# Testing the surveys and visual and track censuses of Eurasian otters (Lutra lutra)

> How well do otter tracks and droppings reveal how many otters are really there? Using radio-tagged otters released into rivers in north-east Spain, researchers found that a single otter leaves signs over dozens of kilometres, and that counting only fresh footprints matched the number of otters seen at dawn and dusk. Counting old and new prints together doubled the estimate.

Summary by Kersey Lawrence, Original Wisdom. Updated 9 November 2026. Page: https://www.originalwisdom.com/tracking-literature/2001-ruiz-olmo-saavedra-testing-surveys-visual-track/

## Citation and access

- **Authors:** Jordi Ruiz-Olmo, Deli Saavedra and Juan Jiménez
- **Year published:** 2001
- **Journal or source:** Journal of Zoology
- **Full citation:** Ruiz-Olmo, J., Saavedra, D. and Jiménez, J. 2001. Testing the surveys and visual and track censuses of Eurasian otters (Lutra lutra). Journal of Zoology 253(3): 359-369.
- **DOI:** 10.1017/S0952836901000334
- **Access status:** Publisher link only
- **Where to find it:** https://doi.org/10.1017/S0952836901000334

## Tags

- **Species:** Mammals > Eurasian otter
- **Region:** Europe > Spain
- **Tracking:** Individual identification; Scat; Sign surveys; Spoor age estimation; Track counts; Track measurements; Tracking; Tracks; Tracks and signs identification; Trailing
- **Behaviour and ecology:** Behaviour; Communication; Daily travel distance; Home range; Movement; Range expansion
- **Methods and technology:** Chi-square test; Detectability; Direct observation; Distribution mapping; Linear regression; Mann–Whitney U test; Pearson correlation; Population estimation; Radio telemetry
- **Data type:** Existing data; New field data
- **Observer qualification:** Not reported
- **Discipline:** Behaviour and ecology; Wildlife tracking
- **Year published:** 2001

## Detailed keywords

- **Species:** Studied: Eurasian otter (Lutra lutra), wild and reintroduced. Other otters mentioned from earlier survey and census work: southern river otter (Lontra provocax), Lutra maculicollis, Aonyx capensis, smooth-coated otter (Lutra perspicillata), Lontra canadensis (also given as Lutra canadensis), sea otter (Enhydra lutris) and Lutra longicaudis.
- **Location and habitat:** North-eastern and eastern Spain. Reintroduction area: the Muga and Fluvià river basins and the Aiguamolls de l’Empordà marshes (over 5000 ha of freshwater canals and lagoons with reeds and bulrushes), Girona Province, short Mediterranean rivers with riverine woodland of ash and poplar. Wild otters: River Bergantes (tributary of the Guadalope, Ebro basin), an arid mountain area; rivers Noguera Ribagorçana and Noguera Pallaresa in the Spanish Pyrenees, Lleida Province. Otters for reintroduction were caught in Extremadura, south-west Spain.
- **Topics:** otter sign surveys (spraints and footprints at 600 m sites); positive and negative sites; false negatives; detection probability and number of otters; river width and detection; home ranges versus survey results; visual (crepuscular) censuses; footprint censuses by forefoot and heel pad length; telling fresh from old prints; overestimation from old tracks; census unit length (10 km stretches); reintroduction and recolonization; scent marking with spraints; spraints per latrine.
- **Statistics or models:** chi-square tests for contingency tables; conditional probability of finding otters at consecutive surveyed sites; Mann–Whitney U test (spraints per latrine); linear regression and Pearson correlation (otters seen versus otters detected by tracks: new tracks y = 1.27x - 0.348, r = 0.921; all tracks y = 1.39x + 0.44, r = 0.863; n = 16, P < 0.001). Inconsistencies in the paper: (1) the text says one early released otter was killed by a car in December 1995, but Table 1 gives the only December death (F1) as “killed by fish net”; (2) the chi-square for one otter versus two or more (3.51, 1 degree of freedom) is reported with P = 0.006, which does not match that value (it would be about 0.06); (3) the Fig. 5(a) caption names triangles and squares, but the plot shows triangles and diamonds, and the curve reaching 1 fastest looks like it should be the two-or-more-otters group; (4) the Discussion says the first four otters were found at 40% of sites along 80 km, while the Results give 23.2% along 80 km in early 1996 and 39% along 135 km in winter 1996; (5) home ranges and movement: the Discussion says otters “will often move up to 200 km a day”, later “> 20 km/day and even 100 km on some days”, with average daily movements of 4-7 km.
- **Data type:** New field data: five otter surveys of 83 sites (spring 1996 to summer 1997); radio-tracking of 22 otters (November 1995 to August 1997); 24 visual and track censuses (602 individual watches over 1067 hours) between 1995 and 1998, eight of them with radio-tagged otters in 1997 and 1998. Existing data: earlier otter surveys in 1984 (30 sites), 1989 (34 sites) and 1994 (144 sites) for comparison.

## Synopsis

### 1. What they did

Since the late 1970s, otters across Europe have been surveyed by checking short stretches of riverbank (usually 600 m) for their tracks and spraints (otter droppings), and recording each site as positive or negative. Counting otters is harder, and researchers have argued for years about how much these surveys and censuses really tell us. The authors set out to test them against otters whose whereabouts were known from radio-tracking.

- Radio-tracking. As part of a reintroduction of Eurasian otters to the Muga and Fluvià river basins in north-east Spain, otters caught in Extremadura were fitted with radio transmitters and released from November 1995. Wild otters on the River Bergantes were also tagged. In all, 22 otters were radio-tracked, located daily from the ground, and by small plane when the signal was lost. Some were followed in 24-hour sessions, with a location at least every 30 minutes.

- Otter surveys. A network of 83 sites, each 600 m long, was surveyed five times between spring 1996 and summer 1997 in the reintroduction area. The results were compared with the otters’ home ranges, and with three earlier surveys from 1984, 1989 and 1994, before the otters were brought back.

- Visual censuses. Teams of 18 to 24 observers, spaced about 500 m apart along 9 to 12 km of river, watched for otters for about two hours at dawn and two hours at dusk, between mid-May and mid-July.

- Track censuses. On the same days, otter footprints along the same stretches were measured (forefoot length with and without the nail, and heel pad length) to tell individuals apart. Fresh prints (under one day old, sometimes two) were separated from old ones.

In total, 24 visual and track censuses were carried out between 1995 and 1998: eight where radio-tracking showed at least one otter had been present the day before, and 16 on Pyrenean rivers with no tagged otters.

### 2. What they found

Surveys and the reintroduced otters

- Before the reintroduction, none of the surveyed sites were positive. With only four otters, 23.2% of sites were positive in early 1996, spread over about 80 km of river and 940 ha of marsh. By winter 1996 to 1997 this had risen to 39%, over 135 km.

- Releasing a further 14 otters did not raise the share of positive sites further (39.2% in spring 1997). A few otters spread their signs over large areas, and new arrivals tended to use the same places.

- The otters always left spraints and prints over large distances (dozens of kilometres). One female used 35.5 km of river in 233 days, and one male 90 km in 588 days.

Surveys compared with home ranges

- Survey results and radio-tracked home ranges matched closely, but not exactly. 37.5% of sites outside the estimated home ranges were positive, and 24.5% of negative sites lay inside home ranges.

- Where only one otter lived, 70.9% of site surveys were positive. Where two or more lived, 96.9% were.

- Detection was lowest on medium rivers (15 to 40 m wide), at 55.5%. On small rivers it was 93.3%, and in lakes and marshes 90.5%. Young and older otters did not differ.

- Surveying two or three consecutive sites was enough to find otters every time, except on medium rivers, which needed seven (three sites found them 92% of the time, four sites 97%).

- The average number of spraints per latrine (1.25 to 2.5) did not change across the five surveys.

Visual censuses

- Every radio-tagged otter present in a census stretch (7 otters) was seen during the standard visual censuses.

- Tagged otters were out of their rest sites during 76.2% of dawn watches and 95.5% of dusk watches, so a census with both a dawn and a dusk watch would have seen every otter in the observers’ field of view.

Visual and track censuses compared

- The number of otters seen and the number told apart by footprint size were closely related.

- Using only fresh prints (from the night before) gave almost the same density (otters per km) as the visual census. Using old and new prints together overestimated the number of otters by about two times.

- Some otters were found only by their prints (they were inactive that day, moved outside the watch times, or were missed by an observer), and some were seen but not found by their prints.

### 3. What they concluded

- In small and medium waterways, a single otter marks so many kilometres that they can be found at most survey sites. Several negative sites in a row can be read as the otters being absent from that area.

- Otters passing through, or using a stretch only to move between core areas, are not picked up well by surveys. A single spraint in an unusual place should be treated with caution until it is repeated.

- The number of positive sites and the number of spraints do not rise in step with the number of otters; spraints quickly reach saturation. They give only a rough idea of abundance.

- Fresh-footprint and visual censuses give numbers close to the real number of otters. Visual censuses need experienced census takers working at set times with a standard method.

- At high densities (over 0.6 otters per km), footprint censuses may undercount, because two otters of the same sex and similar size can have prints that are confused. At low densities (under 0.1 per km), visual censuses may miss otters that move out of the census stretch.

- Ten-kilometre stretches seem adequate as census units when only one-day-old tracks or one activity period are used, though 20 to 30 km may be more accurate. Counting every stretch with tracks a few days old could suggest up to 10 otters where only one lives.

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

Tracking was central to the study. The otter surveys relied on finding and identifying otter tracks and spraints at each site. In the track censuses, several prints in each otter’s trail were measured (forefoot length with and without the nail, and heel pad length) to tell individuals apart, following Sidorovich (1992). Fresh prints were told from old ones by (a) sometimes doing a footprint census before the visual census, (b) noting whether prints were in new mud after rain or on a wet morning, and (c) checking whether prints were dry, since in late spring and summer prints there dry within one or two days. Prints were also used to follow a radio-tagged mother and her cub after her transmitter failed: spraints and adult and cub footprints showed them living apart from the other otters. The authors note that at low density telling individuals apart by prints “caused no difficulty”, but at high density prints of two otters of the same sex and similar size may be confused.

### 5. Tracker or observer certification

Not reported. The paper does not say who did the sign surveys or measured the footprints, or how experienced they were. The authors stress that visual censuses need experienced census takers and cite earlier work in which an experienced observer almost doubled the chance of seeing otters, but they give no test of the observers’ or trackers’ skill in this study.

### 6. Other technologies, skills or innovations

- Radio transmitters (Telonics IMP/300/L, ATS 203-12 and Wagener) weighing 36 to 40 g, located from four-wheel-drive vehicles and, when the signal was lost, from small planes.

- Using a reintroduced population of known size as a test of survey and census methods.

- Conditional probability to estimate how many consecutive sites need surveying to find otters.

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

- On the edges of the species’ range, a single otter can be easily detected in small and medium waterways, so repeated failure to find otters in an area over time suggests they are absent.

- When otters recolonize a river, naturally or after reintroduction, they reach most waterbodies in the basin quickly, often within one to four years, because of their large home ranges.

- The traditional image of a single otter, or a pair, living in a short and hard-to-find stretch of mountain stream is not realistic.

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

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

- The study gives a clear, measured example of why track age matters: counting old and new otter prints together roughly doubled the estimate, while fresh prints matched what observers saw.

- Footprint measurements worked to tell individual otters apart at low density but may confuse similar animals at high density, a limit worth keeping in mind for any count based on track size.

## Cite this summary

Lawrence, K. 2026. Summary of: Testing the surveys and visual and track censuses of Eurasian otters (Lutra lutra) (Ruiz-Olmo and Saavedra et al., 2001). Original Wisdom, Tracking Literature. https://www.originalwisdom.com/tracking-literature/2001-ruiz-olmo-saavedra-testing-surveys-visual-track/