Adelie penguin diet composition, fish species and number, 1991-2021

The fundamental long-term objective of the seabird component of the Palmer LTER (PAL) has been to identify and understand the mechanistic processes that regulate the mean fitness (population growth rate) of regional penguin populations. Two hypotheses have guided this research, with one suggesting that population mean fitness is best explained by changes in regional krill biomass, and the other proposing that long-term changes in sea ice affects mean fitness by tipping the balance in favor of one species over another in accordance with species-specific evolved life history affinities to sea ice. Although these hypotheses are not mutually exclusive, current evidence in the PAL region tends to favor the latter over the former. Since the inception of PAL, Adélie penguin populations have effectively collapsed, while those of gentoo and chinstrap penguins have increased dramatically, trends that are spatially and temporally coherent with decreasing regional sea ice duration. Adélie penguins are an ice-obligate polar species whose life history is intimately linked to the presence of sea ice, while chinstrap and gentoo penguins are ice-intolerant species whose life histories evolved in the sub-Antarctic, where sea ice is a less permanent feature of the marine ecosystem. In contrast, although krill constitute the most important component of the summer diets by mass of these three penguin species, changes in PAL krill abundances have exhibited no long-term trends, and thus fail to explain the divergent patterns in penguin populations evident in our time series. Fish occur frequently in Adélie penguin diets but rarely as whole specimens, hence recording their presence in sorted subsamples (see DIET) is generally limited to noting the incidence of skin, flesh, bones, eyes and especially otoliths. With the exception of otoliths, the weights of these items are obtained if warranted by the sample size. Otoliths can be used to identify individual fish species, and in combination with regression equations based on otolith length and width, can also be used to reconstitute fish length and mass. However, because the expertise needed to identify fish from otoliths does not exist within the PAL program, otolith identification is performed by off-site experts as time and funding allow. This results in long lags between the time otoliths are collected and integrated with the appropriate databases, meaning that understanding the role of fish in Adélie penguin diets still remains a longer-term objective of PAL. Dr. Megan Cimino took over as PI of the LTER seabird project in 2020 from Dr. William Fraser. Field data collection between 2020-2022 has remained consistent with previous years. No samples collected during the 2021-2022 season due to the Palmer Station Pier Build.