Emperors on Thin Ice: Three Years of Breeding Failure at Halley Bay PETER T

Antarctic Science 31(3), 133–138 (2019) © Antarctic Science Ltd 2019 doi:10.1017/S0954102019000099

Emperors on thin ice: three years of breeding failure at Halley Bay PETER T. FRETWELL and PHILIP N. TRATHAN British Antarctic Survey, NERC, Madingley Road, Cambridge CB3 0ET [email protected]

Abstract: Satellite imagery is used to show that the world's second largest emperor penguin colony, at Halley Bay, has suffered three years of almost total breeding failure. Although, like all emperor colonies, there has been large inter-annual variability in the breeding success at this site, the prolonged period of failure is unprecedented in the historical record. The observed events followed the early breakup of the fast ice in the ice creeks that the birds habitually used for breeding. The initial breakup was associated with a particularly stormy period in September 2015, which corresponded with the strongest El Niño in over 60 years, strong winds, and a record low sea-ice year locally. Conditions have not recovered in the two years since. Meanwhile, during the same three-year period, the nearby Dawson-Lambton colony, 55 km to the south, has seen a more than tenfold increase in penguin numbers. The authors associate this with immigration from the birds previously breeding at Halley Bay. Studying this 'tale of two cities' provides valuable information relevant to modelling penguin movement under future climate change scenarios.

Received 31 October 2018, accepted 1 February 2019

Key words: El Niño, ENSO, emperor penguins, Halley, penguins, sea ice

Introduction population and, as it is situated at high latitudes, it plausibly represents an important climate change refugia The emperor penguin (Aptenodytes forsteri Gray) colonyat (Ainley et al. 2010,Jenouvrieret al. 2017). Halley Bay (75°33′S, 27°32′W) was one of the largest The nearest colony to the Halley site is the colonies in Antarctica, second only in size to that at Dawson-Lambton colony, some 55 km to the south, Coulman Island in the Ross Sea (Fretwell et al. 2012). located where the Brunt Ice Shelf joins the continental The colony is located on the northern side of the Brunt coast (Fig. 1). Geographically this is an unusually small Ice Shelf (Fig. 1) and, for the past two decades, has been distance between emperor colonies (Ancel et al. 2017). situated in a bay, locally named 'Windy Creek'. Although Only the Mertz Ice Shelf colonies have a smaller no organized science has been conducted on the colony, distance between them, and these two colonies it has been visited by staff from the Halley Research originated from a single site before the recent calving of Station sporadically from 1956–2012 and estimates of the Mertz Ice Tongue in 2010 (Ancel et al. 2014). size vary between approximately 14 300–23 000 pairs However, recent monitoring has shown that the Halley (Woehler 1993, British Antarctic Survey (BAS) Bay colony has suffered catastrophic breeding failure, unpublished data, H.J. Gillett personal communication whilst the nearby Dawson-Lambton colony has 2018). It is likely that the colony is associated with markedly increased in size. In this paper very high foraging on the shallow McDonald Bank and resolution (VHR) satellite imagery is used to estimate McDonald Ice rumples, to the north and east of the site population changes at the two sites over the last ten years. and the coastal polynya that forms north of the Brunt Ice shelf each summer season (Hodgson et al. 2018). Although this polynya is a consistent feature, the Methods and materials sheltered bays bordering the ice shelf usually retain fast ice until December and often the ice remains all Population size was estimated using VHR satellite images summer. This ensures that emperors are able to raise from the WorldView2 and WorldView3 satellites. An their chicks at the site as their young ﬂedge between identical methodology to that used by Fretwell et al. mid-December and early January. (2012) was employed. In brief a multivariate supervised Although the recorded population has varied, the colony classiﬁcation in ArcGIS (ESRI®ArcMAP™ 10.4.1) to is consistently the largest in the Weddell Sea, over twice the distinguish the area of penguins in each image from size of any other colony in the region. There have been no guano, shadows and snow was used. The area of previously recorded instances of total breeding failure at penguins is then converted to a population estimate via a the site. It possibly represents 6.5–8.5% of the total global robust-regression analysis using ground truthing from 11 133

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Fig. 1. Overview of the Brunt Ice Shelf, showing the location of Halley Bay and Dawson-Lambton emperor penguin colonies. The underlying image is a Landsat8 image from October 2016.

sites, which gives an indication of the number of pixels per hundred adults present at the breeding site (∼2% of the penguin in the imagery. The variance of each estimate is recent population). Investigation of the local area in the calculated using a Monte Carlo analysis based upon the VHR imagery revealed several thousand birds at image quality and ease of counting of each satellite the sea-ice edge. The exact number is difficult to estimate image. Each estimate refers to the number of penguins at due to the rough ice surface confusing the automated the site at the time of the image acquisition. This image analysis. The best estimate is that around five includes both adults and chicks. As with Fretwell et al. times more birds were at, or within ∼100 m of the (2012), the images used were taken during the crèche sea-ice edge than at the main colony site. Many of the period of the emperors' breeding cycle; the assumption penguins were on refrozen brash ice or newly formed was made that there will be one adult per breeding pair grease ice. This does not include the lines of birds at the colony site. Although the ratio of adults to chicks moving between the colony site and the ice edge, which changes through the season, the diminishing number can easily be identified as birds in transit. Emperors do of adults is compensated for by the higher visibility not breed or habitually feed their young at the ice edge of the chicks. In essence, therefore, these counts are as its position is dynamic and the high risk of breakup comparable to the number of breeding pairs at each site. would pose a danger to unfledged chicks. Whether the VHR satellite imagery, at either 30 cm or 50 cm adult birds here were failed breeders or non-breeders is resolution has been collected and analysed at each site, difficult to assess from imagery alone. Subsequent between mid-September and early December every year Landsat8 and Sentinel2 imagery shows that by 29 since 2009 (with the exception of Halley Bay 2016 when November 2018, all of the fast ice on the north side of all land-fast ice was lost early in the season making the Brunt Ice Shelf had gone, highlighting a third year of analysis irrelevant). This has allowed the construction of probable total breeding failure. These assumed failed a ten-year time sequence of population changes (image breeding events are of a scale that is not apparent in the details are given in Supplemental material see http://dx. long, but sporadic record from the site (H.J. Gillett doi.org/10.1017/S0954102019000099). The timing of personal communication 2018, BAS records). the imagery coincides with the crèche period of the emperors' breeding cycle and, for all but one image Dawson-Lambton (the Dawson-Lambton image in 2016), is well before the fledging period of the chicks. Due to the variable timing Between 2016 and 2018, a massive increase in the numbers of the imagery a small amount of mortality between of birds at the nearby Dawson-Lambton Glacier colony earlier imagery and later images would be expected. (76°04′S, 26°40′W), some 55 km to the south (Fig. 2) However, mortality during this period tends to be low was found. The number of pairs estimated by satellite at (Barber-Meyer et al. 2008) (For a further critique of this colony had steadily decreased from 3690 pairs in possible uncertainties, refer to Fretwell et al. 2012). 2010 to 1280 pairs in 2015. However, numbers suddenly To supplement the VHR imagery and to give a more increased to 5315 in 2016, 11 117 pairs in 2017 and complete picture of the sea ice conditions Sentinel2, 14 612 pairs in 2018, an increase of over 1000% from the Sentinel1 and Landsat8 images have also been assessed. 2015 estimate (images taken 7 December, 9 December and 25 October respectively, for 2016, 2017 and 2018; for further details on imagery please see supplemental Results files). The satellite images used to estimate bird numbers showed that, in 2016 and 2017, although breeding birds Halley Bay were evident at Dawson-Lambton, the distribution of Satellite imagery over a ten-year period has confirmed birds around the colony was non-typical. Usually considerable variation in colony size (Fig. 2). More breeding birds congregate in loose groups, but in the importantly, the images have revealed that since 2015 the images from these two years, most birds were widely population has crashed with almost no breeding success dispersed over the sea ice, often individually or in groups in 2016, 2017 and 2018. In 2016 and 2017, low sea ice of less than ten. Many thousands of them were extent and early breakout of sea ice in spring (October/ positioned along cracks in the ice and the lack of guano November; the crèche period of the emperor's breeding staining in these areas (as opposed to the areas around cycle) resulted in complete breeding failure. Satellite the more usual breeding location) indicated that they observations for this phenomenon were verified by had not been there for long. In 2018, this pattern had ground observations from staff at the Halley VI research changed; there was no loose dispersion of birds and station in November 2016. In 2017 early breakout of fast imagery indicated that they had congregated into two ice in mid-November confirmed another year of distinct groups, with heavy guano staining at each site. breeding failure (Fig. 2). In 2018 (VHR image date 25 One group was in approximately the position occupied October) the colony reformed, but with only a few over the last five years, but a larger group, approximately

Fig. 2. Variability in the emperor penguin population breeding at Windy Creek, Halley Bay (solid line), and Dawson-Lambton colony (dashed line). Estimates made from very high resolution satellite imagery following the methods of Fretwell et al.(2012); upper and lower 95% conﬁdence intervals are shown.

2.5 times the size of the other, was in a new site located significant proportion of the regional emperor population 1.4 km to the west. (on average approximately 23% of the population of the colonies between 90–0°W) and complete breeding failure Discussion in three consecutive seasons will have been important. Dispersion and immigration Environmental drivers From the loose pattern of birds visible at Dawson- Lambton in 2017, it is inferred that many of the birds in The breeding failure and reasons for relocation are 2016 and 2017 were not breeding at the site and had almost certainly linked to the early breakup of sea ice possibly arrived later in the season. In 2018, however, at the Halley Bay site (Barbraud & Weimerskirch 2001, the close-huddled groups and heavy guano staining Barbraud et al. 2011), but exactly why that breakup suggest that the majority of the population had been occurred is unknown. It is interesting that the first there for some time; this was more consistent with the year of poor sea-ice conditions immediately followed pattern seen at other breeding colonies. the strongest El Niño in over 60 years, one of the It appears that many of the birds from Halley Bay have most positive values of the Southern Annular Mode relocated to Dawson-Lambton, with the rest remaining at (SAM) and a record low sea-ice year for Halley Bay. Halley Bay, but not breeding successfully. This number (Ocean Niño Index (ONI) data available from https:// seems to be diminishing on an annual basis as more climatedataguide.ucar.edu/climate-data/nino-sst-indices- failed breeders move to the nearby colony. It is possible nino-12-3-34-4-oni-and-tni) (see supplemental Fig. 1 that some emperors could have formed a new colony at http://dx.doi.org/10.1017/S0954102019000099,data elsewhere, but an exhaustive search using Sentinel2 available from https://legacy.bas.ac.uk/met/READER/ imagery shows no new colony locations in the region. It surface/stationpt.html; SAM data available from http:// is also possible that some birds could have moved to www.nerc-bas.ac.uk/icd/gjma/sam.html). However, without other colony locations further away, but due to the a longer time series of population data and sea-ice natural variability in colony sizes, small increases in records, it is impossible to ascertain whether similar population would be difficult to detect. Another population declines have followed previous years with alternative explanation is that some adults have skipped strong ONI or SAM signals. A long time series would breeding over these years and will return to breed at the also be required to ascertain whether the origin of the original site when conditions there improve. Observations colony at Halley Bay followed similar events elsewhere made in the present study emphasize that emperor that led to colony failure and resettlement over longer penguins move between colony sites (LaRue et al. 2015) time periods than the historical record. and, when large colonies move, it can take a number of What is certain is that in September 2016 just prior to years for resettlement to occur, similar to the recent the satellite image shown in Fig. 2b, average atmospheric situation at the Mertz colony (Ancel et al. 2014). pressure for the month was the lowest for September in It is highly likely that the regional population in the over 30 years, whilst average wind speed was the highest Weddell Sea was impacted following the loss of at least in September over a similar time frame. Temperatures three consecutive breeding seasons. Halley comprised a were also higher than average. Wind speeds at Halley

Fig. 3. Medium resolution satellite imagery of the Windy Creek breeding site for the years between spring 2015 and spring 2018.

tend to be strongest during August, September and the present authors, and from earlier papers (Kooyman October (Turner et al. 2009), and the timing of strong et al. 2007, Ancel et al. 2014), points to the fact that wind events in relation to key aspects of breeding will stochastic impacts upon emperors may be vital, even for almost certainly be important. high-latitude locations. Strong winds, or storm events Understanding how environmental drivers, such as can create coastal leads or polynyas that are beneﬁcial to changes in SAM, sea ice, or wind speed, direction and foraging, but prolonged periods of extreme winds can velocity, impact upon the breeding colony at Halley, or also lead to breakup and dissipation of fast ice, which indeed elsewhere, remains a key challenge. Further, can cause total breeding failure when it occurs at a exploring how extremes of such events lead to breeding sensitive time for the penguins. failure is vital for projecting future population At Halley, another important factor inﬂuencing the trajectories in a warming environment. The relationship stability of the fast ice around the colony could be the between climate change and El Niño events, or positive dynamic nature of the creek in which it is located. Until SAM anomalies is still a matter of active research recently, the colony was situated within a sheltered ice (Trenberth & Hoar 1997, Turner 2004, Turner et al. creek, on the northern side of the Brunt Ice Shelf, 2005,Yehet al. 2009, Collins et al. 2010, Bracegirdle informally named Windy Creek. Over the past 60 years, 2013, Cai et al. 2015). Recent research suggests that the the colony has occasionally moved to other adjacent frequency of El Niño and La Niña events are predicted sheltered creeks (H.J. Gillett personal communication to increase, while ENSO-related catastrophic weather 2018, BAS records). With the fast-ice breakout in 2016, events are also likely to occur more frequently with ice shelf morphology changed (Fig. 3) and the resulting unabated greenhouse gas emissions (Cai et al. 2015). more open nature of the creek may now be less suitable However, other evidence cautions that it is not yet for fast-ice retention. The Brunt Ice Shelf is a fast-moving possible to say whether ENSO activity will be enhanced and dynamic environment (Hodgson et al. 2018). Over or damped, or if the frequency of events will change the last two decades the creek has gradually moved (Collins et al. 2010). The evidence from observations of westwards by over 600 m per year and it is possible that

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