Introduction

Increasing global carbon emissions cause a cascade of related climatic and topographic changes, including glacial and ice sheet loss. These changes can have profound effects on the ecology of the affected area. Among the susceptible ecological processes, trophic dynamics are some of the most significant. Antarctica has some of the most well-documented instances of ice loss, as well as the surrounding land masses in the Southern Ocean \cite{Rignot_2013,Pritchard_2012,Chen_2013}. In these ecosystems, much of the food web is dominated by one planktonic organism: krill. Specifically, the Antarctic krill (Euphausia superba) provides a primary food source for a variety of small fish and sea birds as well as cetaceans and pinnipeds \cite{NICOL_2006}. This means that krill population densities directly influence a vast array of species which depend on them \cite{Croxall_1999}. Of those species, brush-tailed penguins-- Adelie (Pygoscelis adeliae), chinstrap (P. antarcticus), and gentoo (P. papua)-- are recently and heavily dependent on krill for food \cite{Volkman_1980,Ju_res_2016a}
Beginning as early as the 1820s, krill availability to brush-tailed penguins in the Southern Ocean began to rise as a response to an assortment of ecological variables. Mainly, historic finfish and marine mammal fishing practices produced a "krill surplus" up to the 1980s \cite{Laws_1977,Emslie_2007,Emslie_2013}. The reduction in large krill predators, like rorquals and fur seals, lightened top-down pressures on krill populations, thus allowing higher availability to lower trophic level predators, like penguins. In fact, it is hypothesized that this surplus gradually shifted the diets of brush-tailed penguins from a fish-krill mixture to one of predominantly krill in the last few centuries \cite{Emslie_2007,Emslie_2013}. In particular, Adelie and chinstrap appear to be more heavily reliant on krill than gentoo penguins \cite{Kadye_2011}.
However, availability of krill began to decline around the turn of the 20th century as modern economic exploitation of krill and unfavorable climatic conditions rose. Additionally,  the marine mammal populations previously devastated by overharvest had probably began to recover \cite{Trivelpiece_2011,Surma_2014}. We propose that continued climate change coupled with gradual reestablishment of higher-trophic-level predators at the turn of the 20th century, which may be negatively affecting krill abundance, will be reflected in Antarctic brush-tailed penguin abundance estimates. Furthermore, we hypothesize that if there are significant relations with krill abundance, they will be strongest with the more krill-reliant penguins, Adelie and chinstrap.