Juvenile dispersal to Antarctic habitats: the “ice age”
In this study we present the first tracking data on the post-fledging dispersal of Eudyptes penguins. Our results on the macaroni penguin, the largest consumer of global marine resources, show that fledglings from Kerguelen Is. target the Antarctic MIZ across an extended southeastward journey. Although it remains unknown for how long exactly juvenile macaroni penguins use the MIZ, this result is fundamentally new for the species, compared to the habitats previously known to be used by the adults, year-round (e.g., Trathan et al. 2006; Bost et al. 2009a; Bon et al. 2015). Even at South Georgia and Bouvetøya islands, located south of the APF in the Atlantic sector, the adults move north after breeding, away from sea ice (Waluda et al. 2010; Lowther et al. 2014). This exclusive and coherent use of Antarctic ice habitats by the juveniles is thus an important element that not only improves knowledge about the species’ at-sea ecology, but also challenges our perception of the Southern Ocean food webs. Indeed, this annual migration of an extremely abundant consumer such as the macaroni penguin, implies that considerable levels of predation on Antarctic prey biomass have remained unaccounted for.
Our results are nevertheless coherent with direct observations of juvenile macaroni penguins in the Southern Ocean. From ship-based surveys, Reid et al. (1999) observed juveniles > 1,300 km to the southeast of the Kerguelen Plateau during March, significantly farther south than the adults, a result that is remarkably consistent with our tracking study. Further, young macaroni penguins (“likely 1 year old”; Golubev 2016) have been repeatedly observed ashore in the Drygalski region near Mirny research station (66°33’S, 93°01’E), whereas none was recorded at any other East Antarctic site over 1954–1988 (Woehler 1992). Thus, despite relatively low sample sizes in our study, fledglings’ tracks from Kerguelen Is. are coherent with independent observations of macaroni penguins in the region, and may provide an adequate basis for population-wide inference.
At Kerguelen Is., the successful breeders leave the colony for their pre-molt trip just after the chicks fledge and move in the same direction as the latter (Thiebot et al. 2014a; this study), initially following the local oceanographic circulation (Park et al. 2009). It is thus plausible that juveniles and pre-molting adults move jointly at sea, but only until mid-March when adults move back for molt. Our study shows it is not possible however that the juveniles may be guided by the pre-molting adults to the ice edge, as they move 2.7 times farther than the adults to reach the ice. This element strongly suggests the innate nature of this ice-bound dispersal in juvenile macaroni penguins.
Expected costs and benefits to exploiting the MIZ
Macaroni penguins’ post-fledging dispersal to the MIZ is intriguing for two reasons. First, living in extremely cold Antarctic waters bears significant thermoregulation costs for endotherms, and especially for juvenile penguins which are smaller than adults and likely less efficient at foraging (Williams and Croxall 1991; Orgeret et al. 2019). In contrast, in the emperor penguin the juveniles move from Antarctic to more temperate waters during their initial dispersal, which may help minimizing such age-specific thermoregulation costs (Kooyman et al. 1996). Second, macaroni penguins are visual pursuit-feeders, and as such are constrained by daylight hours to feed: they dive from sunrise to sunset (Green et al. 2005). Yet the further south the juveniles go, the shorter the daylength in April–September. These elements (cold, darkness), and the cost of moving over great distances, suggest that the fledglings may in fact find highly favorable conditions in the MIZ, allowing to offset these adverse effects. Several hypotheses may be proposed in this context:
(1) High prey biomass. The MIZ is a target feeding habitat for an abundant community of predators moving from afar (e.g., Péron et al. 2010), with high local biomass of prey including Antarctic krill Euphausia superba. In April-May, the tracked juveniles were in contact with the distribution of krill-specialist Adélie penguins Pygoscelis adeliae migrating from East Antarctica (Thiebot et al. 2019). Juvenile macaroni penguins may thus target this habitat to feed intensively on Antarctic prey including E. superba as well. This hypothesis is coherent with the facts that their distance from the colony stabilized when approaching this habitat, and that Antarctic krill is found only from 59°S in our study area (Miquel 1991; Nicol et al. 2000; Jarvis et al. 2010).
(2) Increased prey accessibility. At Kerguelen Is., the breeding adults dive to > 40 m deep to feed on subantarctic krill E. vallentini (Sutton et al. 2021), and dive depth increased after summer at South Georgia (Green et al. 2005). Subantarctic prey may thus become inaccessible to fledglings with lesser diving skills after summer, whereas crustaceans remain available at shallower depths in Antarctic habitats, particularly during winter (Ainley et al. 1991).
(3) Facilitated prey capture. Endothermic predators typically hold a larger advantage in movement ability over ectothermic prey in colder versus warmer waters (Grady et al. 2019). Thus, juvenile macaroni penguins may take advantage of the prey’s lower maneuverability in colder waters, to forage adequately despite their developing skills.
(4) Using ice floes. Drifting ice provides the penguins with opportunities to haul out and rest, socialize and preen (Wienecke et al. 2010), all under decreased thermoregulation costs and increased protection from subsurface predators. As naïve juveniles may be particularly prone to predation by lack of behavioral compensation compared to adults (Newton 1998), the ability of fledgling penguins to rest and haul out when reaching the MIZ may indeed represent an adaptive strategy improving survival rates.
(5) Minimized intraspecific competition. Southern Ocean predators often exhibit latitudinally-structured habitat use between juveniles and adults (e.g., seals: Field et al. 2005; albatrosses: Weimerskirch et al. 2006; petrels: Trebilco et al. 2008; penguins: Houstin et al. 2022; although not always, e.g. Clarke et al. 2003; Hinke et al. 2019; Orgeret et al. 2019). This mechanism presumably allows reducing risks of intraspecific competition for food. In macaroni penguins, only the juveniles might be able to reach the MIZ, owing to their longer time at sea compared to adults (Reid et al. 1999; Bost et al. 2009a; Thiebot et al. 2013). To clarify this point, it would be desirable to track adults that skipped breeding or failed early in the season, and examine whether these birds may also take advantage of distant Antarctic habitats in this situation of extended time available at sea.
Implications for management and conservation
We have now established that outside the breeding season, macaroni penguins from Kerguelen archipelago disperse into different habitats according to age class (Bost et al. 2009a; Thiebot et al. 2014a; this study), as observed in other penguin species (Sherley et al. 2017; Houstin et al. 2022; Young et al. 2022). This heterogeneity in the species’ distribution implies, first, a spatial decoupling in the environmental conditions that may affect the population’s dynamics. For example, juvenile African penguins Spheniscus demersus were found to migrate to exclusive areas characterized by poor feeding conditions, which clarified a population sink (Sherley et al. 2017). In macaroni penguins, our study newly provides the basis to (1) examine links between climate-induced changes in the Antarctic ice environment and population trajectories measured at the colony (as observed in other marine predators that also rely seasonally on Antarctic sea ice, e.g. Hindell et al. 2016), and (2) anticipate the effects predicted from such environmental driver on future population trends. Also, wide and stage-dependent patterns in the at-sea distribution of marine predators may reveal spatial gaps in protection policies across jurisdictions, and this discontinuity may hence reduce the efficacy of local conservation efforts dedicated to these populations (Harrison et al. 2018; Young et al. 2022). In this context, the post-fledging dispersal of macaroni penguins is particularly informative. After the breeding season at Kerguelen, macaroni penguins from all life-cycle stages quickly move out of the French and Australian EEZs (Bost et al. 2009a; Thiebot et al. 2014a; this study): thus, it is clear that national jurisdictions alone offer limited leverage to protect this population. In contrast, the tracked penguins spent a considerable time in ABNJ, where it has been legally more challenging to implement areas protecting marine biodiversity from harmful human activities (De Santo 2018). This result further highlights that across their life-cycle stages, penguins in general and macaroni penguins especially are oceanic migrants, and as such the current progress towards an international treaty for the high seas may be highly significant for their conservation at the global scale (Thiebot and Dreyfus 2021). The time spent in ABNJ comprised periods predominantly inside the CCAMLR-application area. This emphasizes the key role of this organization in the preservation of marine predators such as macaroni penguins (Agnew 1997; Reid et al. 2005); however, the short periods outside this area by all tracked individuals revealed that fledglings are also at risk from unregulated human activities that may operate or develop in ABNJ outside the CCAMLR-application area (Dunn et al. 2018).
Importantly, one-third of the juvenile macaroni penguins that were tracked over more than two months temporarily distributed in the Drygalski area, one of the three regions selected in the East Antarctic Representative System of MPAs (CCAMLR 2011). The Drygalski area was previously known to include significant foraging areas of both Antarctic and allochthonous marine predators (Péron et al. 2010; Raymond et al. 2015; Bestley et al. 2019). Given the estimated population size of 1.8 million pairs of macaroni penguins at Kerguelen Is., hundreds of thousands of fledglings may distribute in this area seasonally. Our tracking study thus adds further support to the ecological importance of the Drygalski area, which connects several distant food webs through an array of long-ranged migratory predators. In turn, our study emphasizes that the conservation of Antarctic habitats, such as the Drygalski area, may benefit to vulnerable key consumers even from beyond Antarctic ecosystems.
In conclusion, our results reveal that during their post-fledging dispersal, juvenile macaroni penguins use a distinct habitat from that of breeding or nonbreeding adults. Ecological drivers behind this age class-specific pattern are not clear, and it remains unknown for how long the juveniles exploit these Antarctic habitats. Nevertheless, this result provides new and important information for population management and the conservation of marine biodiversity. Adequate marine spatial planning requires comprehensive scientific knowledge on the species’ distribution (Barton et al. 2015; Hays et al. 2019; Houstin et al. 2022). We thus emphasize the need to continue collecting data on individuals across life-cycle stages, especially in the case of mobile biodiversity components with global conservation concern.