# Stable isotopes document the winter foraging ecology of king penguins and highlight connectivity between subantarctic and Antarctic ecosystems

Yves Cherel, Charline Parenteau, Paco Bustamante, Charles-André Bost

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Yves Cherel, Charline Parenteau, Paco Bustamante, Charles-André Bost. Stable isotopes document the winter foraging ecology of king penguins and highlight connectivity between subantarctic and Antarctic ecosystems. Ecology and Evolution, 2018, 8 (5), pp.2752-2765. 10.1002/ece3.3883. hal-02024974v2

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# Stable isotopes document the winter foraging ecology of king penguins and highlight connectivity between subantarctic and Antarctic ecosystems

Yves Cherel<fnref n="1" /> ID

Charline Parenteau<fnref n="1" />

Paco Bustamante<fnref n="2" /> ID

Charles-André Bost<fnref n="1" />

## Correspondence

Yves Cherel, Centre d'Etudes Biologiques de Chizé (CEBC), UMR 7372 du CNRS-Université de La Rochelle, Villiers-en-Bois, France.
Email: cherel@cebc.cnrs.fr

## Funding information

Institut Polaire Français Paul Emile Victor (IPEV); Terres Australes et Antarctiques Françaises (TAAF)

## Abstract

The poorly known winter foraging ecology of the king penguin, a major Southern Ocean consumer, was investigated at the subantarctic Crozet Islands where the largest global population breeds. Blood δ¹³C and δ¹⁵N values were used as proxies of the birds' foraging habitat and diet, respectively, and circulating prolactin levels helped in determining the birds' reproductive status. Plasma prolactin concentrations showed that king penguin adults of unknown breeding status (n = 52) that were present at the colony in winter were in fact breeders and failed breeders, but were not non-breeders. Circulating prolactin was neither related to δ¹³C nor δ¹⁵N values, thus suggesting that both breeders and failed breeders used the same foraging habitats and fed on the same prey. Plasma and blood cell isotopic values depicted four new relevant biological features on the feeding strategies of king penguins during the critical winter period: (1) 42% of the birds foraged in the distant Antarctic Zone, but 58% fed primarily in subantarctic waters (δ¹³C), (2) they preyed upon myctophids in both zones (δ¹⁵N), (3) individuals were consistent in their foraging strategies over the winter months (δ¹³C and δ¹⁵N), and (4) a higher proportion of females (77%-80%) than males (27%-31%) favored feeding in distant Antarctic waters (δ¹³C). This study highlights trophic connectivity between subantarctic and Antarctic ecosystems and hence the key role of energy export from Antarctic waters to sustain breeding populations of subantarctic predators, including during the Austral winter.

## KEYWORDS

diet, myctophid, prolactin, seabird, Southern Ocean

# 1 | INTRODUCTION

Environmental variability influences avian population dynamics. In seabirds, climatic variations during winter affect fluctuations in population numbers, in many cases operating on variation in adult survival (Barbraud & Weimerskirch, 2003; Grosbois & Thompson, 2005; Sandvik, Erikstad, Barrett, & Yoccoz, 2005). A major challenge in identifying the underlying biological mechanisms is the lack of knowledge

on foraging habits during winter, when most seabirds disperse far from their breeding grounds. The lack of winter habitat-use and dietary information is particularly relevant for penguins, because the swimming and diving behavior of these flightless predators make them cryptic organisms when at sea. Consequently, little is known about their winter biology, although increasing use of bio-logging has provided new insights into their diving patterns (Green, Boyd, Woakes, Warren, & Butler, 2005) and foraging grounds (Ballard et al., 2010; Bost, Thiébot,

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

© 2018 The Authors. *Ecology and Evolution* published by John Wiley & Sons Ltd.

<footnote n="1">¹Centre d'Etudes Biologiques de Chizé (CEBC), UMR 7372 du CNRS-Université de La Rochelle, Villiers-en-Bois, France</footnote>

<footnote n="2">²Littoral, Environnement et Sociétés (LIENSs), UMR 7266 du CNRS-Université de la Rochelle, La Rochelle, France</footnote>
Pinaud, Cherel, & Trathan, 2009). This lack of information is of special concern, because (1) penguins number ~113 millions of individuals (van Franeker, Bathmann, & Mathot, 1997) and form 90% of seabird biomass in the Southern Ocean (Woehler, 1993), where they constitute a key group of marine consumers within the pelagic ecosystem (de Brooke, 2004; Woehler, 1995), and (2) populations of many species have declined substantially in the past two decades, with penguins being now the most threatened seabird taxon after albatrosses (Trathan et al., 2015).

The subantarctic king penguin is the sixth largest seabird consumer worldwide (de Brooke, 2004). The breeding cycle of the king penguin is unique amongst avian species since it spans >1 year, thus including the winter period. At that time, most adults desert the colonies and chicks fast (Figure 1), being fed at irregular intervals by the returning parents (Saraux, Friess, Le Maho, & Le Bohec, 2012). King penguins prey upon myctophids in spring, summer and autumn (Cherel, Fontaine, Richard, & Labat, 2010; Cherel, Verdon, & Ridoux, 1993; Raclot, Groscolas, & Cherel, 1998), but the diet of chicks indicates a shift to more diverse fish prey and to cephalopods in winter (Cherel, Ridoux, & Rodhouse, 1996; Moore, Robertson, & Wienecke, 1998). Winter is also the key period driving population dynamics of the species, because adult mortality occurs almost exclusively at that time (Le Bohec et al., 2007; Olsson & van der Jeugd, 2002).

The king penguin is one of the most thoroughly studied seabirds as a behavioral, ecological, and physiological model of a colonial and diving animal (Aubin & Jouventin, 1998; Bost et al., 2015; Handrich et al., 1997; Kooyman et al., 1992). However, despite the overwhelming biological importance of the winter season, most foraging studies related to this species focused on the summer months. At that time, breeding adults from the subantarctic Crozet Islands (its main breeding ground) forage mainly in the Subantarctic Zone (SAZ) to the Polar Front (PF) (Bost et al., 1997, 2015; Charrassin & Bost, 2001). In winter,

no information is available on the at-sea behavior of non breeders and failed breeders, but the few tracked breeders showed a different strategy. They perform very long trips to the south, crossing the PF and feeding for themselves in the Antarctic Zone (AZ) on still unknown prey (Bost, Charrassin, Clerquin, Ropert-Coudert, & Le Maho, 2004; Charrassin & Bost, 2001; Pütz, 2002). However, trips were 30% longer for equipped than unequipped birds and desertion occurred in many cases (Bost et al., 2004); hence, more investigations using complementary methods are needed to record the foraging behavior of king penguins during the critical winter period.

The main goal of the present work was to document the food and feeding ecology of king penguins during the Austral winter using the stable isotope method (δ¹³C and δ¹⁵N) on a representative numbers (~50) of adult birds of unknown breeding status (hereafter UBS birds) that are found in the colony at that time (Figure 1). Reproductive status of UBS penguins (presumably a pool of breeders, failed breeders, and non breeders) was a *posteriori* investigated by measuring their plasma concentration of prolactin, a hormone involved in parental care in birds (Angelier, Wingfield, Tartu, & Chastel, 2016). The use of δ¹³C and δ¹⁵N values was validated on consumers from the southern Indian Ocean. Tissue δ¹³C values of predators reflect the decreasing δ¹³C gradient at the base of the food web from the tropics to Antarctic waters and thus indicate the predator latitudinal foraging habitat (Cherel & Hobson, 2007; Jaeger, Lecomte, Weimerskirch, Richard, & Cherel, 2010). Tissue δ¹⁵N values of consumers change according to their trophic position in the increasing order crustacean eaters < myctophid eaters < fish and squid eaters (Cherel et al., 2010). This method complements biologging, with the main advantage that initial marking of individuals is not necessary and that every capture will provide foraging information before tissue sampling, and hence with no deleterious effect on the recorded behavior. We posed the following four predictions about the foraging habitat, diet, and sex-specific strategies of UBS king penguins during the winter months.

<figure><img src="image_2.png" /><figcaption>**FIGURE 1** The king penguin colony of La Baie du Marin (Possession Island, Crozet Archipelago) in winter. Two crèches of large chicks are in the foreground and background, adults with unknown reproductive status rest near the river, and a few breeders with their corresponding single chicks stand in the periphery of the crèches. Small white birds are lesser sheathbills (*Chionis minor*)</figcaption></figure>

1. **Prediction 1. King penguins forage in Antarctic waters in winter.** As previously tracked king penguins foraged within the AZ, most UBS birds would present low blood δ¹³C values that characterize feeding on Antarctic prey. Based on penguin blood isoscapes, δ¹³C values <-22.5 ‰, from -22.5 to -19.7 ‰, and >-19.7 ‰ were considered to correspond to the AZ, SAZ, and Subtropical (STZ) zones, respectively (Cherel & Hobson, 2007; Jaeger, Lecomte, et al., 2010). Hence, δ¹³C values < -22.5 ‰ record foraging within the AZ.

2. **Prediction 2. King penguins feed on Antarctic krill in winter.** In the southern Indian Ocean, most tracked breeders reached the northern sea ice edge and beyond within the Seasonal Ice Zone (SIZ, as far south as 63-65°S; Moore, Wienecke, & Robertson, 1999; Charrassin & Bost, 2001; Pütz, 2002), where it was hypothesized that king penguins shift from a fish- to a krill-based diet (Bost et al., 2004; Le Bohec et al., 2008). While tracking give no dietary information, blood δ¹⁵N values differentiate penguins that feed on low versus higher (krill vs. pelagic fish) trophic level prey (Cherel, 2008; Cherel, Hobson, Guinet, & Vanpé, 2007).
<table><caption>TABLE 1 Food and blood δ¹³C and δ¹⁵N values of king penguins during the Austral winter. Lipids were removed from food samples and plasma, but not from RBC and whole blood (see text)</caption><thead><tr><th>Groups</th><th>n</th><th>Tissue</th><th>δ¹³C (‰)</th><th>δ¹⁵N (‰)</th><th>C:N mass ratio</th></tr></thead><tbody><tr><td colspan="6">Winter 2002</td></tr><tr><td>Food samples</td><td>10</td><td>Digested fish</td><td>-22.4 ± 0.3 (-22.7 to -21.8)</td><td>7.3 ± 0.3 (6.9-7.7)</td><td>3.68 ± 0.10</td></tr><tr><td>Chicks</td><td>10</td><td>Whole blood</td><td>-21.2 ± 0.2 (-21.6 to -20.9)</td><td>11.1 ± 0.3 (10.6-11.5)</td><td>3.47 ± 0.05</td></tr><tr><td>Breeders</td><td>10</td><td>Whole blood</td><td>-21.9 ± 0.4 (-22.4 to -21.2)</td><td>9.9 ± 0.4 (9.4-10.5)</td><td>3.45 ± 0.04</td></tr><tr><td colspan="6">Winter 2008</td></tr><tr><td>Chicks</td><td>10</td><td>Whole blood</td><td>-21.9 ± 0.5 (-22.4 to -20.9)</td><td>10.7 ± 0.3 (10.4-11.3)</td><td>3.38 ± 0.07</td></tr><tr><td rowspan="2">Breeders</td><td>9</td><td>Blood cells</td><td>-21.4 ± 0.6 (-22.1 to -20.5)</td><td>10.0 ± 0.2 (9.8-10.4)</td><td>3.26 ± 0.03</td></tr><tr><td>8</td><td>Plasma</td><td>-21.5 ± 0.7 (-22.4 to -20.8)</td><td>10.7 ± 0.3 (10.2-11.2)</td><td>3.45 ± 0.05</td></tr><tr><td rowspan="2">UBS adults</td><td>52</td><td>Blood cells</td><td>-22.3 ± 0.6 (-23.5 to -21.2)</td><td>10.3 ± 0.4 (9.1-11.0)</td><td>3.26 ± 0.04</td></tr><tr><td>48</td><td>Plasma</td><td>-22.2 ± 0.8 (-23.9 to -20.7)</td><td>11.1 ± 0.4 (9.9-11.8)</td><td>3.43 ± 0.05</td></tr></tbody></table>

UBS, unknown breeding status (see text).

Values are means ± SD with ranges in parentheses.

3. **Prediction 3. Individual king penguins show consistency in their winter foraging strategies**, a common behavior in many seabirds that are faithful to their wintering grounds (Phillips, Lewis, Gonzalez-Solis, & Daunt, 2017). Comparison of δ¹³C and δ¹⁵N values in tissues that record trophic information at different time scales depicts trophic variations at the individual level (Hobson & Bond, 2012; Martinez del Rio, Sabat, Anderson-Sprecher, & Gonzalez, 2009). Positive linear relationships between isotopic values in plasma (short-term integration, days to weeks) versus red blood cells (RBC, medium-term integration, weeks to months; Hobson & Clark, 1993; Barquete, Strauss, & Ryan, 2013) would indicate consistency in the foraging habitat and diet of individual king penguins over the winter months. Conversely, contrasted (unrelated) plasma and RBC values would highlight time-related changes in their winter strategy (Cherel, Connan, Jaeger, & Richard, 2014).

4. **Prediction 4. Females forage more in Antarctic waters than males.** The rationale is that females perform fewer and longer foraging trips than males in winter (Descamps, Gauthier-Clerc, Gendner, & Le Maho, 2002; Le Vaillant, Ropert-Coudert, Le Maho, & Le Bohec, 2016; Saraux et al., 2012). Hence, females have more time than males to reach and feed within the distant AZ. Unfortunately, the sex of all but one of the previously winter-tracked king penguins was unknown (Bost et al., 2004; Charrassin & Bost, 2001; Jouventin, Capdeville, Cuénot-Chaillet, & Boiteau, 1994; Moore et al., 1999; Pistorius et al., 2017; Pütz, 2002).

## 2 | MATERIALS AND METHODS

The study took place at the king penguin colony of La Baie du Marin, Possession Island, Crozet Archipelago. The Crozet Islands (46-47°S) are located within the SAZ of the southern Indian Ocean. The Southern Ocean is here defined as the ocean south of the Subtropical Front

(STF), and the AZ, SAZ, and STZ as the zones south of the Polar Front (PF), between the PF and STF, and north of the STF, respectively.

Fieldwork was carried out during two Austral winters (2002 and 2008). At the very end of winter 2002 (early September), randomly chosen breeding adults (n = 10) and their corresponding chicks were blood sampled, and the adults were stomach-flushed one time to collect a representative subsample (100-400 g) of the food provisioned to their single offspring. In winter 2008 (July-August), three groups of birds were blood sampled: chicks (n = 10), breeders (n = 9), and adults of unknown status (UBS birds, n = 52). Blood was collected into a heparinized syringe by venepuncture of a flipper vein. In 2008 (not 2002), whole blood was centrifuged to separate plasma from RBC. Blood and food samples were kept frozen at -20°C until analysis in France.

Food samples were thawed overnight over a sieve to remove the liquid fraction. The solid fraction was then placed in a large flat-bottomed tray, and fresh remains were divided into broad prey classes (fish and cephalopods), which were weighed to estimate their proportions by mass in the diet. Total numbers of each prey item were counted in each individual food sample. Prey was identified using published keys and descriptions and by comparison with material (squid beaks and fish bones and otoliths) held in our own reference collection (Cherel et al., 1996; Xavier & Cherel, 2009).

Tissue δ¹³C and δ¹⁵N values were determined on food samples, and, depending on study groups, on whole blood, RBC, and plasma (Table 1). Food and blood samples were freeze-dried and powdered. King penguins feed on fatty fishes (Raclot et al., 1998) and plasma, unlike whole blood and RBC, contains a high and variable lipid content that affect its δ¹³C values (Cherel, Hobson, Bailleul, & Groscolas, 2005; Cherel, Hobson, & Hassani, 2005; Cherel, Hobson, & Weimerskirch, 2005). C:N mass ratios indicated that cyclohexane efficiently extracted lipids from fish muscle (Cherel et al., 2010), but not from plasma (Cherel, Connan, et al., 2014), which thus required a stronger delipidation using chloroform/methanol (Cherel, Hobson, & Weimerskirch, 2005). Tissue sub samples were weighed with a microbalance, packed in tin containers, and nitrogen and carbon isotope
ratios were subsequently determined by a continuous flow mass spectrometer (Thermo Scientific Delta V Advantage) coupled to an elemental analyzer (Thermo Scientific Flash EA 1112). Results are presented in the usual δ notation relative to Vienna PeeDee Belemnite and atmospheric N₂ for δ¹³C and δ¹⁵N, respectively. Replicate measurements of internal laboratory standards (acetanilide and peptone) indicate measurement errors <0.15 ‰ for both δ¹³C and δ¹⁵N values.

Sexing UBS king penguins is challenging because (1) unlike breeders, they do not perform their sex-specific call to communicate with their single chicks or mates (Derenne, Jouventin, & Mougin, 1979), and (2) the species is only slightly dimorphic, with males being larger than, but overlapping in size with females (Barrat, 1976). Hence, UBS birds and breeders from 2008 were genetically sexed using a molecular method (Fridolfsson & Ellegren, 1999) on either whole blood or RBC. Prolactin concentration was measured using a heterologous radioimmunoassay on plasma (Cherel, Mauget, Lacroix, & Gilles, 1994) to determine the breeding status of UBS adults sampled in winter 2008. In king penguins, circulating prolactin is high during incubation and the long chick-rearing period that includes winter; it remains at moderate levels in failed breeders but is low during molt, pairing, and courtship (Cherel et al., 1994; Garcia, Jouventin, & Mauget, 1996; Jouventin & Mauget, 1996).

Data were statistically analyzed using SYSTAT 13. Values are means ± SD. δ¹³C, and δ¹⁵N values of whole blood were considered to be comparable to those of RBC, because RBC contains more organic matter than plasma and consequently whole blood has δ¹³C and δ¹⁵N values very close to those of RBC (Cherel, Hobson, Bailleul, et al., 2005). To help interpret blood δ¹³C and δ¹⁵N values of king penguins, they were compared to blood values of the two truly Antarctic penguins, the emperor and Adélie penguins, which forage at high latitudes and feed mainly on pelagic fish and krill, respectively (Cherel, 2008).

## 3 | RESULTS

In late winter 2002, the food of king penguin chicks was dominated by fish (98.7% and 97.8% by mass and number, respectively), with cephalopods accounting for the remaining 1.3%-2.2%. Fishes were primarily myctophids (94.2% by number). Two species, *Krefftichthys anderssoni* and *Protomyctophum tenisoni*, were present in all the food samples (n = 10) and almost equally formed the main part of the diet (49.0% and 44.3% by number, respectively). Two other prey were found in most samples (n = 8), namely the paralepidid fish *Arctozenus risso* (3.1%) and the onychoteuthid squid *Kondakovia longimana* (1.5%).

Food samples and groups of king penguins (whole blood or RBC) segregated by both their δ¹³C and δ¹⁵N values (ANOVA, $F_{5,95} = 11.97$ and 174.27, respectively, both *p* < .0001). δ¹⁵N value was lower in food samples than in blood (Tukey's Honestly-Significant-Difference tests, all *p* < .0001) (Figure 2). Blood δ¹⁵N values varied slightly among penguin groups, with chicks being significantly ¹⁵N-enriched when compared to breeders and UBS adults (all *p* ≤ .004). Overall the range of mean δ¹³C values was low. In contrast to breeders and chicks, some UBS birds presented very ¹³C-depleted values (Table 1). Accordingly,

<figure><img src="image_3.png" /><figcaption>**FIGURE 2** Food and blood δ¹³C (upper panel) and δ¹⁵N (lower panel) values of king penguins during the Austral winter. Values not sharing the same superscripted letter are significantly different at *p* < .05 (Tukey's Honestly-Significant-Difference tests). Values are means ± SD. UBS, unknown breeding status (see text)</figcaption></figure>

mean plasma δ¹³C value was lower in UBS adults than in breeders sampled in winter 2008 (two-sample t-test, *t* = 2.45, *p* = .018).

Most breeders sampled in winter 2008 were males (8 of 9). Sex ratio also differed significantly from parity amongst the 52 sampled UBS adults, which included more males than females (71% vs. 29%, respectively; single proportion test, Z = 3.05, *p* = .002). Plasma prolactin concentration (range: 25–115 ng/ml) was significantly higher in UBS females than males (Table 2), and it was higher in breeding males than in UBS males (n = 7 and 35, 54 ± 13 and 42 ± 10 ng/ml, Mann-Whitney U-test, U = 187.0, *p* = .029). Circulating prolactin in UBS females and males showed a continuous range of values that were not linearly correlated with either plasma δ¹³C or δ¹⁵N values (least squares method and associated ANOVA, females: $F_{1,11} = 0.08$ and 0.50, *p* = .785 and 0.495; males: $F_{1,33} = 0.16$ and 4.15, *p* = .693 and 0.050, respectively).

No sex-related differences were found in either RBC or plasma δ¹⁵N values of UBS adults, but RBC and plasma δ¹³C values were significantly lower in females than in males (Table 2). Both RBC and plasma δ¹³C values indicated that proportionally more UBS females than males foraged within the AZ (RBC: 12 of 15 [80%] vs. 10 of 37 [27%], plasma: 10 of 13 [77%] vs. 11 of 35 [31%], equality of proportion tests, Z = 3.50 and 2.82, *p* < .0001 and *p* = .005, respectively). Conversely, their blood δ¹³C values showed that more males than females foraged in the SAZ (Figure 3). When foraging within the same
<table><caption>**TABLE 2** Blood δ¹³C and δ¹⁵N values and circulating prolactin of UBS king penguins during the Austral winter 2008. See text for isotopic delineation of water masses, and thus of birds foraging either in the Antarctic or Subantarctic zones</caption><thead><tr><th rowspan="2"></th><th rowspan="2">Females</th><th rowspan="2">Males</th><th colspan="2">Two sample t-tests</th></tr><tr><th>t</th><th>p</th></tr></thead><tbody><tr><td colspan="5"><strong>Red blood cells</strong></td></tr><tr><td colspan="5"><strong>All UBS adults</strong></td></tr><tr><td>n</td><td>15</td><td>37</td><td></td><td></td></tr><tr><td>δ¹³C (‰)</td><td>-22.8 ± 0.5</td><td>-22.1 ± 0.5</td><td>3.97</td><td>&lt;.0001</td></tr><tr><td>δ¹⁵N (‰)</td><td>10.3 ± 0.4</td><td>10.3 ± 0.4</td><td>0.09</td><td>.928</td></tr><tr><td>C:N mass ratio</td><td>3.26 ± 0.03</td><td>3.26 ± 0.05</td><td>0.24</td><td>.810</td></tr><tr><td colspan="5"><strong>Antarctic zone</strong></td></tr><tr><td>n</td><td>12</td><td>10</td><td></td><td></td></tr><tr><td>δ¹³C (‰)</td><td>-22.9 ± 0.3</td><td>-22.8 ± 0.3</td><td>0.84</td><td>.409</td></tr><tr><td>δ¹⁵N (‰)</td><td>10.3 ± 0.4</td><td>10.4 ± 0.2</td><td>0.62</td><td>.543</td></tr><tr><td colspan="5"><strong>Subantarctic zone</strong></td></tr><tr><td>n</td><td>3</td><td>27</td><td></td><td></td></tr><tr><td>δ¹³C (‰)</td><td>-22.1 ± 0.5</td><td>-21.9 ± 0.4</td><td>na</td><td>na</td></tr><tr><td>δ¹⁵N (‰)</td><td>10.2 ± 0.3</td><td>10.2 ± 0.4</td><td>na</td><td>na</td></tr><tr><td colspan="5"><strong>Plasma</strong></td></tr><tr><td colspan="5"><strong>All UBS adults</strong></td></tr><tr><td>n</td><td>13</td><td>35</td><td></td><td></td></tr><tr><td>δ¹³C (‰)</td><td>-22.9 ± 0.6</td><td>-22.0 ± 0.8</td><td>4.14</td><td>&lt;.0001</td></tr><tr><td>δ¹⁵N (‰)</td><td>11.1 ± 0.4</td><td>11.1 ± 0.4</td><td>0.01</td><td>.996</td></tr><tr><td>C:N mass ratio</td><td>3.43 ± 0.04</td><td>3.43 ± 0.06</td><td>0.15</td><td>.880</td></tr><tr><td>Prolactin (ng/ml)</td><td>77 ± 26</td><td>42 ± 10</td><td>6.84</td><td>&lt;.0001</td></tr><tr><td colspan="5"><strong>Antarctic Zone</strong></td></tr><tr><td>n</td><td>10</td><td>11</td><td></td><td></td></tr><tr><td>δ¹³C (‰)</td><td>-23.2 ± 0.3</td><td>-22.9 ± 0.4</td><td>2.06</td><td>.053</td></tr><tr><td>δ¹⁵N (‰)</td><td>11.3 ± 0.2</td><td>11.3 ± 0.2</td><td>0.41</td><td>.683</td></tr><tr><td>Prolactin (ng/ml)</td><td>77 ± 28</td><td>40 ± 10</td><td>4.15</td><td>.001</td></tr><tr><td colspan="5"><strong>Subantarctic Zone</strong></td></tr><tr><td>n</td><td>3</td><td>24</td><td></td><td></td></tr><tr><td>δ¹³C (‰)</td><td>-22.0 ± 0.1</td><td>-21.6 ± 0.4</td><td>na</td><td>na</td></tr><tr><td>δ¹⁵N (‰)</td><td>10.5 ± 0.3</td><td>11.1 ± 0.5</td><td>na</td><td>na</td></tr><tr><td>Prolactin (ng/ml)</td><td>77 ± 23</td><td>43 ± 11</td><td>na</td><td>na</td></tr></tbody></table>

Statistical differences are marked in bold. Values are means ± *SD*. UBS, unknown breeding status (see text).

oceanographic zone, there were no significant differences in RBC and plasma δ¹³C and δ¹⁵N values between UBS males and females (Table 2).

Plasma and RBC δ¹³C values were positively and linearly cor- related in UBS adults and breeders sampled in winter 2008 (except

<figure><img src="image_4.png" /><figcaption>**FIGURE 3** Delipidated plasma δ¹⁵N versus δ¹³C values in UBS adults and breeders of king penguins sampled in the Austral winter 2008. Whole blood isotopic values of two high-Antarctic seabirds that feed primarily on fish (breeding emperor penguins in spring) and crustaceans (breeding Adélie penguins in spring) were included in the figure to help interpreting king penguin δ¹³C and δ¹⁵N values (Cherel, 2008). Following Cherel and Hobson (2007), dashed lines correspond to the δ¹³C estimation of the Polar Front (PF) and of the Subtropical Front (STF), which delimit the Antarctic (AZ), Subantarctic (SAZ), and Subtropical Zones. UBS, unknown breeding status (see text)</figcaption></figure>

two outliers; see discussion). Plasma and RBC δ¹⁵N values were also positively and linearly related among individuals of the two groups (Figure 4). Paired t-tests indicated that δ¹³C values in RBC and plasma were not significantly different (n = 54 [without the two outliers], t = 0.43, *p* = .667), but δ¹⁵N values were on average 0.80 ± 0.24 ‰ higher in plasma than in RBC (n = 56, t = 24.51, *p* < .0001).

## 4 | DISCUSSION

The present work exemplifies the usefulness of stable isotopes to complement the direct methods of bio-logging and dietary analysis to reveal previously unknown foraging strategies during cryptic stages in the life cycle of animals (Cherel, Kernaléguen, Richard, & Guinet, 2009). Blood δ¹³C and δ¹⁵N values depict four relevant biological features on the foraging ecology of king penguins during the critical winter period: (1) overall, birds forage both in Antarctic (AZ) and sub-antarctic (SAZ) waters, (2) they primarily prey upon mesopelagic fish in both zones, (3) individuals are consistent in their foraging strategies over the winter months and (4) females have a greater tendency than males to feed in distant Antarctic waters.

Despite their long breeding cycle (>1 year), king penguins attempt to breed every year but none breed successfully in two consecutive seasons. Early breeders that are successful on year 1 become late breeders that fail during incubation or during the early chick-rearing period on year 2, being thus again early breeders on year 3 (van Heezik, Seddon, Cooper, & Plös, 1994; Le Bohec et al., 2007; Olsson, 1996). Accordingly, both breeders and failed breeders are commonly found in the colony in winter (Descamps et al., 2002). Circulating prolactin in our study animals confirmed that pattern, with no UBS king penguins
**FIGURE 4** Delipidated plasma δ¹³C versus RBC δ¹³C values (left panel) and delipidated plasma δ¹⁵N versus RBC δ¹⁵N values (right panel) in UBS adults and breeders of king penguins sampled in the Austral winter 2008 (for outliers, see text). Regression lines (least squares method and associated ANOVA): δ¹³C (without outliers), y = 1.13x + 3.00, F₁,₅₂ = 345.83, p < .0001; δ¹⁵N, y = 1.01x + 0.74, F₁,₅₄ = 121.31, p < .0001. UBS, unknown breeding status (see text)

<figure><img src="image_5.png" /></figure>

sampled in 2008 having low hormone levels that characterize non
breeders (Garcia et al., 1996). This suggests that the few non breed-
ers disperse widely and remain at sea without coming ashore, as most
oceanic penguins (Bost et al., 2009; Thiébot, Cherel, Trathan, & Bost,
2012). Plasma prolactin concentration was elevated in all sampled
UBS king penguins, and it was higher in females than in males, which is
in agreement with previous investigations (Cherel et al., 1994; Garcia
et al., 1996). No bimodality occurred in hormone levels, thus preclud-
ing a clear-cut differentiation between breeders and failed breeders.
Instead, the continuum of hormone values indicates that sampled UBS
birds included unknown proportions of breeders and failed breeders.
Within that context, a relevant finding is that circulating prolactin was
neither related to blood δ¹³C nor δ¹⁵N values, thus suggesting that
both breeders and failed breeders used the same foraging habitats and
fed on the same prey during the winter months.

**4.1 | Prediction 1. King penguins forage in Antarctic waters in winter**

King penguin chicks and breeders that were sampled in the winters
of 2002 and 2008 showed limited variations in their isotopic values.
Blood δ¹³C values indicated foraging within the SAZ and at the PF,
which is in agreement with the many birds sampled at the Crozet
Islands during the spring and summer months (Cherel, Hobson,
Bailleul, et al., 2005; Cherel et al., 2007; Cherel, Y., unpublished
data). Mean δ¹³C values of UBS adults sampled in winter 2008 are
close to those of chicks and breeders, but they showed a larger
range of individual values that illustrates three notable features
about their feeding grounds:

1. Based on the known latitudinal δ¹³C gradient, low blood δ¹³C values indicated Antarctic foraging grounds for the species. Forty-two percent of UBS adults foraged within the AZ, with six birds having RBC δ¹³C values <-23 %. However, an isotopic comparison with the two true Antarctic penguins, the emperor and Adélie penguins, shows that king penguins did not reach high-Antarctic waters (Figure 3). Overall, the isotopic data are in close agreement with the few winter-tracked breeders that forage within the SIZ, but not in dense pack ice further south

(Bost et al., 2004; Charrassini & Bost, 2001; Pistorius et al., 2017; Pütz, 2002).

2. No UBS adults foraged in the subtropics, which confirms the many satellite-tracking and isotopic investigations over sites and years that never showed any king penguin crossing the STF in the southern Indian Ocean (Bost et al., 1997; Charrassini & Bost, 2001; Cherel et al., 2007; Pistorius et al., 2017; Pütz, 2002).

3. Instead, both RBC and plasma δ¹³C showed a continuum of values indicating that winter foraging grounds of king penguins ranged from subantarctic to Antarctic waters. Importantly, blood δ¹³C values of a majority of UBS adults (58%) were similar to those found during summer, thus indicating that birds foraged primarily within the SAZ and at the PF in winter. In contrast, all but one winter satellite-tracked breeders crossed the PF and foraged within the AZ (Bost et al., 2004; Moore et al., 1999; Pistorius et al., 2017; Pütz, 2002). This mismatch, together with dietary indication of feeding very close to the colony to feed the chicks (Cherel et al., 1996), highlights the need of further investigations using a combination of isotopic and bio-logging methods on the same individuals to better detail the various foraging strategies of king penguins during winter.

**4.2 | Prediction 2. King penguins feed on Antarctic krill in winter**

When foraging within the SAZ and at the PF in winter, king penguin
breeders fed primarily on myctophids for themselves, as indicated
by their blood δ¹⁵N values that were identical to the many previous
measurements performed on the species at the Crozet Islands (Cherel,
Hobson, Bailleul, et al., 2005; Cherel et al., 2007) and elsewhere
(Cherel, Pütz, & Hobson, 2002; Cherel et al., 2010). A myctophid-
based diet was verified by the stepwise lower δ¹⁵N value of food de-
livered to chicks in early September 2002 (Figure 2). At that time, food
was mainly composed of two myctophid species (K. anderssoni and
P. tenisoni) that form the bulk of the chick diet in spring (Cherel et al.,
1993) and temporally correspond to an increase in parent feeding
visits from early September onward (Saraux et al., 2012). The slightly
higher δ¹⁵N values of chicks than breeders result from two cumulative
isotopic explanations: (1) Chicks fast most of the time in winter, thus using endogenous protein reserves that induces a progressive $ ^{15}N $ enrichment of their tissues (Barrat, 1976; Cherel, Stahl, & Le Maho, 1987; Cherel, Hobson, Bailleul, et al., 2005), and (2) winter food of chicks includes squids that have higher $ ^{15}N $ values than fish (Cherel, Ducatez, Fontaine, Richard, & Guinet, 2008; Cherel et al., 1996).

Blood $ ^{15}N $ values of UBS adults indicate they also fed on fish whatever their $ ^{13}C $ values and hence their foraging zones (Figure 3). Feeding on myctophids in both the SAZ and AZ did not verify the hypothesis that king penguins shift to Antarctic krill when foraging in Antarctic waters (Bost et al., 2004; Le Bohec et al., 2008). This new finding is supported by several isotopic arguments.

1. Penguins feeding on low trophic level prey, that is, swarming crustaceans, have low blood $ ^{15}N $ values that do not fit with the higher king penguin values. Those species include pygoscelid penguins feeding on Antarctic krill in the AZ (Juares, Santos, Mennucci, Coria, & Mariano-Jelicich, 2016; Polito et al., 2016) and crested penguins preying upon euphausiids and hyperiids within the SAZ, including at the Crozet Islands (Cherel et al., 2007).

2. Seabirds feeding on high trophic level prey (large fish, squids, and carrion) have higher $ ^{15}N $ values than king penguins, as illustrated by large petrels and albatrosses (Blévin et al., 2013).

3. Instead, blood $ ^{15}N $ values of UBS king penguins foraging within the AZ are identical to those of birds foraging in the SAZ (Cherel et al., 2007; this study) and close to the values of other myctophid eaters, including Antarctic and subantarctic fur seals, elephant seals, and blue petrels (Cherel et al., 2007, 2008; Cherel, Connan, et al., 2014). Hence, the king penguin is a meso-predator that specializes on myctophids all year long. Identifying fish species targeted by king penguin in the AZ remains to be investigated, but all penguin $ ^{15}N $ values notably were >9.9 and >11.0 ‰ in RBC and plasma, respectively (Figure 3). Taking into account the king penguin blood-diet discrimination factor (the differences in isotopic composition between blood and diet, here 2.1 ‰; Cherel, Hobson, & Hassani, 2005), these relatively high $ ^{15}N $ values suggest king penguins fed less on the lower trophic level *Krefftichthys anderssoni* (6.6–7.6 ‰) and *P. tenisoni* (6.9–8.1 ‰), and more on *Electrona antarctica* (8.3–8.9 ‰), the most abundant myctophid in Antarctic waters (Hulley, 1990; Cherel et al., 2010, unpublished data).

## 4.3 | Prediction 3. Individual king penguins show consistency in their winter foraging strategies

Another relevant finding is that king penguins (both UBS adults and breeders) showed individual consistency in their winter foraging strategies, as indicated by the highly significant linear relationships between RBC and plasma isotopic values (Xavier et al., 2017). Individual birds foraged within the same isotopic habitat ($ ^{13}C $) where they fed on the same isotopic diet ($ ^{15}N $) during the months preceding blood sampling (Figure 4). This means that individuals foraging in

Antarctic waters did not feed significantly within the SAZ but, instead fasted and relied on their energy reserves built up in the AZ to cover the energetic cost of travelling to the colony and remaining ashore, a “capital” strategy similar to that of some male Antarctic fur seals (Cherel et al., 2009). Fidelity to feeding habitats and prey presumably increases foraging success in winter, being thus beneficial during a period marked by a decrease in the availability of marine resources. Two birds showed a different strategy, however (outliers in Figure 4). Their more positive $ ^{13}C $ values in plasma than in RBC indicate that they shifted to more northern feeding grounds over the last weeks before blood sampling. The two king penguins exemplify the foraging plasticity of Southern Ocean seabirds facing various environmental conditions at different spatio temporal scales (Cherel, Connan, et al., 2014; Tremblay & Cherel, 2003).

Plasma was enriched in $ ^{15}N $ (but not in $ ^{13}C $) when compared to RBC (Figure 4). This plasma-RBC difference is in agreement with the values calculated in captive birds feeding on isotopically controlled diets (Federer, Hollmen, Esler, Wooller, & Wang, 2010; Hahn, Hoye, Korthals, & Klaassen, 2012; Kurle et al., 2013). The $ ^{15}N $ differences arise from tissue-specific isotopic discrimination factors due primarily to tissue-specific protein, and hence amino acid, composition (Wolf, Carleton, & Martinez del Rio, 2009). Such tissue-specific discrimination factors preclude comparing raw isotopic values of different tissues without correcting them first (Cherel, Jaquemet, Maglio, & Jaeger, 2014).

## 4.4 | Prediction 4. More females than males forage in Antarctic waters

A last new finding is that, as expected, females rather than males foraged within the AZ in winter. Females feeding further away than males are likely the explanation of chicks being visited two times less by females than by males in winter (Descamps et al., 2002). This means that theoretically ~1/3 of breeders present at the colony are female birds, a biased sex ratio that fit well with the proportion of females observed among the sampled UBS adults in winter 2008 (29%). Why females favor distant foraging grounds is difficult to interpret. In seabirds, sex-related segregation in food and feeding ecology is linked to sexual size dimorphism with the underlying mechanisms being related to sexual specialization in diet or habitat, or size-mediated competitive exclusion (Phillips, McGill, Dawson, & Bearhop, 2011). Whatever the driving factor is for king penguin, the finding potentially has important demographic consequences. During normal years, survival rate is high and identical for both sexes (Le Bohec et al., 2007, 2008; Olsson & van der Jeugd, 2002), but females survive less well than males following a catastrophic year marked by food shortage (Olsson & van der Jeugd, 2002). The sex-related difference in survival can be explained by the sex-related difference in winter foraging grounds because (1) king penguin mortality is negatively affected by warm events within the AZ (Le Bohec et al., 2008), and (2) females forage more within the AZ than males (present study), thus resulting in a skewed sex ratio favoring males in the whole population (Olsson & van der Jeugd, 2002).
<table><caption>**TABLE 3** Review of dietary, tracking, and stable isotopes evidences that pinnipeds and seabirds breeding in subantarctic islands (southern Indian Ocean) forage in Antarctic waters. Dietary bio-indicators are two endemic Antarctic prey species, the Antarctic krill *Euphausia superba* and oceanic squid *Psychroteuthis glacialis*</caption><thead><tr><th>Species</th><th>Breeding localities</th><th>Life stages</th><th>Diet</th><th>Tracking</th><th>Stable isotopes</th><th>References</th></tr></thead><tbody><tr><td>Southern elephant seal</td><td>Marion</td><td>Post-breeding, post-molt</td><td>+</td><td></td><td></td><td>(McIntyre, Bornemann, Plötz, Tosh, & Bester, 2011; McIntyre, Tosh, Plötz, Bornemann, & Bester, 2010; McIntyre et al., 2011)</td></tr><tr><td>(Mirounga leonina)</td><td>Kerguelen</td><td>Post-breeding, post-molt</td><td>+</td><td>+</td><td></td><td>(Authier, Dragon, Cherel, & Guinet, 2012; Bailleul, Charrassin, Ezraty, et al., 2007; Bailleul, Charrassin, Monestiez, et al., 2007; Chaigne, Authier, Richard, Cherel, & Guinet, 2013; Labrousse et al., 2015)</td></tr><tr><td>Antarctic fur seal (Arctocephalus gazella)</td><td>Marion</td><td>Winter</td><td>+</td><td>+</td><td></td><td>(Arthur et al., 2016)</td></tr><tr><td></td><td>Crozet</td><td>Breeding, all year long</td><td>+</td><td></td><td></td><td>(Cherel et al., 2007, 2009; Kernaléguen et al., 2012; 2016)</td></tr><tr><td>King penguin (Aptenodytes patagonicus)</td><td>Marion</td><td>Chick-rearing (winter)</td><td>+</td><td></td><td></td><td>(Pistorius et al., 2017)</td></tr><tr><td></td><td>Crozet</td><td>Chick-rearing (winter)</td><td>+</td><td>+</td><td></td><td>(Charrassin & Bost, 2001; Pütz, 2002; Bost et al., 2004; present study)</td></tr><tr><td>Macaroni penguin (Eudyptes chrysolophus)</td><td>Marion</td><td>Pre-molt</td><td>+</td><td>+</td><td></td><td>(Whitehead, Connan, Ropert-Coudert, & Ryan, 2017; Whitehead, Kato, Ropert-Coudert, & Ryan, 2016)</td></tr><tr><td></td><td>Crozet</td><td>Pre-molt, winter migration</td><td>+</td><td>+</td><td></td><td>(Thiébot, Cherel, Trathan, & Bost, 2011; Thiébot et al., 2014)</td></tr><tr><td></td><td>Kerguelen</td><td>pre-molt, winter migration</td><td>+</td><td></td><td></td><td>(Thiébot et al., 2011, 2014)</td></tr><tr><td>Rockhopper penguin (Eudyptes chrysocome filholi)</td><td>Marion</td><td>Pre-molt</td><td>+</td><td>+</td><td></td><td>(Whitehead et al., 2016, 2017)</td></tr><tr><td>Wandering albatross (Diomedea exulans)</td><td>Crozet</td><td>Incubation</td><td>+</td><td>+</td><td></td><td>(Jaeger, Lecomte, et al., 2010; Lecomte et al., 2010; Weimerskirch, Salamolard, Sarrazin, & Jouventin, 1993; Weimerskirch et al., 2014)</td></tr><tr><td></td><td>Kerguelen</td><td>Incubation</td><td>+</td><td></td><td></td><td>(Pinaud & Weimerskirch, 2007)</td></tr><tr><td>Black-browed albatross (Thalassarche melanophris)</td><td>Kerguelen</td><td>Incubation</td><td>+</td><td></td><td></td><td>(Pinaud & Weimerskirch, 2007)</td></tr><tr><td>Grey-headed albatross (Thalassarche chrysostoma)</td><td>Marion</td><td>Chick-rearing, molt</td><td>+</td><td>+</td><td>+</td><td>(Connan, McQuaid, Bonnevie, Smale, & Cherel, 2014; Jaeger et al., 2013; Nel et al., 2001; Richoux, Jaquemet, Bonnevie, Cherel, & McQuaid, 2010)</td></tr><tr><td></td><td>Kerguelen</td><td>Chick-rearing</td><td>+</td><td></td><td></td><td>(Cherel, Weimerskirch, & Trouvé, 2002)</td></tr><tr><td>Light-mantled sooty albatross (Phoebetria palpebrata)</td><td>Marion</td><td>Chick-rearing, molt</td><td>+</td><td>+</td><td>+</td><td>(Berruti & Marcus, 1978; Connan et al., 2014; Cooper & Klages, 1995; Jaeger et al., 2013)</td></tr><tr><td></td><td>Crozet</td><td>Chick-rearing, molt</td><td>+</td><td>+</td><td>+</td><td>(Jaeger, Connan, Richard, & Cherel, 2010; Pinaud & Weimerskirch, 2007; Ridoux, 1994)</td></tr><tr><td></td><td>Kerguelen</td><td>Molt</td><td></td><td></td><td>+</td><td>(Jaeger et al., 2013)</td></tr></tbody></table>

(Continues)
<table><caption>TABLE 3 (Continued)</caption><thead><tr><th>Species</th><th>Breeding localities</th><th>Life stages</th><th>Diet</th><th>Tracking</th><th>Stable isotopes</th><th>References</th></tr></thead><tbody><tr><td>Sooty albatross</td><td>Marion</td><td>Chick-rearing</td><td>+</td><td></td><td></td><td>(Berruti &amp; Hercus, 1978; Cooper &amp; Klages, 1995)</td></tr><tr><td>(Phoebetria fusca)</td><td>Crozet</td><td>Chick-rearing</td><td>+</td><td></td><td>+</td><td>(Pinaud &amp; Weimerskirch, 2007; Ridoux, 1994)</td></tr><tr><td>Southern giant petrel</td><td>Crozet</td><td>Incubation, chick-rearing</td><td></td><td>+</td><td></td><td>(Thiers et al., 2014)</td></tr><tr><td>(Macronectes giganteus)</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>White-chinned petrel</td><td>Crozet</td><td>Incubation, chick-rearing</td><td>+</td><td>+</td><td>+</td><td>(Catard, Weimerskirch, &amp; Cherel, 2000; Connan, Cherel, &amp; Mayzaud, 2007; Jaeger, Connan, et al., 2010; Ridoux, 1994)</td></tr><tr><td>(Procellaria aequinoctialis)</td><td>Kerguelen</td><td>Incubation, chick-rearing</td><td>+</td><td>+</td><td>+</td><td>(Delord et al., 2010; Jaeger et al., 2013; Péron et al., 2010)</td></tr><tr><td>White-headed petrel</td><td>Kerguelen</td><td>Chick-rearing</td><td></td><td></td><td>+</td><td>(Blévin et al., 2013)</td></tr><tr><td>(Pterodroma lessoni)</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Kerguelen petrel</td><td>Crozet</td><td>Chick-rearing</td><td>+</td><td></td><td></td><td>(Ridoux, 1994)</td></tr><tr><td>(Aphrodroma brevirostris)</td><td>Kerguelen</td><td>Chick-rearing, molt</td><td></td><td></td><td>+</td><td>(Blévin et al., 2013; unpublished data)</td></tr><tr><td>Blue petrel</td><td>Crozet</td><td>Chick-rearing</td><td>+</td><td></td><td></td><td>(Ridoux, 1994)</td></tr><tr><td>(Halobaena coerulea)</td><td>Kerguelen</td><td>All year long</td><td>+</td><td>+</td><td>+</td><td>(Cherel, Bocher, Trouvé, et al., 2002; Cherel, Connan, et al., 2014; Cherel et al., 2016; Connan, Mayzaud, Trouvé, Barbraud, &amp; Cherel, 2008; Quillfeldt, Cherel, Delord, &amp; Weimerskirch, 2015)</td></tr><tr><td>Antarctic prion</td><td>Kerguelen</td><td>Chick-rearing, molt</td><td>+</td><td></td><td>+</td><td>(Cherel, Bocher, de Broyer, et al., 2002; Cherel et al., 2016; Weimerskirch, Fradet, &amp; Cherel, 1999)</td></tr><tr><td>(Pachyptila desolata)</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Thin-billed prion</td><td>Kerguelen</td><td>All year long</td><td>+</td><td>+</td><td>+</td><td>(Cherel, Bocher, de Broyer, et al., 2002; Cherel, Connan, et al., 2014; Cherel et al., 2016; Quillfeldt et al., 2015)</td></tr><tr><td>(Pachyptila belcheri)</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Common diving petrel</td><td>Kerguelen</td><td>Spring, molt</td><td></td><td></td><td>+</td><td>(Bocher, Cherel, &amp; Hobson, 2000; Cherel, Phillips, Hobson, &amp; McGill, 2006; Cherel, Connan, et al., 2014)</td></tr><tr><td>(Pelecanoides urinatrix)</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>South Georgian diving petrel</td><td>Kerguelen</td><td>Molt</td><td></td><td></td><td>+</td><td>(Bocher et al., 2000; unpublished data)</td></tr><tr><td>(Pelecanoides georgicus)</td><td></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>

## 4.5 | Trophic connectivity between subantarctic and Antarctic ecosystems

King penguins largely foraged in Antarctic waters where they fed on mesopelagic fish. Breeding north the PF and foraging south of it appears a common life history trait of many warm-blooded vertebrates. Dietary investigation, satellite-tracking and stable isotopes concur to

indicate that many subantarctic marine mammals and seabirds foraged in the AZ during parts of their life cycle (Table 3). They belong to a large diversity of taxa (pinnipeds, penguins, albatrosses, and petrels), which show a large range in size (from diving petrels to elephant seals), in flying ability (from flightless to dynamic soaring), and in feeding methods (pursuit diving and surface seizing being the commonest). Ecologically, they include major consumers of the Southern Ocean
both in terms of numbers and in terms of biomass, thus emphasizing the key role of the nutritional flux from Antarctic waters to sustain breeding populations of subantarctic predators. Hence, the PF does not act as a biogeographic barrier for pinnipeds and seabirds, and there is a strong trophic connectivity between the AZ oceanic and SAZ island ecosystems.

When feeding in the AZ, king penguins foraged primarily within the Permanent Open Ocean Zone (POOZ, Tréguer & Jacques, 1992), with some of them reaching the northern limits of the SIZ (Bost et al., 2004; Pistorius et al., 2017). Other species favor foraging within the SIZ, as indicated by the importance of Antarctic krill in the diet of Antarctic fur seals (Cherel et al., 2009) and of various procellariform seabirds (Cherel, Bocher, de Broyer, & Hobson, 2002; Cherel, Bocher, Trouvé, & Weimerskirch, 2002; Cherel, Quillfeldt, Delord, & Weimerskirch, 2016; Delord et al., 2010). Finally, some far-ranging Kerguelen elephant seals transit through the POOZ and SIZ to feed within the productive but distant Coastal and Continental Shelf Zone (Labrousse et al., 2015). Consequently, population dynamics of subantarctic predators are shaped by Antarctic climatic indices (Barbraud & Weimerskirch, 2003; Le Bohec et al., 2008; Nevoux & Barbraud, 2006), making them at risk from natural or human-driven environmental changes negatively affecting the biomass and distribution of Antarctic marine resources.

## ACKNOWLEDGMENTS

The authors thank F. Pawlowski and M. Berlincourt for collecting samples in the field in 2002 and 2008, respectively. They also thank M. Brault-Favrou and G. Guillou from the Plateforme de Spectrométrie Isotopique (IRMS) from LIENS for preparing and running stable isotope samples, respectively, A. Lacroix for performing the prolactin radioimmunoassay, S. Ruault for sexing genetically the penguins, and C. Trouvé for her help in food analysis. The present work was supported financially and logistically by the Institut Polaire Français Paul Emile Victor (IPEV, programme no. 109, H. Weimerskirch), and the Terres Australes et Antarctiques Françaises (TAAF). The Contrat de Plan Etat Région is thanked for funding the IRMS. The Institut Universitaire de France (IUF) is acknowledged for its support to P. Bustamante.

## CONFLICT OF INTEREST

None declared.

## AUTHOR CONTRIBUTIONS

YC and CAB conceived and designed the study. Prolactin measurement was performed by CP, and PB supervised stable isotope analysis. YC analyzed the data and wrote the manuscript, which was revised and improved by PB and CAB.

## ORCID

Yves Cherel http://orcid.org/0000-0001-9469-9489

Paco Bustamante http://orcid.org/0000-0003-3877-9390

## REFERENCES

Angelier, F., Wingfield, J. C., Tartu, S., & Chastel, O. (2016). Does prolactin mediate parental and life-history decisions in response to environmental conditions in birds? A review. *Hormones & Behavior, 77*, 18-29.
https://doi.org/10.1016/j.yhbeh.2015.07.014

Arthur, B., Hindell, M., Bester, M. N., Oosthuizen, W. C., Wege, M., & Lea, M. A. (2016). South for the winter? Within-dive foraging effort reveals the trade-offs between divergent foraging strategies in a free-ranging predator. *Functional Ecology*, 30, 1623-1637. https://doi.org/10.1111/1365-2435.12636

Aubin, T., & Jouventin, P. (1998). Cocktail-party effect in king penguin colonies. *Proceedings of the Royal Society B*, 265, 1665-1673. https://doi.org/10.1098/rspb.1998.0486

Authier, M., Dragon, A. C., Cherel, Y., & Guinet, C. (2012). How large is large: estimating ecologically meaningful isotopic differences in observational studies of wild animals. *Rapid Communications in Mass Spectrometry*, 26, 2657-2664. https://doi.org/10.1002/rcm.6389

Bailleul, F., Charrassin, J. B., Ezraty, R., Girard-Ardhuin, F., McMahon, C., Field, I., & Guinet, C. (2007). Southern elephant seals from Kerguelen Islands confronted by Antarctic sea ice. Changes in movements and in diving behaviour. *Deep-Sea Research II*, 54, 343-355. https://doi.org/10.1016/j.dsr2.2006.11.005

Bailleul, F., Charrassin, J. B., Monestiez, P., Roquet, F., Biuw, M., & Guinet, C. (2007). Successful foraging zones of southern elephant seals from the Kerguelen Islands in relation to oceanographic conditions. *Philosophical Transactions of the Royal Society B*, 362, 2169-2181. https://doi.org/10.1098/rstb.2007.2109

Ballard, G., Toniolo, V., Ainley, D. G., Parkinson, C. L., Arrigo, K. R., & Trathan, P. N. (2010). Responding to climate change: Adélie Penguins confront astronomical and ocean boundaries. *Ecology*, 91, 2056-2069. https://doi.org/10.1890/09-0688.1

Barbraud, C., & Weimerskirch, H. (2003). Climate and density shape population dynamics of a marine top predator. *Proceedings of the Royal Society B*, 270, 2111-2116. https://doi.org/10.1098/rspb.2003.2488

Barquete, V., Strauss, V., & Ryan, P. G. (2013). Stable isotope turnover in blood and claws: a case study in captive African penguins. *Journal of Experimental and Marine Biology and Ecology*, 448, 121-127. https://doi.org/10.1016/j.jembe.2013.06.021

Barrat, A. (1976). Quelques aspects de la biologie et de l'écologie du Manchot royal (Aptenodytes patagonicus) des Iles Crozet. *Comité national français des recherches antarctiques*, 40, 9-52.

Berruti, A., & Hercus, T. (1978). Cephalopod prey of the sooty albatrosses *Phoebetria fusca* and *P. palpebrata* at Marion Island. *South African Journal of Antarctic Research*, 8, 99-103.

Blévin, P., Carravieri, A., Jaeger, A., Chastel, O., Bustamante, P., & Cherel, Y. (2013). Wide range of mercury contamination in chicks of Southern Ocean seabirds. *PLoS ONE*, 8, e54508. https://doi.org/10.1371/journal.pone.0054508

Bocher, P., Cherel, Y., & Hobson, K. A. (2000). Complete trophic segregation between South Georgian and common diving petrels during breeding at Iles Kerguelen. *Marine Ecology Progress Series*, 208, 249-264. https://doi.org/10.3354/meps208249

Bost, C. A., Charrassin, J. B., Clerquin, Y., Ropert-Coudert, Y., & Le Maho, Y. (2004). Exploitation of distant marginal ice zones by king penguins during winter. *Marine Ecology Progress Series*, 283, 293-297. https://doi.org/10.3354/meps283293

Bost, C. A., Cotté, C., Terray, P., Barbraud, C., Bon, C., Delord, K., ... Weimerskirch, H. (2015). Large-scale climatic anomalies affect marine predator foraging behaviour and demography. *Nature Communications*, 6, 8220. https://doi.org/10.1038/ncomms9220

Bost, C. A., Georges, J. Y., Guinet, C., Cherel, Y., Pütz, K., Charrassin, J. B., ... Le Maho, Y. (1997). Foraging habitat and food intake of satellite-tracked king penguins during the austral summer at Crozet Archipelago.
*Marine Ecology Progress Series*, 150, 21-33. https://doi.org/10.3354/meps150021

Bost, C. A., Thiébot, J. B., Pinaud, D., Cherel, Y., & Trathan, P. N. (2009). Where do penguins go during the inter-breeding period? Using geolocation to track the winter dispersion of the macaroni penguin. *Biology Letters*, 5, 473-476. https://doi.org/10.1098/rsbl.2009.0265

de Brooke, M. L. (2004). The food consumption of the world's seabirds. *Proceedings of the Royal Society B (Supplement)*, 271, S246-S248. https://doi.org/10.1098/rsbl.2003.0153

Catard, A., Weimerskirch, H., & Cherel, Y. (2000). Exploitation of distant Antarctic waters and close shelf-break waters by white-chinned petrels rearing chicks. *Marine Ecology Progress Series*, 194, 249-261. https://doi.org/10.3354/meps194249

Chaigne, A., Authier, M., Richard, P., Cherel, Y., & Guinet, C. (2013). Shift in foraging grounds and diet broadening during ontogeny in southern elephant seals from Kerguelen Islands. *Marine Biology*, 160, 977-986. https://doi.org/10.1007/s00227-012-2149-5

Charrassain, J. B., & Bost, C. A. (2001). Utilisation of the oceanic habitat by king penguins over the annual cycle. *Marine Ecology Progress Series*, 221, 285-297. https://doi.org/10.3354/meps221285

Cherel, Y. (2008). Isotopic niches of emperor and Adélie penguins in Adélie Land, Antarctica. *Marine Biology*, 154, 813-821. https://doi.org/10.1007/s00227-008-0974-3

Cherel, Y., Bocher, P., de Broyer, C., & Hobson, K. A. (2002). Food and feeding ecology of the sympatric thin-billed *Pachyptila belcheri* and Antarctic *P. desolata* prions at Iles Kerguelen, Southern Indian Ocean. *Marine Ecology Progress Series*, 228, 263-281. https://doi.org/10.3354/meps228263

Cherel, Y., Bocher, P., Trouvé, C., & Weimerskirch, H. (2002). Diet and feeding ecology of blue petrels *Halobaena caerulea* at Iles Kerguelen, Southern Indian Ocean. *Marine Ecology Progress Series*, 228, 283-299. https://doi.org/10.3354/meps228283

Cherel, Y., Connan, M., Jaeger, A., & Richard, P. (2014). Seabird year-round and historical feeding ecology: blood and feather δ¹³C and δ¹⁵N values document foraging plasticity of small sympatric petrels. *Marine Ecology Progress Series*, 505, 267-280. https://doi.org/10.3354/meps10795

Cherel, Y., Ducatez, S., Fontaine, C., Richard, P., & Guinet, C. (2008). Stable isotopes reveal the trophic position and mesopelagic fish diet of female southern elephant seals breeding on the Kerguelen Islands. *Marine Ecology Progress Series*, 370, 239-247. https://doi.org/10.3354/meps07673

Cherel, Y., Fontaine, C., Richard, P., & Labat, J. P. (2010). Isotopic niches and trophic levels of myctophid fishes and their predators in the Southern Ocean. *Limnology and Oceanography*, 55, 324-332. https://doi.org/10.4319/lo.2010.55.1.0324

Cherel, Y., & Hobson, K. A. (2007). Geographical variation in carbon stable isotope signatures of marine predators: a tool to investigate their foraging areas in the Southern Ocean. *Marine Ecology Progress Series*, 329, 281-287. https://doi.org/10.3354/meps329281

Cherel, Y., Hobson, K. A., Bailleul, F., & Groscolas, R. (2005). Nutrition, physiology, and stable isotopes: New information from fasting and molting penguins. *Ecology*, 86, 2881-2888. https://doi.org/10.1890/05-0562

Cherel, Y., Hobson, K. A., Guinet, C., & Vanpé, C. (2007). Stable isotopes document seasonal changes in trophic niches and winter foraging individual specialization in diving predators from the Southern Ocean. *Journal of Animal Ecology*, 76, 826-836. https://doi.org/10.1111/j.1365-2656.2007.01238.x

Cherel, Y., Hobson, K. A., & Hassani, S. (2005). Isotopic discrimination between food and blood and feathers of captive penguins: Implications for dietary studies in the wild. *Physiological and Biochemical Zoology*, 78, 106-115. https://doi.org/10.1086/425202

Cherel, Y., Hobson, K. A., & Weimerskirch, H. (2005). Using stable isotopes to study resource acquisition and allocation in procellariiform

seabirds. *Oecologia*, 145, 533-540. https://doi.org/10.1007/s00442-005-0156-7

Cherel, Y., Jaquemet, S., Maglio, A., & Jaeger, A. (2014). Differences in δ¹³C and δ¹⁵N values between feathers and blood of seabird chicks: Implications for non-invasive isotopic investigations. *Marine Biology*, 161, 229-237. https://doi.org/10.1007/s00227-013-2314-5

Cherel, Y., Kernaléguen, L., Richard, P., & Guinet, C. (2009). Whisker isotopic signature depicts migration patterns and multi-year intra- and inter-individual foraging strategies in fur seals. *Biology Letters*, 5, 830-832. https://doi.org/10.1098/rsbl.2009.0552

Cherel, Y., Mauget, R., Lacroix, A., & Gilles, J. (1994). Seasonal and fasting-related changes in circulating gonadal steroids and prolactin in king penguins *Aptenodytes patagonicus*. *Physiological Zoology*, 67, 1154-1173. https://doi.org/10.1086/physzool.67.5.30163887

Cherel, Y., Phillips, R. A., Hobson, K. A., & McGill, R. (2006). Stable isotope evidence of diverse species-specific and individual wintering strategies in seabirds. *Biology Letters*, 2, 301-303. https://doi.org/10.1098/rsbl.2006.0445

Cherel, Y., Pütz, K., & Hobson, K. A. (2002). Summer diet of king penguins (*Aptenodytes patagonicus*) at the Falkland Islands, southern Atlantic Ocean. *Polar Biology*, 25, 898-906.

Cherel, Y., Quillfeldt, P., Delord, K., & Weimerskirch, H. (2016). Combination of at-sea activity, geolocation and feather stable isotopes documents where and when seabirds molt. *Frontiers in Ecology and Evolution*, 4, 3.

Cherel, Y., Ridoux, V., & Rodhouse, P. G. (1996). Fish and squid in the diet of king penguin chicks, *Aptenodytes patagonicus*, during winter at sub-Antarctic Crozet Islands. *Marine Biology*, 126, 559-570. https://doi.org/10.1007/BF00351323

Cherel, Y., Stahl, J. C., & Le Maho, Y. (1987). Ecology and physiology of fasting in king penguin chicks. *The Auk*, 104, 254-262.

Cherel, Y., Verdon, C., & Ridoux, V. (1993). Seasonal importance of oceanic myctophids in king penguin diet at Crozet Islands. *Polar Biology*, 13, 355-357.

Cherel, Y., Weimerskirch, H., & Trouvé, C. (2002). Dietary evidence for spatial foraging segregation in sympatric albatrosses (*Diomedea spp.*) rearing chicks at Iles Nuageuses, Kerguelen. *Marine Biology*, 141, 1117-1129.

Connan, M., Cherel, Y., & Mayzaud, P. (2007). Lipids from stomach oil of procellariiform seabirds document the importance of myctophid fish in the Southern Ocean. *Limnology and Oceanography*, 52, 2445-2455. https://doi.org/10.4319/lo.2007.52.6.2445

Connan, M., Mayzaud, P., Trouvé, C., Barbraud, C., & Cherel, Y. (2008). Interannual dietary changes and demographic consequences in breeding blue petrels from Kerguelen Islands. *Marine Ecology Progress Series*, 373, 123-135. https://doi.org/10.3354/meps07723

Connan, M., McQuaid, C. D., Bonnevie, B. T., Smale, M. J., & Cherel, Y. (2014). Combined stomach content, lipid and stable isotope analyses reveal spatial and trophic partitioning among three sympatric albatrosses from the Southern Ocean. *Marine Ecology Progress Series*, 497, 259-272. https://doi.org/10.3354/meps10606

Cooper, J., & Klages, N. T. W. (1995). The diets and dietary segregation of sooty albatrosses (*Phoebetria spp.*) at subantarctic Marion Island. *Antarctic Science*, 7, 15-23.

Delord, K., Cotté, C., Péron, C., Marteau, C., Pruvost, P., Gasco, N., ... Weimerskirch, H. (2010). At-sea distribution and diet of an endangered top predator: Relationship between white-chinned petrels and commercial longline fisheries. *Endangered Species Research*, 13, 1-16. https://doi.org/10.3354/esr00309

Derenne, M., Jouventin, P., & Mougin, J. L. (1979). Le chant du Manchot royal (*Aptenodytes patagonica*) et sa signification évolutive. *Le Gerfaut*, 69, 211-224.

Descamps, S., Gauthier-Clerc, M., Gendner, J. P., & Le Maho, Y. (2002). The annual breeding cycle of unbanded king penguins *Aptenodytes patagonicus* on Possession Island (Crozet). *Avian Science*, 2, 87-98.
Federer, R. N., Hollmen, T. E., Esler, D., Wooller, M. J., & Wang, S. W. (2010). Stable carbon and nitrogen isotope discrimination factors from diet to blood plasma, cellular blood, feathers, and adipose tissue fatty acids in spectacled eiders (Somateria fischeri). *Canadian Journal of Zoology*, **88**, 866-874. https://doi.org/10.1139/Z10-052

van Franeker, J. A., Bathmann, U. V., & Mathot, S. (1997). Carbon fluxes to Antarctic top predators. *Deep-Sea Research II*, **44**, 435-455. https://doi.org/10.1016/S0967-0645(96)00078-1

Fridolfsson, A. K., & Ellegren, H. (1999). A simple and universal method for molecular sexing of non-ratite birds. *Journal of Avian Biology*, **30**, 116-121. https://doi.org/10.2307/3677252

Garcia, V., Jouventin, P., & Mauget, R. (1996). Parental care and the prolactin secretion pattern in the king penguin: an endogenously timed mechanism? *Hormones and Behavior*, **30**, 259-265. https://doi.org/10.1006/hbeh.1996.0031

Green, J. A., Boyd, I. L., Woakes, A. J., Warren, N. L., & Butler, P. J. (2005). Behavioural flexibility during year-round foraging in macaroni penguins. *Marine Ecology Progress Series*, **296**, 183-196. https://doi.org/10.3354/meps296183

Grosbois, V., & Thompson, P. M. (2005). North Atlantic climate variation influences survival in adult fulmars. *Oikos*, **109**, 273-290. https://doi.org/10.1111/j.0030-1299.2005.13774.x

Hahn, S., Hoye, B. J., Korthals, H., & Klaassen, M. (2012). From food to offspring down: tissue-specific discrimination and turn-over of stable isotopes in herbivorous waterbirds and other avian foraging guilds. *PLoS ONE*, **7**, e30242. https://doi.org/10.1371/journal.pone.0030242

Handrich, Y., Bevan, R. M., Charrassin, J. B., Butler, P. J., Pütz, K., Woakes, A. J., ... Le Maho, Y. (1997). Hypothermia in foraging king penguins. *Nature*, **388**, 64-67. https://doi.org/10.1038/40392

van Heezik, Y. M., Seddon, P. J., Cooper, J., & Plöss, A. L. (1994). Interrelationships between breeding frequency, timing and outcome in king penguins *Aptenodytes patagonicus*: Are king penguins biennial breeders? *Ibis*, **136**, 279-284. https://doi.org/10.1111/j.1474-919X.1994.tb01096.x

Hobson, K. A., & Bond, A. L. (2012). Extending an indicator: Year-round information on seabird trophic ecology from multiple-tissue stable-isotope analyses. *Marine Ecology Progress Series*, **461**, 233-243. https://doi.org/10.3354/meps09835

Hobson, K. A., & Clark, R. G. (1993). Turnover of ¹³C in cellular and plasma fractions of blood: Implications for non destructive sampling in avian dietary studies. *The Auk*, **110**, 638-641. https://doi.org/10.2307/4088430

Hulley, P. A. (1990). Family Myctophidae. In O. Gon, & P. C. Heemstra (Eds.), *Fishes of the Southern Ocean* (pp. 146-178). Grahamstown, South Africa: JLB Smith Institute of Ichthyology.

Jaeger, A., Connan, M., Richard, P., & Cherel, Y. (2010). Use of stable isotopes to quantify seasonal changes of trophic niche and levels of population and individual specialisation in seabirds. *Marine Ecology Progress Series*, **401**, 269-277. https://doi.org/10.3354/meps08380

Jaeger, A., Jaquemet, S., Phillips, R. A., Wanless, R. M., Richard, P., & Cherel, Y. (2013). Stable isotopes document inter- and intra-specific variation in feeding ecology of nine large southern Procellariiformes. *Marine Ecology Progress Series*, **490**, 255-266. https://doi.org/10.3354/meps10436

Jaeger, A., Lecomte, V. J., Weimerskirch, H., Richard, P., & Cherel, Y. (2010). Seabird satellite tracking validates the use of latitudinal isoscapes to depict predators' foraging areas in the Southern Ocean. *Rapid Communications in Mass Spectrometry*, **24**, 3456-3460. https://doi.org/10.1002/rcm.4792

Jouventin, P., Capdeville, D., Cuénot-Chaillet, F., & Boiteau, C. (1994). Exploitation of pelagic resources by a non-flying seabird: Satellite tracking of the king penguin throughout the breeding cycle. *Marine Ecology Progress Series*, **106**, 11-19. https://doi.org/10.3354/meps106011

Jouventin, P., & Mauget, R. (1996). The endocrine basis of the reproductive cycle in the king penguin (*Aptenodytes patagonicus*). *Journal of Zoology*, **238**, 665-678. https://doi.org/10.1111/j.1469-7998.1996.tb05421.x

Juares, M. A., Santos, M., Mennucci, J. A., Coria, N. R., & Mariano-Jelichic, R. (2016). Diet composition and foraging habitats of Adélie and gentoo penguins in three different stages of their annual cycle. *Marine Biology*, **163**, 105. https://doi.org/10.1007/s00227-016-2886-y

Kernaléguen, L., Arnould, J. P. Y., Guinet, C., Cazelles, B., Richard, P., & Cherel, Y. (2016). Early-life sexual segregation: Ontogeny of isotopic niche differentiation in the Antarctic fur seal. *Scientific Reports*, **6**, 33211. https://doi.org/10.1038/srep33211

Kernaléguen, L., Cazelles, B., Arnould, J. P. Y., Richard, P., Guinet, C., & Cherel, Y. (2012). Long-term species, sexual and individual variations in foraging strategies of fur seals revealed by stable isotopes in whiskers. *PLoS ONE*, **7**, e32916. https://doi.org/10.1371/journal.pone.0032916

Kooyman, G. L., Cherel, Y., Le Maho, Y., Croxall, J. P., Thorson, P. H., Ridoux, V., & Kooyman, C. A. (1992). Diving behavior and energetics during foraging cycles in king penguins. *Ecological Monographs*, **62**, 143-163. https://doi.org/10.2307/2937173

Kurle, C. M., Finkelstein, M. E., Smith, K. R., George, D., Ciani, D., Koch, P. L., & Smith, D. R. (2013). Discrimination factors for stable isotopes of carbon and nitrogen in blood and feathers from chicks and juveniles of the California Condor. *The Condor*, **115**, 492-500. https://doi.org/10.1525/cond.2013.120107

Labrousse, S., Vacqué-Garcia, J., Heerah, K., Guinet, C., Sallée, J. B., Authier, M., ... Charrassin, J. B. (2015). Winter use of sea ice and ocean water mass habitat by southern elephant seals: The length and breadth of the mystery. *Progress in Oceanography*, **137**, 52-68. https://doi.org/10.1016/j.pocean.2015.05.023

Le Bohec, C., Durant, J. M., Gauthier-Clerc, M., Stenseth, N. C., Park, Y. H., Pradel, R., ... Le Maho, Y. (2008). King penguin population threatened by Southern Ocean warming. *Proceedings of the National Academy of Sciences of the United States of America*, **105**, 2493-2497. https://doi.org/10.1073/pnas.0712031105

Le Bohec, C., Gauthier-Clerc, M., Grémillet, D., Pradel, R., Béchet, A., Gendner, J. P., & Le Maho, Y. (2007). Population dynamics in a long-lived seabird: I. Impact of breeding activity on survival and breeding probability in unbanded king penguins. *Journal of Animal Ecology*, **76**, 1149-1160. https://doi.org/10.1111/j.1365-2656.2007.01268.x

Le Vaillant, M., Ropert-Coudert, Y., Le Maho, Y., & Le Bohec, C. (2016). Individual parameters shape foraging activity in breeding king penguins. *Behavioral Ecology*, **27**, 352-362. https://doi.org/10.1093/beheco/arv146

Lecomte, V. J., Sorci, G., Cornet, S., Jaeger, A., Faivre, B., Arnoux, E., ... Weimerskirch, H. (2010). Patterns of aging in the long-lived wandering albatross. *Proceedings of the National Academy of Sciences of the United States of America*, **107**, 6370-6375. https://doi.org/10.1073/pnas.0911181107

Martinez del Rio, C., Sabat, P., Anderson-Sprecher, R., & Gonzalez, S. P. (2009). Dietary and isotopic specialization: The isotopic niche of three *Cinclodes* ovenbirds. *Oecologia*, **161**, 149-159. https://doi.org/10.1007/s00442-009-1357-2

McIntyre, T., Ansorge, I. J., Bornemann, H., Plötz, J., Tosh, C. A., & Bester, M. N. (2011). Elephant seal dive behaviour is influenced by ocean temperature: Implications for climate change impacts on an ocean predator. *Marine Ecology Progress Series*, **441**, 257-272. https://doi.org/10.3354/meps09383

McIntyre, T., Bornemann, H., Plötz, J., Tosh, C. A., & Bester, M. N. (2011). Water column use and forage strategies of female southern elephant seals from Marion Island. *Marine Biology*, **158**, 2125-2139. https://doi.org/10.1007/s00227-011-1719-2

McIntyre, T., Tosh, C. A., Plötz, J., Bornemann, H., & Bester, M. N. (2010). Segregation in a sexually dimorphic mammal: A mixed-effects modelling analysis of diving behaviour in southern elephant seals. *Marine Ecology Progress Series*, **412**, 293-304. https://doi.org/10.3354/meps08680

Moore, G. J., Robertson, G., & Wienecke, B. (1998). Food requirements of breeding king penguins at Heard Island and potential
overlap with commercial fisheries. *Polar Biology*, 20, 293-302. https://doi.org/10.1007/s00300050306

Moore, G. J., Wienecke, B., & Robertson, G. (1999). Seasonal change in foraging areas and dive depths of breeding king penguins at Heard Island. *Polar Biology*, 21, 376-384. https://doi.org/10.1007/s00300050376

Nel, D. C., Lutjeharms, J. R. E., Pakhomov, E. A., Ansorge, I. J., Ryan, P. G., & Klages, N. T. W. (2001). Exploitation of mesoscale oceanographic features by grey-headed albatross *Thalassarche chrysostoma* in the southern Indian Ocean. *Marine Ecology Progress Series*, 217, 15-26. https://doi.org/10.3354/meps217015

Nevoux, M., & Barbraud, C. (2006). Relationships between sea ice concentration, sea surface temperature and demographic traits of thin-billed prions. *Polar Biology*, 29, 445-453. https://doi.org/10.1007/s00300-005-0075-4

Olsson, O. (1996). Seasonal effects of timing and reproduction in the king penguin: A unique breeding cycle. *Journal of Avian Biology*, 27, 7-14. https://doi.org/10.2307/3676955

Olsson, O., & van der Jeugd, H. P. (2002). Survival in king penguins *Aptenodytes patagonicus*: Temporal and sex-specific effects of environmental variability. *Oecologia*, 132, 509-516. https://doi.org/10.1007/s00442-002-0985-6

Péron, C., Delord, K., Phillips, R. A., Charbonnier, Y., Marteau, C., Louzao, M., & Weimerskirch, H. (2010). Seasonal variation in oceanographic habitat and behaviour of white-chinned petrels *Procellaria aequinoctialis* from Kerguelen Island. *Marine Ecology Progress Series*, 416, 267-284. https://doi.org/10.3354/meps08785

Phillips, R. A., Lewis, S., Gonzalez-Solis, J., & Daunt, F. (2017). Causes and consequences of individual variability and specialization in foraging and migration strategies of seabirds. *Marine Ecology Progress Series*, 578, 117-150. https://doi.org/10.3354/meps12217

Phillips, R. A., McGill, R. A. R., Dawson, D. A., & Bearhop, S. (2011). Sexual segregation in distribution, diet and trophic level of seabirds: Insights from stable isotope analysis. *Marine Biology*, 158, 2199-2208. https://doi.org/10.1007/s00227-011-1725-4

Pinaud, D., & Weimerskirch, H. (2007). At-sea distribution and scale-dependent foraging behaviour of petrels and albatrosses: A comparative study. *Journal of Animal Ecology*, 76, 9-19. https://doi.org/10.1111/j.1365-2656.2006.01186.x

Pistorius, P., Hindell, M., Crawford, R., Makhado, A., Dyer, B., & Reisinger, R. (2017). At-sea distribution and habitat use in king penguins at sub-Antarctic Marion Island. *Ecology and Evolution*, 7, 3894-3903. https://doi.org/10.1002/ece3.2833

Polito, M. J., Brasso, R. L., Trivelpiece, W. Z., Karnovsky, N., Patterson, W. P., & Emslie, S. D. (2016). Differing foraging strategies influence mercury (Hg) exposure in an Antarctic penguin community. *Environmental Pollution*, 218, 196-206. https://doi.org/10.1016/j.envpol.2016.04.097

Pütz, K. (2002). Spatial and temporal variability in the foraging areas of breeding king penguins. *The Condor*, 104, 528-538. https://doi.org/10.1650/0010-5422(2002)104[0528:SATVIT]2.0.CO;2

Quillfeldt, P., Cherel, Y., Delord, K., & Weimerskirch, H. (2015). Cool, cold, or colder? Spatial segregation of prions and blue petrels is explained by differences in preferred sea surface temperatures. *Biology Letters*, 11, 20141090. https://doi.org/10.1098/rsbl.2014.1090

Raclot, T., Groscolas, R., & Cherel, Y. (1998). Fatty acid evidence for the importance of myctophid fishes in the diet of king penguins, *Aptenodytes patagonicus*. *Marine Biology*, 132, 523-533. https://doi.org/10.1007/s002270050418

Richoux, N. B., Jaquemet, S., Bonnevie, B. T., Cherel, Y., & McQuaid, C. D. (2010). Trophic ecology of grey-headed albatrosses from Marion Island, Southern Ocean: Insights from stomach contents and diet tracers. *Marine Biology*, 157, 1755-1766. https://doi.org/10.1007/s00227-010-1448-y

Ridoux, V. (1994). The diets and dietary segregation of seabirds at the sub-antarctic Crozet Islands. *Marine Ornithology*, 22, 1-192.

Sandvik, H., Erikstad, K. E., Barrett, R., & Yoccoz, N. G. (2005). The effect of climate on adult survival in five species of North Atlantic seabirds. *Journal of Animal Ecology*, 74, 817-831. https://doi.org/10.1111/j.1365-2656.2005.00981.x

Saraux, C., Friess, B., Le Maho, Y., & Le Bohec, C. (2012). Chick-provisioning strategies used by king penguins to adapt to a multiseasonal breeding cycle. *Animal Behavior*, 84, 675-683. https://doi.org/10.1016/j.anbehav.2012.06.024

Thiébot, J. B., Cherel, Y., Acqueberge, M., Prudor, A., Trathan, P. N., & Bost, C. A. (2014). Adjustment of pre-moult foraging strategies in macaroni penguins *Eudyptes chrysolophus* according to locality, sex and breeding status. *Ibis*, 156, 511-522. https://doi.org/10.1111/ibi.12151

Thiébot, J. B., Cherel, Y., Trathan, P. N., & Bost, C. A. (2011). Inter-population segregation in the wintering areas of macaroni penguins. *Marine Ecology Progress Series*, 421, 279-290. https://doi.org/10.3354/meps08907

Thiébot, J. B., Cherel, Y., Trathan, P. N., & Bost, C. A. (2012). Coexistence of oceanic predators on wintering areas explained by population-scale foraging segregation in space or time. *Ecology*, 93, 122-130. https://doi.org/10.1890/11-0385.1

Thiers, L., Delord, K., Barbraud, C., Phillips, R. A., Pinaud, D., & Weimerskirch, H. (2014). Foraging zones of the two sibling species of giant petrels in the Indian Ocean throughout the annual cycle: Implication for their conservation. *Marine Ecology Progress Series*, 499, 233-248. https://doi.org/10.3354/meps10620

Trathan, P. N., Garcia-Borboroglu, P., Boersma, D., Bost, C. A., Crawford, R. J. M., Crossin, G. T., ... Wienecke, B. (2015). Pollution, habitat loss, fishing, and climate change as critical threats to penguins. *Conservation Biology*, 29, 31-41. https://doi.org/10.1111/cobi.12349

Tréguer, P., & Jacques, G. (1992). Dynamics of nutrients and phytoplankton, and fluxes of carbon, nitrogen and silicon in the Antarctic Ocean. *Polar Biology*, 12, 149-162.

Tremblay, Y., & Cherel, Y. (2003). Geographic variation in the foraging behaviour, diet and chick growth of rockhopper penguins. *Marine Ecology Progress Series*, 251, 279-297. https://doi.org/10.3354/meps251279

Weimerskirch, H., Cherel, Y., Delord, K., Jaeger, A., Patrick, S. C., & Riottelambert, L. (2014). Lifetime foraging patterns of the wandering albatross: Life on the move! *Journal of Experimental Marine Biology and Ecology*, 450, 68-78. https://doi.org/10.1016/j.jembe.2013.10.021

Weimerskirch, H., Fradet, G., & Cherel, Y. (1999). Natural and experimental changes in chick provisioning in a long-lived seabird, the Antarctic prion. *Journal of Avian Biology*, 30, 165-174. https://doi.org/10.2307/3677126

Weimerskirch, H., Salamolard, M., Sarrazin, F., & Jouventin, P. (1993). Foraging strategy of wandering albatrosses through the breeding season: A study using satellite telemetry. *The Auk*, 110, 325-342.

Whitehead, T. O., Connan, M., Ropert-Coudert, Y., & Ryan, P. G. (2017). Subtle but significant segregation in the feeding ecology of sympatric penguins during the critical pre-moult period. *Marine Ecology Progress Series*, 565, 227-236. https://doi.org/10.3354/meps12017

Whitehead, T. O., Kato, A., Ropert-Coudert, Y., & Ryan, P. G. (2016). Habitat use and diving behaviour of macaroni *Eudyptes chrysolophus* and eastern rockhopper *E. chrysocome filholi* penguins during the critical pre-moult period. *Marine Biology*, 163, 19-37. https://doi.org/10.1007/s00227-015-2794-6

Woehler, E. J. (1993). The distribution and abundance of Antarctic and sub-antarctic penguins (p. 76). Cambridge, UK: Scientific Committee on Antarctic Research, Scott Polar Research Institute.

Woehler, E. J. (1995). Consumption of Southern Ocean marine resources by penguins. In P. Dann, I. Norman, & P. Reilly (Eds.), *The penguins* (pp. 266-294). Chipping Norton, UK: Surrey Beatty & Sons.
Wolf, N., Carleton, S. A., & Martinez del Rio, C. (2009). Ten years of experimental animal isotopic ecology. *Functional Ecology*, 23, 17–26. https://doi.org/10.1111/j.1365-2435.2009.01529.x

Xavier, J. C., & Cherel, Y. (2009). *Cephalopod beak guide for the Southern Ocean* (p. 129). Cambridge, UK: British Antarctic Survey.

Xavier, J. C., Trathan, P. N., Ceia, F. R., Tarling, G. A., Adlard, S., Fox, D., ... Cherel, Y. (2017). Sexual and individual foraging segregation in gentoo penguins from the Southern Ocean during an abnormal winter. *PLoS ONE*, 12, e0174850. https://doi.org/10.1371/journal.pone.0174850

**How to cite this article:** Cherel Y, Parenteau C, Bustamante P, Bost C-A. Stable isotopes document the winter foraging ecology of king penguins and highlight connectivity between subantarctic and Antarctic ecosystems. *Ecol Evol*. 2018;8:2752–2765. https://doi.org/10.1002/ece3.3883