Estimating Age Ratios and Size of Pacific Walrus Herds on Coastal Haulouts using Video Imaging

Abstract

During Arctic summers, sea ice provides resting habitat for Pacific walruses as it drifts over foraging areas in the eastern Chukchi Sea. Climate-driven reductions in sea ice have recently created ice-free conditions in the Chukchi Sea by late summer causing walruses to rest at coastal haulouts along the Chukotka and Alaska coasts, which provides an opportunity to study walruses at relatively accessible locations. Walrus age can be determined from the ratio of tusk length to snout dimensions. We evaluated use of images obtained from a gyro-stabilized video system mounted on a helicopter flying at high altitudes (to avoid disturbance) to classify the sex and age of walruses hauled out on Alaska beaches in 2010–2011. We were able to classify 95% of randomly selected individuals to either an 8- or 3-category age class, and we found measurement-based age classifications were more repeatable than visual classifications when using images presenting the correct head profile. Herd density at coastal haulouts averaged 0.88 walruses/m2 (std. err. = 0.02), herd size ranged from 8,300 to 19,400 (CV 0.03–0.06) and we documented ∼30,000 animals along ∼1 km of beach in 2011. Within the herds, dependent walruses (0–2 yr-olds) tended to be located closer to water, and this tendency became more pronounced as the herd spent more time on the beach. Therefore, unbiased estimation of herd age-ratios will require a sampling design that allows for spatial and temporal structuring. In addition, randomly sampling walruses available at the edge of the herd for other purposes (e.g., tagging, biopsying) will not sample walruses with an age structure representative of the herd. Sea ice losses are projected to continue, and population age structure data collected with aerial videography at coastal haulouts may provide demographic information vital to ongoing efforts to understand effects of climate change on this species.

Demography of the Pacific walrus (Odobenus rosmarus divergens) in a changing Arctic

“…Such haul‐outs can number in the tens of thousands (Monson et al . , Fischbach et al . ) and can be far from offshore feeding areas.…”

Stable isotope differences of polar bears in the Southern Beaufort Sea and Chukchi Sea

“…Alternatively, differences in diet and available prey between the CS and SBS polar bear subpopulations could also be driving the isotopic separation shown. The Chukchi Sea has a higher primary productivity than the Beaufort Sea (Grebmeier et al 2006), which is reflected in the food web as increased availability of preferred prey, specifically for bears in CS (Stirling et al 1977, Fischbach et al 2009, Jay et al 2012, Monson et al 2013, Boveng et al 2017). This is further supported by the better body condition and larger body size of the bears in CS compared to the SBS subpopulation (Rode et al 2014).…”

“…Overall abundance of ice seals, namely ringed ( Pusa hispida ) and bearded seals, which are the preferred prey of polar bears, is greater in the CS than in the SBS (Stirling et al 1977, Boveng et al 2017). Furthermore, walrus haul‐outs in the Point Lay, Alaska area, and on the Chukotka peninsula (CS subpopulation territory; USFWS 2019), provide feeding opportunities for polar bears in the CS subpopulation (Fischbach et al 2009, Jay et al 2012, Monson et al 2013). Polar bears in the CS subpopulation also have a greater access to gray whales ( Eschrichtius robustus ) from beach‐cast carcasses along the Chukotkan coast and Wrangel Island (Laidre et al 2018).…”

Camera technology for monitoring marine biodiversity and human impact

“…Miniaturized and lightweight cameras installed on unmanned aerial vehicles (UAVs) have been deployed to survey marine vertebrates on land (Pacific walrus [ Odobenus rosmarus divergens ]; Monson et al . ) and in water (dugongs [ Dugong dugon ]; Hodgson et al . ).…”

“…Autonomous and unmanned vehicles are being used for remote sensing in both sea and air (Williams et al 2012;Anderson and Gaston 2013). Miniaturized and lightweight cameras installed on unmanned aerial vehicles (UAVs) have been deployed to survey marine vertebrates on land (Pacific walrus [Odobenus rosmarus divergens]; Monson et al 2013) and in water (dugongs [Dugong dugon]; Hodgson et al 2013). Aerial imagery techniques have previously been utilized for population estimates (see Buckland et al 2012), Environmental Impact Assessments (Thaxter and Burton 2009), and coastal habitat mapping (Lathrop et al 2006), but the improved spatial and temporal resolution provided by UAVs offer many other applications in marine systems (Anderson and Gaston 2013).…”