{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,2,14]],"date-time":"2026-02-14T12:33:32Z","timestamp":1771072412124,"version":"3.50.1"},"reference-count":42,"publisher":"MDPI AG","issue":"21","license":[{"start":{"date-parts":[[2021,10,23]],"date-time":"2021-10-23T00:00:00Z","timestamp":1634947200000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"DOI":"10.13039\/100000203","name":"U.S. Geological Survey","doi-asserted-by":"publisher","award":["Ecosystems Mission Area"],"award-info":[{"award-number":["Ecosystems Mission Area"]}],"id":[{"id":"10.13039\/100000203","id-type":"DOI","asserted-by":"publisher"}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Remote Sensing"],"abstract":"<jats:p>Pacific walruses (Odobenus rosmarus divergens) are using coastal haulouts in the Chukchi Sea more often and in larger numbers to rest between foraging bouts in late summer and autumn in recent years, because climate warming has reduced availability of sea ice that historically had provided resting platforms near their preferred benthic feeding grounds. With greater numbers of walruses hauling out in large aggregations, new opportunities are presented for monitoring the population. Here we evaluate different types of satellite imagery for detecting and delineating the peripheries of walrus aggregations at a commonly used haulout near Point Lay, Alaska, in 2018\u20132020. We evaluated optical and radar imagery ranging in pixel resolutions from 40 m to ~1 m: specifically, optical imagery from Landsat, Sentinel-2, Planet Labs, and DigitalGlobe, and synthetic aperture radar (SAR) imagery from Sentinel-1 and TerraSAR-X. Three observers independently examined satellite images to detect walrus aggregations and digitized their peripheries using visual interpretation. We compared interpretations between observers and to high-resolution (~2 cm) ortho-corrected imagery collected by a small unoccupied aerial system (UAS). Roughly two-thirds of the time, clouds precluded clear optical views of the study area from satellite. SAR was unaffected by clouds (and darkness) and provided unambiguous signatures of walrus aggregations at the Point Lay haulout. Among imagery types with 4\u201310 m resolution, observers unanimously agreed on all detections of walruses, and attained an average 65% overlap (sd 12.0, n 100) in their delineations of aggregation boundaries. For imagery with ~1 m resolution, overlap agreement was higher (mean 85%, sd 3.0, n 11). We found that optical satellite sensors with moderate resolution and high revisitation rates, such as PlanetScope and Sentinel-2, demonstrated robust and repeatable qualities for monitoring walrus haulouts, but temporal gaps between observations due to clouds were common. SAR imagery also demonstrated robust capabilities for monitoring the Point Lay haulout, but more research is needed to evaluate SAR at haulouts with more complex local terrain and beach substrates.<\/jats:p>","DOI":"10.3390\/rs13214266","type":"journal-article","created":{"date-parts":[[2021,10,24]],"date-time":"2021-10-24T22:07:11Z","timestamp":1635113231000},"page":"4266","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":18,"title":["Evaluation of Satellite Imagery for Monitoring Pacific Walruses at a Large Coastal Haulout"],"prefix":"10.3390","volume":"13","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-6555-865X","authenticated-orcid":false,"given":"Anthony S.","family":"Fischbach","sequence":"first","affiliation":[{"name":"U.S. Geological Survey Alaska Science Center, 4210 University Dr., Anchorage, AK 99508, USA"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-0186-1104","authenticated-orcid":false,"given":"David C.","family":"Douglas","sequence":"additional","affiliation":[{"name":"U.S. Geological Survey Alaska Science Center, 250 Egan Dr., Juneau, AK 99801, USA"}]}],"member":"1968","published-online":{"date-parts":[[2021,10,23]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"291","DOI":"10.1007\/s00300-012-1259-3","article-title":"Potential population-level effects of increased haulout-related mortality of Pacific walrus calves","volume":"36","author":"Udevitz","year":"2013","journal-title":"Polar. Biol."},{"key":"ref_2","unstructured":"Garlich-Miller, J., MacCracken, J.G., Snyder, J., Meehan, R., Myers, M., Wilder, J.M., Lance, E., and Matz, A. (2011). Status Review of the Pacific Walrus (Odobenus Rosmarus Divergens)."},{"key":"ref_3","doi-asserted-by":"crossref","first-page":"20","DOI":"10.1111\/mms.12013","article-title":"Estimation of calf: Cow ratios of Pacific walruses for use in population modeling and monitoring","volume":"30","author":"Citta","year":"2014","journal-title":"Mar. Mammal. Sci."},{"key":"ref_4","doi-asserted-by":"crossref","first-page":"514","DOI":"10.1111\/j.1748-7692.2010.00419.x","article-title":"Results and evaluation of a survey to estimate Pacific walrus population size","volume":"27","author":"Speckman","year":"2011","journal-title":"Mar. Mammal. Sci."},{"key":"ref_5","unstructured":"Battaile, B.C., and Jay, C.V. (2021). Walrus Haulout Photographs Near Pt. Lay Alaska, September 2014, U.S. Geological Survey Data Release."},{"key":"ref_6","doi-asserted-by":"crossref","unstructured":"Fischbach, A.S., Kochnev, A.A., Garlich-Miller, J.L., and Jay, C.V. (2016). Pacific Walrus Coastal Haulout Database, 1852\u20132016\u2014Background Report.","DOI":"10.3133\/ofr20161108"},{"key":"ref_7","doi-asserted-by":"crossref","first-page":"4917","DOI":"10.1175\/JCLI-D-17-0427.1","article-title":"Seasonal and regional manifestation of Arctic sea ice loss","volume":"31","author":"Onarheim","year":"2018","journal-title":"J. Clim."},{"key":"ref_8","doi-asserted-by":"crossref","first-page":"1","DOI":"10.3354\/meps10057","article-title":"Walrus areas of use in the Chukchi Sea during sparse sea ice cover","volume":"468","author":"Jay","year":"2012","journal-title":"Mar. Ecol. Prog. Ser."},{"key":"ref_9","unstructured":"Burn, D.M., and Cody, M.B. (2005, January 12\u201316). Use of Satellite Imagery to Estimate Walrus Abundance at Round Island, Alaska. Proceedings of the 16th Biennial Conference on the Biology of Marine Mammals, San Diego, CA, USA."},{"key":"ref_10","first-page":"89","article-title":"Study of Atlantic walruses in the Barents Sea using space technologies","volume":"10","author":"Semenova","year":"2011","journal-title":"Earth Space"},{"key":"ref_11","doi-asserted-by":"crossref","first-page":"197","DOI":"10.1134\/S1062359013020106","article-title":"The possibility of using high resolution satellite images for detection of marine mammals","volume":"40","author":"Platonov","year":"2013","journal-title":"Biol. Bull."},{"key":"ref_12","unstructured":"Zinglersen, K.B., Garde, E., Langley, K., and M\u00e4tzler, E. (2019). RemoteID: Identification of Atlantic Walrus at Haul Out Sites in Greenland Using High-Resolution Satellite Images, Greenland Institute of Natural Resources. Technical Report No. 111."},{"key":"ref_13","doi-asserted-by":"crossref","first-page":"1727","DOI":"10.1007\/s00300-011-1023-0","article-title":"Satellite imagery can be used to detect variation in abundance of Weddell seals (Leptonychotes weddellii) in Erebus Bay, Antarctica","volume":"34","author":"LaRue","year":"2011","journal-title":"Polar Biol."},{"key":"ref_14","doi-asserted-by":"crossref","unstructured":"McMahon, C.R., Howe, H., van den Hoff, J., Alderman, R., Brolsma, H., and Hindell, M.A. (2014). Satellites, the all-seeing eyes in the sky: Counting elephant seals from space. PLoS ONE, 9.","DOI":"10.1371\/journal.pone.0092613"},{"key":"ref_15","doi-asserted-by":"crossref","first-page":"760","DOI":"10.1093\/biosci\/bix059","article-title":"Google haul out: Earth observation imagery and digital aerial surveys in coastal wildlife management and abundance estimation","volume":"67","author":"Moxley","year":"2017","journal-title":"BioScience"},{"key":"ref_16","unstructured":"(2021, July 29). Landsat Missions, Available online: https:\/\/usgs.gov\/landsat."},{"key":"ref_17","unstructured":"(2021, July 29). Sentinel Online. Available online: https:\/\/sentinel.esa.int."},{"key":"ref_18","unstructured":"(2021, July 29). Discover Digital Globe. Available online: https:\/\/discover.digitalglobe.com."},{"key":"ref_19","unstructured":"(2021, July 29). Airbus. Available online: https:\/\/www.intelligence-airbusds.com."},{"key":"ref_20","unstructured":"(2021, July 29). Planet. Available online: https:\/\/www.planet.com."},{"key":"ref_21","unstructured":"(2021, July 29). CubeSat. Available online: https:\/\/www.cubesat.org."},{"key":"ref_22","doi-asserted-by":"crossref","first-page":"59","DOI":"10.1016\/j.paerosci.2016.11.002","article-title":"CubeSat evolution: Analyzing CubeSat capabilities for conducting science missions","volume":"88","author":"Poghosyan","year":"2017","journal-title":"Prog. Aerosp. Sci."},{"key":"ref_23","doi-asserted-by":"crossref","first-page":"172","DOI":"10.1126\/science.348.6231.172","article-title":"Startup liftoff","volume":"348","author":"Hand","year":"2015","journal-title":"Science"},{"key":"ref_24","doi-asserted-by":"crossref","first-page":"10017","DOI":"10.1002\/2017WR022240","article-title":"CubeSats in hydrology: Ultrahigh-resolution insights into vegetation dynamics and terrestrial evaporation","volume":"53","author":"McCabe","year":"2017","journal-title":"Water Resour. Res."},{"key":"ref_25","unstructured":"Fischbach, A.S., Jay, C.V., Monette, C.J., and Adams, J.D. (2021). Walrus Haulout Outlines and Count Data Apparent from Aerial Survey Images Collected Near Point Lay Alaska, Autumn 2018 and 2019, U.S. Geological Survey Data Release."},{"key":"ref_26","unstructured":"(2021, July 29). Eskimo Walrus Commission. Available online: https:\/\/eskimowalruscommission.org."},{"key":"ref_27","unstructured":"Eineder, M., Fritz, T., Mittermayer, J., Roth, A., Boerner, E., Breit, H., Braeutigam, T., and Kraus, T. (2013). TerraSAR-X Ground Segment, German Aerospace Center (DLR). Available online: https:\/\/sss.terrasar-x.dlr.de\/docs\/TX-GS-DD-3302.pdf."},{"key":"ref_28","unstructured":"(2021, July 29). Google Earth Engine. Available online: https:\/\/earthengine.google.com."},{"key":"ref_29","doi-asserted-by":"crossref","first-page":"29","DOI":"10.1111\/j.1365-2451.2006.00546.x","article-title":"Google Earth: A new geological resource","volume":"22","author":"Lisle","year":"2006","journal-title":"Geol. Today"},{"key":"ref_30","doi-asserted-by":"crossref","first-page":"12","DOI":"10.1109\/MPRV.2008.80","article-title":"Openstreetmap: User-generated street maps","volume":"7","author":"Haklay","year":"2008","journal-title":"IEEE Pervasive Comput."},{"key":"ref_31","unstructured":"Fischbach, A.S., Douglas, D.C., and Monette, C.J. (2020). Walrus Haulout Outlines Apparent from Satellite Imagery Near Point Lay Alaska, Autumn 2018, 2019 and 2020, U.S. Geological Survey data release."},{"key":"ref_32","doi-asserted-by":"crossref","first-page":"1359","DOI":"10.1007\/s00300-016-2060-5","article-title":"Evaluation of a method using survey counts and tag data to estimate the number of Pacific walruses (Odobenus rosmarus divergens) using a coastal haulout in northwestern Alaska","volume":"40","author":"Battaile","year":"2017","journal-title":"Polar Biol."},{"key":"ref_33","unstructured":"(2021, June 20). National Centers for Environmental Information, Climate Data Online, Available online: https:\/\/www.ncdc.noaa.gov\/cdo-web."},{"key":"ref_34","unstructured":"(2021, June 20). Global Monitoring Laboratory, Solar Calculator, Available online: https:\/\/gml.noaa.gov."},{"key":"ref_35","unstructured":"Kochnev, A.A. (2010, January 11\u201315). The haulout of Pacific walruses (Odobenus rosmarus divergens) on Cape Serdtse-Kamen, the Chukchi Sea. Proceedings of the Marine Mammals of the Holarctic, Collection of Scientific Papers after the Sixth International Conference, Kaliningrad, Russia. Available online: https:\/\/www.researchgate.net\/profile\/Anatoly-Kochnev\/publication\/317256277_The_haulout_of_Pacific_walruses_Odobenus_rosmarus_divergens_on_Cape_Serdtse-Kamen_the_Chukchi_Sea\/links\/592e79b5a6fdcc89e75ca081\/The-haulout-of-Pacific-walruses-Odobenus-rosmarus-divergens-on-Cape-Serdtse-Kamen-the-Chukchi-Sea.pdf."},{"key":"ref_36","doi-asserted-by":"crossref","first-page":"6","DOI":"10.1109\/MGRS.2013.2248301","article-title":"A tutorial on synthetic aperture radar","volume":"1","author":"Moreira","year":"2013","journal-title":"IEEE Geosci. Remote Sens. Mag."},{"key":"ref_37","unstructured":"Copernicus Space Component Mission Management Team (2019). Sentinel High Level Operations Plan, European Space Agency. Available online: https:\/\/sentinels.copernicus.eu\/documents\/247904\/685154\/Sentinel_High_Level_Operations_Plan."},{"key":"ref_38","unstructured":"(2021, June 20). RADARSAT Constellation Mission. Available online: https:\/\/www.asc-csa.gc.ca\/eng\/satellites\/radarsat."},{"key":"ref_39","unstructured":"(2021, June 20). COSMO-SkyMed. Available online: https:\/\/www.asi.it\/en\/earth-science\/cosmo-skymed."},{"key":"ref_40","unstructured":"(2021, July 29). IceEye. Available online: https:\/\/www.iceye.com."},{"key":"ref_41","unstructured":"(2021, July 29). Capella Space. Available online: https:\/\/www.capellaspace.com."},{"key":"ref_42","unstructured":"(2021, July 29). Umbra. Available online: https:\/\/umbra.space."}],"container-title":["Remote Sensing"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/www.mdpi.com\/2072-4292\/13\/21\/4266\/pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2025,10,11]],"date-time":"2025-10-11T07:21:58Z","timestamp":1760167318000},"score":1,"resource":{"primary":{"URL":"https:\/\/www.mdpi.com\/2072-4292\/13\/21\/4266"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2021,10,23]]},"references-count":42,"journal-issue":{"issue":"21","published-online":{"date-parts":[[2021,11]]}},"alternative-id":["rs13214266"],"URL":"https:\/\/doi.org\/10.3390\/rs13214266","relation":{},"ISSN":["2072-4292"],"issn-type":[{"value":"2072-4292","type":"electronic"}],"subject":[],"published":{"date-parts":[[2021,10,23]]}}}