Abstract
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 2018–2020. 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 4–10 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.
Highlights
Pacific walruses (Odobenus rosmarus divergens) haulout on the coast to rest when sea ice is unavailable over continental-shelf waters
We evaluate the repeatability of interpreting the occurrence, aerial extent, and location of walrus haulouts from a representative sampling of available Earth observing satellite imagery
Because none of the Earth observing satellite imagery available to this study enabled the counting of walruses when gathered closely together in a haulout, and because walrus density varies substantially at coastal haulouts, we did not seek to estimate abundance [25]
Summary
Pacific walruses (Odobenus rosmarus divergens) haulout on the coast to rest when sea ice is unavailable over continental-shelf waters. When hauled out on land, walruses are at risk from human activities including disturbances that can result in death from stampeding [1]. Mitigating disturbance or pollution risks requires near-real-time information about the presence and relative size of coastal haulouts during ice-free periods. Large aggregations at coastal haulout sites contrast with small, dispersed walrus groups that rest on ice floes when sea ice is present over their offshore feeding grounds [3,4]. Large aggregations at terrestrial haulouts present an opportunity to survey and enumerate walruses with greater precision than may be achieved with offshore surveys [5]. Coastal haulout locations themselves are typically remote and challenging to access, making on-site survey efforts costly and sporadic at best [6]
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