# Interpreting Carnivore Scent-Station Surveys

> For decades, wildlife managers have counted carnivore tracks at scented patches of smoothed earth to follow population trends. Using eight years of Minnesota data, these authors showed that a single wolf, coyote or fox often leaves tracks at several stations in a row, and that earlier tests of the method were too weak to say whether it works. Their advice: count lines rather than stations, look for long-term trends, and use scent stations alongside other information.

Summary by Kersey Lawrence, Original Wisdom. Updated 9 November 2026. Page: https://www.originalwisdom.com/tracking-literature/1998-sargeant-johnson-interpreting-carnivore-scent-station/

## Citation and access

- **Authors:** Glen A. Sargeant, Douglas H. Johnson and William E. Berg
- **Year published:** 1998
- **Journal or source:** The Journal of Wildlife Management
- **Full citation:** Sargeant, G. A., Johnson, D. H. and Berg, W. E. 1998. Interpreting carnivore scent-station surveys. The Journal of Wildlife Management 62(4): 1235-1245.
- **DOI:** 10.2307/3801987
- **Access status:** Hosted (freely available elsewhere)
- **PDF:** https://www.originalwisdom.com/wp-content/uploads/2026/11/1998_Sargeant_and_Johnson_etal_interpreting-carnivore-scent-station-surveys.pdf

## Tags

- **Species:** Mammals > Bobcat; Mammals > Coyote; Mammals > Eastern spotted skunk; Mammals > Grey wolf; Mammals > Raccoon; Mammals > Red fox; Mammals > Striped skunk
- **Region:** North America > United States
- **Tracking:** Sign surveys; Tracking; Tracks; Tracks and signs identification
- **Behaviour and ecology:** Behaviour; Movement
- **Methods and technology:** Chi-square test; Linear regression; Logistic regression; Population trends; Power analysis; Simulation
- **Data type:** Existing data; Modelling or theoretical
- **Observer qualification:** Not reported
- **Discipline:** Population trends; Wildlife tracking
- **Year published:** 1998

## Detailed keywords

- **Species:** Studied in the Minnesota survey: gray wolf (Canis lupus), coyote (Canis latrans), red fox (Vulpes vulpes), skunks (mostly Mephitis mephitis, but also Spilogale putorius), raccoon (Procyon lotor) and bobcat (Felis rufus, as named in the paper). Reanalysed from earlier studies: raccoons on Davies Island, Tennessee (Smith et al. 1994) and bobcats introduced to Cumberland Island, Georgia (Diefenbach et al. 1994). Mentioned from another study: opossum (Didelphis virginiana).
- **Location and habitat:** Minnesota, United States: 441 scent-station lines along unpaved roads across the state, analysed by biogeographic section (Red River, Aspen Parkland, Peatland, North Superior, Lake Plain, Oak Savanna, West Superior, South Superior, Glaciated Plain and Driftless; South Superior was combined with West Superior). Reanalysed studies: Davies Island, Tennessee, and Cumberland Island, Georgia, United States.
- **Topics:** scent-station surveys; population indices; population trend monitoring; tracks at scented baits; station index versus line index; spatial correlation between stations; repeat visits by the same individual (pseudoreplication); binomial distribution; statistical power; confounding (weather, season, habitat, human activity); validation of index methods; survey design and cost; observer bias and observer error.
- **Statistics or models:** Linear regression of rank-transformed index values on year (trend tests, with section by year interaction); chi-square goodness-of-fit tests against the binomial distribution; Fisher’s inverse chi-square test; indicator variograms; simulation of 10,000 binomial datasets (SAS RANBIN) to estimate the power of Spearman’s rank correlation and logistic analysis; multiple regression of inverse visitation rate on abundance and time period. Statewide means weighted by section area. Note: several test values repeat in the text and Table 1 (for example F = 37.56 for wolf and raccoon station indices and red fox line indices; F = 4.23 for bobcat station and coyote line indices). This is plausible for rank-based tests on 8 years of data and is not necessarily an error. Text and Table 1 trend directions agree.
- **Data type:** Existing data: the Minnesota Department of Natural Resources carnivore scent-station survey, 1986 to 1993 (lines run one night a year between late August and mid-October; 1992 to 1993 data excluded from the spatial analysis), and survey data from Diefenbach et al. (1994). Modelling: simulations patterned after Smith et al. (1994).

## Synopsis

### 1. What they did

For decades, wildlife managers have counted the tracks left at scented baits (scent stations) to follow trends in carnivore numbers. Carnivores are secretive and usually live at low densities, so counting them directly is hard and costly. Scent stations seemed a cheap answer, but attempts to check whether they really track abundance had given mixed results.

The authors used data from one of the most intensive long-term scent-station surveys: Minnesota, 1986 to 1993. Each station was a circle of smoothed earth 0.9 m across with a fatty-acid scent tablet in the middle. Ten stations, 480 m apart along unpaved roads, made one line, and 441 lines were spread across the state. Each line ran for one night a year, and the next day the presence or absence of each species’ tracks was recorded at every station.

They asked:

- How do scent-station data behave statistically? Are visits to nearby stations independent, or does one animal visit several stations in a row?

- How should the data be analysed? They compared a station index (share of stations visited) with a line index (share of lines with at least one visit), and tested for trends over the years in each part of the state.

- Do scent stations reflect abundance? They reanalysed two of the most rigorous earlier validation studies: raccoons on Davies Island, Tennessee, and bobcats introduced to Cumberland Island, Georgia.

### 2. What they found

Visits to nearby stations are linked

- For every species except raccoons, lines had more stations visited, and fewer lines were visited, than expected if each visit were independent.

- Results from stations closer than about 2 to 2.5 km were similar. The authors put this down mainly to single animals visiting several stations in succession. The effect was strongest for wide-ranging, territorial species (wolves, coyotes, red foxes, bobcats) and weakest for skunks and raccoons.

- Stations at opposite ends of a line also gave surprisingly similar results, which the authors could not explain.

- Counting each line as simply visited or not visited removed the problem: the number of lines visited fitted a binomial distribution for all species.

Trends in Minnesota, 1986 to 1993

- Statewide station indices rose for wolves, coyotes, red foxes, raccoons and bobcats, and fell for skunks. Line indices showed the same directions.

- Statewide figures hid regional differences. Coyote station indices fell in the West Superior and Peatland sections but rose elsewhere. Raccoon indices fell only in the Peatland section. Skunk line indices rose in three sections and fell in the rest.

The validation studies

- Raccoons, Davies Island: even if raccoon visits truly rose in line with numbers, the original tests would have detected it in only 18.3% (rank correlation) and 12.6% (logistic analysis) of 10,000 simulations. The original study’s failure to find a link was most simply explained by low statistical power.

- Bobcats, Cumberland Island: abundance explained 73% of the variation in visit rates. But within each of the two time periods, visits did not rise with numbers, and a model that only separated the two periods explained 79%. Other factors that differed between periods could explain the result as well as abundance could.

### 3. What they concluded

- Visits to stations closer than about 2 km are not independent, because one animal can visit several. Treating stations as independent makes tests find differences too often and lets a few busy lines dominate.

- Lines are the right unit. Scoring each line as visited or not gives well-behaved data and gives individual animals fairer weight.

- Testing for trends over several years is more reliable than comparing two years, because many factors other than abundance change visit rates from year to year.

- Regions should be analysed separately, because trends differed between them.

- The validity of scent-station indices has been neither proven nor seriously disproven. Earlier validation studies were limited by low statistical power and confounding.

- Long-term trends in visit rates probably reflect real population changes. But the method has poor spatial and time resolution, is easily confounded, and has low statistical power. It should supplement other information, not replace it.

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

Scent stations are a tracks and signs method: each station is a patch of smoothed earth where visiting animals leave footprints, and observers recorded which species’ tracks were present the day after the station was set. All analyses rest on these track identifications. No trailing was reported. The authors list observer bias and observer error among possible sources of error, but did not test identification accuracy.

### 5. Tracker or observer certification

Not reported. The paper does not say who checked the stations or how their track identification skills were assessed. The authors name observer bias and observer error as possible influences on the results, and note that their simulations did not include observer error.

### 6. Other technologies, skills or innovations

- Indicator variograms, a geostatistical tool, used to show how similarity between stations changes with distance.

- Computer simulation of 10,000 datasets to estimate the statistical power of an earlier study’s tests.

- A simple trend test (regression of ranked index values on year) that the authors found easy to apply and interpret.

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

- Scent stations are best used for broad, long-term trends at a regional scale, not for rare species or for local monitoring of wide-ranging carnivores.

- Reliable results may need hundreds of lines of many stations, so the method may be less cost-effective than often believed.

- Scent-station indices cannot yet be converted to numbers of animals, so it is hard to know from them alone when management action is needed.

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

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

- The whole method depends on correctly identifying tracks in smoothed earth, yet the paper, like many survey papers, says nothing about who identified them or how well. Observer skill is a hidden source of error in track-based indices.

- The finding that one animal can leave tracks at several stations in a row is something trackers can read directly on the ground; following the tracks between stations could show how often this happens.

## Cite this summary

Lawrence, K. 2026. Summary of: Interpreting Carnivore Scent-Station Surveys (Sargeant and Johnson et al., 1998). Original Wisdom, Tracking Literature. https://www.originalwisdom.com/tracking-literature/1998-sargeant-johnson-interpreting-carnivore-scent-station/