Estimating age ratios and size of pacific walrus herds on coastal haulouts using video imaging.

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/m(2) (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

Supplement 1. Excel spreadsheet with example calculations.

<h2>File List</h2><p> <a href="Supplement1.xls">Supplement1.xls</a> (md5: 4202b5bccb5ee828f646f50530394c47) </p> <p> <i>Please be advised that the ESA cannot guarantee the forward migration of proprietary file formats such as Excel (.xls) documents.</i> </p><h2>Description</h2><div> <p>SupplementA.xls is an Excel spreadsheet containing 5 sheets with example calculations. The first 4 sheets (labeled Model 1 - Model 4) contain calculations for models considered in APPLICATION TO YELLOWSTONE BISON:</p> <p>Model 1: Makes no assumptions about equality of survival rates for different age classes.</p> <p>Model 2: Assumes survival rates are equal for ages 0–1, 2–3, 4–5, 6–7, 8–9, 10–11, 12–13.</p> <p>Model 3: Assumes survival rates are equal for ages 0–1, 2–3, 4–5, 6–11, 12–13.</p> <p>Model 4: Assumes survival rates are equal for ages 0–13.</p> <p>The last sheet (labeled 3 Years) contains calculations for a hypothetical example with 3 age classes and 3 years of data, and no assumptions about equality of survival rates.</p> </div

Appendix A. Derivation of explicit estimators.

Derivation of explicit estimators

Numbers of walruses selected for measurement-based aging and resolution of age classifications from hold sequences recorded in 2011.

a<p>Proportions in parenthesis are relative to 1,413 attempted measurements.</p>b<p>Proportions in parenthesis are relative to the column totals given in the 1^st row.</p>1<p>Column only includes larger animals where aging required tusk and snout measurements.</p>2<p>An attempt was made to find a measurable image but none with the requisite perfectly perpendicular front or side view was found, and only a visual estimate of age was made or age was classified as unknown.</p>3<p>Column only includes smaller animals where aging did not require tusk or snout measurements.</p

Appendix B. Example illustrating equivalence of survival rate estimates based on full and conditional likelihoods.

Example illustrating equivalence of survival rate estimates based on full and conditional likelihoods

Walrus haulout location near Pt. Lay, Alaska.

<p>Equally scaled composite images of two walrus herds hauled out near Pt. Lay on the NW coast of Alaska on August 25^th (left photo) and August 26^th (right photo), 2011. Green lines delineate the outline of the herd and yellow circles identify approximate location of the focal point for hold sequences. Red X's indicate the north location of paired reference marks used to scale the images.</p