Geographic Structure of Adélie Penguin Populations: Overlap in Colony-Specific Foraging Areas

Abstract

In an investigation of the factors leading to geographic structuring among Adélie Penguin (Pygoscelis adeliae) populations, we studied the size and overlap of colony‐ specific foraging areas within an isolated cluster of colonies. The study area, in the southwestern Ross Sea, included one large and three smaller colonies, ranging in size from 3900 to 135 000 nesting pairs, clustered on Ross and Beaufort Islands. We used triangulation of radio signals from transmitters attached to breeding penguins to determine foraging locations and to define colony‐specific foraging areas during the chick‐provisioning period of four breeding seasons, 1997–2000. Colony populations (nesting pairs) were determined using aerial photography just after egg‐laying; reproductive success was estimated by comparing ground counts of chicks fledged to the number of breeding pairs apparent in aerial photos. Foraging‐trip duration, meal size, and adult body mass were estimated using RFID (radio frequency identification) tags and an automated reader and weighbridge. Chick growth was assessed by weekly weighing. We related the following variables to colony size: foraging distance, area, and duration; reproductive success; chick meal size and growth rate; and seasonal variation in adult body mass. We found that penguins foraged closest to their respective colonies, particularly at the smaller colonies. However, as the season progressed, foraging distance, duration, and area increased noticeably, especially at the largest colony. The foraging areas of the smaller colonies overlapped broadly, but very little foraging area overlap existed between the large colony and the smaller colonies, even though the foraging area of the large colony was well within range of the smaller colonies. Instead, the foraging areas of the smaller colonies shifted as that of the large colony grew. Colony size was not related to chick meal size, chick growth, or parental body mass. This differed from the year previous to the study, when foraging trips of the large colony were very long, parents lost mass, and chick meals were smaller. In light of existing data on prey abundance in neritic waters in Antarctica suggesting that krill are relatively evenly distributed and in high abundance in the Southern Ross Sea, we conclude that penguins depleted or changed the availability of their prey, that the degree of alteration was a function of colony size, and that the large colony affected the location (and perhaps ultimately the size) of foraging areas for the smaller colonies. It appears, therefore, that foraging dynamics play a role in the geographic structuring of colonies in this species.

Predicting the distribution of a threatened albatross: The importance of competition, fisheries and annual variability

“…This model has received considerable support from some studies (Wanless and Harris, 1993;Grémillet et al, 2004;Masello et al, 2010), but several others documented a broad range of overlaps in foraging areas (Ainley et al, 2004;Wakefield et al, 2011). The very fine-scale data presented here clearly indicate a mixed picture, with a large area of overlap in terms of maximum extent, but obvious avoidance by birds from New Island of the main foraging area used by albatrosses from Steeple Jason, and vice versa.…”

“…However, there are several data sets apparently disproving this hypothesis (Ainley et al, 2004;Wakefield et al, 2011). Moreover, even though ca.…”

“…Research suggests that birds from neighboring colonies can have markedly different at-sea distributions, which may result from intra-specific competition (Huin, 2002;Ainley et al, 2004;Grémillet et al, 2004;Trathan et al, 2006). Furthermore, inter-annual variation in oceanographic conditions may also be important (Kappes et al, 2010).…”

Trophic cascades in the western Ross Sea, Antarctica: revisited

“…On the basis of these results, foraging by penguins and whales appeared to alter the availability of fish and krill to penguins, at least over mesoscale spatial variations in the southwestern Ross Sea. In addition, the seasonally increasing FTD was at least partially due to an expansion over time of the foraging area at the large Crozier colony (Ainley et al 2004, Ford et al 2014. However, the question remained as to how these predators were altering prey availability: by interference competition, with whales driving krill deeper than the diving capability of penguins, or by exploitative competition via intra-and interspecific prey depletion.…”

“…The Crozier colony also has a very extensive foraging 'halo,' much larger than the smaller colonies in the region, the size of the halo being a manifestation of the relative amount of prey depletion near to the colony (cf. Ainley et al 2004, Ballance et al 2009, Ford et al 2014. We sought to learn whether increased FTD was due to a wider (farther from colony, longer duration spent foraging) and/or a more intensive (deeper diving) search for prey as prey became less available owing to pre dation pressure through time (Ainley et al 2004(Ainley et al , 2006.…”