# Evaluation of Satellite Imagery for Monitoring Pacific Walruses at a Large Coastal Haulout  
[PERSON]  
1U.S. Geological Survey Alaska Science Center, 4210 University Dr., Anchorage, AK 99508, USA 1  
[PERSON]  
2U.S. Geological Survey Alaska Science Center, 250 Egan Dr., Juneau, AK 99801, USA; [EMAIL_ADDRESS]  
Footnote 1: email: [PERSON]DRESS]; fischbach@usgs. The periods of ice-free conditions during late summer and autumn in the Arctic are lengthening due to warming [7], especially in the Chukchi Sea where Pacific walruses seasonally range. Since 2007, during most years, sea ice has retreated northward beyond the continental shelf in the late summer and autumn, leaving Pacific walruses with no ice on which to rest between foraging bouts. This fundamental change in the distribution of sea ice across the summer and autumn range of Pacific walruses has resulted in larger numbers coming ashore for longer periods on both the U.S. and Russian coasts of the Chukchi Sea [8]. In the U.S., large haulouts have formed on the barrier islands near Point Lay, Alaska [6], in nearly all years since 2007 (except 2008 and 2012), when sea ice retreated beyond the continental shelf. When walruses haulout on land, they group tightly together in herds that may extend over hundreds or even thousands of meters of shoreline and up to one hundred meters inland. Large walrus aggregations are visually distinct from surrounding coastal features due to their color and texture, even when viewed at great distances or with relatively course resolution imagery. As such, some types of satellite imagery have been used successfully to distinguish walruses. [PERSON] and [PERSON] [9] pioneered detection of walrus haulouts with the use of high-resolution optical imagery from the QuickBird satellite (0.6 m panchromatic and 2.4 m multispectral). They identified walrus groups with as few as two animals and related walrus groups identifiable in the satellite imagery to on-site observer counts. Atlantic walruses on the shores of the Barents Sea were apparent in EROS-B panchromatic 0.7 m imagery [10], and Pacific walruses were detectable in GeoEye 0.5 m panchromatic imagery near Wrangel Island, Russia [11]. The Greenland Institute of Natural Resources examined the capability of detecting small walrus groups through statistical processing of high-resolution imagery from Worldview, Pleiades, and SPOT [12]. Despite these successes visualizing walruses with satellite sensors, systematic reconnaissance of haulouts has been hampered by infrequent data collections, cloud cover, timely access, and cost. High-resolution optical satellite imagery has also proved useful for enumerating other pinniped species such as Weddell seals (_Leptonychotes weddellii_) [13], elephant seals (_Mirounga leonina_) [14], and gray seals (_Halichoerus gypus_) [15], and such applications will likely expand as the number of high-resolution satellite systems increases [15]. No study to date has evaluated the repeatability and validity of interpreting walrus haulouts from imagery that spans a breadth of Earth observing satellite sensors that have recently become operational. Over the past decade, civil access to both optical and SAR imagery increased qualitatively for research and applied purposes. New satellite constellations have increased collection periodicity and improved resolution capability. These constellations support imagery acquired with reduced revisit intervals on the scale of days to hours, moderate (10 to 40 m) or high (0.4 m to 4 m) image resolutions, and some with active radar instruments that can discriminate ground features through clouds and at night. Under clear-sky daytime conditions, walrus haulouts can be imaged with several optical sensors operating in the visible to infra-red spectrum. The joint National Aeronautics and Space Administration (NASA) and U.S. Geological Survey (USGS) Landsat program [16] has collected multispectral imagery continuously since 1972.