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STUDY TITLE: Development of Airborne Remote Sensing Methods for Surveys of Pacific Walrus 

REPORT TITLE: Development of Airborne Remote Sensing Methods for Surveys of Pacific Walrus 

CONTRACT NUMBER: Intra-agency Agreement No. 0103RU72398 

MMS STUDY NUMBER: OCS Study MMS 2006-003 

SPONSERING OCS REGION: Alaska 

APPLICABLE PLANNING AREAS: Alaska 

FISCAL YEAR OF PRODUCT FUNDING: FY2003 

COMPLETION DATE OF REPORT: December 2006 

COSTS: $87,120 

COSTS BY FY: $87,120 (FY2003) 

CUMULATIVE PROJECT COST: $87,120 

 

PROJECT MANAGER: Douglas Burn 

AFFILIATION: U.S. Fish and Wildlife Service, Marine Mammals Management 

ADDRESS: 1011 East Tudor Road, Anchorage, AK 99503 

PRINCIPAL INVESTIGATORS: D.M. Burn, M.S. Udevitz, M.A. Webber and J.L. Garlich-Miller 

KEY WORDS: aerial survey, Bering sea, digital photography, infrared, population estimation, Odobenus 

rosmarus divergens, Pacific walrus, regression estimation, remote sensing, thermal imagery 

BACKGROUND: At any time of year, some or all of the walrus population is associated with sea ice, 

where animals haul out of the water and rest in large aggregations on ice floes. Within minutes of leaving 

the water, their skin temperature becomes noticeably warmer than the background environment, which 

provides excellent thermal contrast. In April 2002, we tested an Airborne Multispectral Scanner (AMS) to 

determine if this technology could be used to detect walrus groups on sea ice and estimate the number of 

walrus present in each group. We collected thermal imagery of 37 walrus groups in the Bering Sea at 

spatial resolutions ranging from 1-4 m, and high resolution digital photographs of the same walrus groups. 

We found that walrus were considerably warmer than the background environment of ice, snow, and 

seawater and easily detected in the thermal imagery. We also found a significant linear relation between 

walrus group size and the amount of heat measured by the thermal sensor at all 4 spatial resolutions tested. 

We hypothesized that the relationship between group size and thermal signature could be used in a doublesampling framework to estimate total walrus numbers in an area, by conducting a thermal survey of an area 

and obtaining photographs from a sub sample of the thermally detected groups. Based upon the successful 

test of the AMS system in 2002, we proposed conducting a pilot survey for Pacific walrus in the spring of 

2003 in the Bering Sea pack ice near St Lawrence Island. The study area was selected because major 

aggregations of walruses typically occur during spring in association with polynyas that form around the 

Island. 
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OBJECTIVES: 1. Conduct a full operational test of the AMS system to evaluate the potential of this 

system as a survey tool for Pacific walruses distributed over a wide geographic area.2. Evaluate the 

effectiveness of the AMS system as a survey tool for other marine mammal and bird species. 

DESCRIPTION: In April 2003, we conducted an operational test of an airborne multispectral scanner 

(AMS) over pack ice in the Bering Sea to evaluate the potential of this system as a survey tool for Pacific 

walruses. We scanned a total of 28,875 km2

 of sea ice habitat at a spatial resolution of 4 m and collected 

high resolution photographs from a subset of the thermally detected walrus groups. We found a significant 

positive relationship between walrus group size and the amount of heat measured by the AMS and used this 

relationship to estimate total walrus numbers in the survey area. The number of walruses hauled out onto 

sea ice in our study area was estimated at 4,785 animals with a 95% confidence interval of 2,499–7,111. 

SIGNIFICANT CONCLUSIONS: We believe that the AMS system as configured for this study would be 

a highly effective tool for surveying large areas of sea ice habitat for walrus groups. With a 6 km swath 

width, it should be possible to sample more 10,000 km2

 in an 8-hr flight. Although walrus groups > 4 

animals were easily detected and enumerated in the 4 m thermal data, the system was unable to detect 

individual walruses or seals (Phoca spp. and Erignathus barbatus). We found that most (94.6%) of the 

walruses photographed in our survey area occurred in groups > 6 animals, therefore we expect the 

magnitude of any bias due to undetected groups of hauled out animals would be relatively small. 

STUDY RESULTS: We sampled a total area of 28,875 km2

 with the AMS system at a spatial resolution 

of 4 m, and encountered many walrus groups scattered throughout the survey area. Survey efficiency, 

measured as the proportion of the survey block sampled with the AMS system, ranged from 21 to 38 

percent. The number of square kilometers scanned per day ranged from 3,622 to 6,051 with a mean of 

4,813. Air speed, which ranged from 333 to 444 km/hr, was not limited by scanner operations but was 

affected by wind speed and direction. During the 2003 survey effort, threshold temperatures ranged from -

2.98 to -4.32 °C. In general, the threshold temperatures were lower when ambient temperatures were lower. 

We photographed a total of 46 walrus groups during the survey period and were able to match 25 of these 

to corresponding hotspots in the thermal imagery. There was a strong linear relationship (X1

2

 =90.02, P < 

0.01) between the numbers of walruses in a group and the index of total walrus heat, with a slope parameter 

estimate of β = 0.67 (standard error = 0.0070). Plots of deviance residuals did not show any lack of fit to 

the linear model or the Poisson variance function. The total number of walruses in the 6 surveyed blocks 

was estimated at 4,785 animals with a standard error of 1,186 (C.V. = 0.25) and a 95 % confidence interval 

of 2,499–7,111. 

STUDY PRODUCTS: 

Burn, D., M. Webber and M. Udevitz. (2006). Application of airborne thermal imagery to surveys of 

Pacific walrus (Odobenus rosmarus divergens). Wildlife Society Bulletin 34:51–58. 

Udevitz, M.S. 2005. Statistical procedures for estimating Pacific walrus population size. Technical report, 

Alaska Science Center, U.S. Geological Survey, Anchorage, Alaska. 9 pp. 

USFWS and USGS. (2006) Study Plan for estimating the size of the Pacific walrus population. 

Udevitz, M.S., D.M. Burn and M.A. Webber. (In prep) Estimation of walrus populations on sea ice with 

infrared imagery and aerial photography. 
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Map showing area of study 

Study area for field data collection of AMS thermal imagery and digital aerial photography indicating 

survey blocks, midlines of strip transects flown and the location of detected walrus groups.