite this summary

Lawrence, K. 2026. Summary of: Evaluating bridge survey ability to detect river otter Lontra canadensis presence: a comparative study (Gallant and Vasseur et al., 2008). Original Wisdom, Tracking Literature. https://www.originalwisdom.com/tracking-literature/2008-gallant-vasseur-evaluating-bridge-survey-ability/