The Adélie penguin (Pygoscelis adeliae) breeding population at Esperanza/Hope Bay, at the tip of the Antarctic Peninsula, is one of the largest Adélie penguin colonies in Antarctica. Nevertheless, the last known published field count during the egg-laying period is from 1985/1986. We counted breeding pairs within the entire colony, consisting of 274 breeding groups, during the 2012/2013 breeding season for comparison with previously published ground and satellite-derived counts. We also counted breeding pairs in 26 breeding groups that have been monitored annually since 1995/1996. We estimated the current population size to be 104,139 (95th-percentile CI 70,126-138,151) breeding pairs for the whole colony. The counts indicate population declines of 15.9% over 27 years in the whole colony and 37.5% over 18 years in the annually monitored subset of breeding groups, respectively. The observed decrease matches recent trends reported in other Adélie penguin colonies throughout the western Antarctic Peninsula and southern Scotia Sea. This population assessment contributes to the current estimates of the total predator populations in the region, which is necessary information for the management of marine living resources.

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Mercury (Hg) is a pervasive environmental contaminant that accumulates in the organs and tissues of seabirds at concentrations capable of causing acute or long-term adverse health effects. In the present study, Hg concentrations in Adélie penguin (Pygoscelis adeliae) egg membranes and chick feathers served as a proxy for Hg bioavailability in the marine environment surrounding the northern Antarctic Peninsula. Stable isotopes were measured in conjunction with Hg to infer information regarding feeding habits (δ 15 N, diet/trophic level; δ 13 C, foraging habitat). The Hg concentrations were low relative to toxicity benchmark values associated with adverse health effects in birds and ranged between 0.006 and 0.080 µg g -1 dry weight (n = 65) in egg membranes and 0.140 to 1.05 µg g -1 fresh weight (n = 38) in feathers. Egg membrane δ 15 N signatures suggested that females from different breeding colonies had similar diets consisting of lower and higher trophic prey prior to arrival to breeding grounds. In contrast, δ 15 N signatures in feathers indicated that chick diet varied by colony. The Hg concentrations demonstrated significant positive relationships with δ 15 N, providing support for the hypothesis of Hg biomagnification up the food chain. The δ 13 C signatures in both tissue types provided evidence of foraging habitat segregation among populations. The differences in Hg exposure and foraging ecology suggest that each colony has localized foraging behaviors by breeding adults that warrant additional investigation.