© 2001 by The Society for Integrative and Comparative Biology
Habitat Use by Weddell Seals and Emperor Penguins Foraging in the Ross Sea, Antarctica1
1 Department of Biological Sciences, University of Alaska Anchorage, Anchorage, Alaska 99508
2 Scholander Hall, 0204, University of California San Diego, La Jolla, California 92093
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SYNOPSIS |
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The only apex predators that live year-round at high latitudes of the Ross Sea are the Weddell seal and emperor penguin. The seasonal distribution, foraging depths, and diet of these two species appear to overlap. What makes it possible for emperor penguins and Weddell seals to co-exist at high latitude throughout the winter when other marine tetrapods apparently cannot? Both species have similar adaptations for exploitation of the deep-water habitat, forage on the same species, and routinely make long and deep dives. Yet, despite these similarities, there is probably little trophic overlap between the adults of both species due to geographical and seasonal differences in habitat use. For example, during the winter months while female emperor penguins are ranging widely in the pack ice, adult seals are foraging and fattening for the upcoming summer fast, literally beneath the feet of the male penguins. However, there is more extensive overlap between juvenile seals and adult penguins, and shifts in prey abundance and/or distribution would likely affect these two groups similarly. In contrast, juvenile penguins appear to avoid inter- and intra- specific competition by leaving the Ross Sea once they molt.
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INTRODUCTION |
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The emperor penguin (Aptenodytes forsteri) and the Weddell seal (Leptonychotes weddellii) are the only two warm-blooded, air-breathing marine predators that live year-round at the high latitudes of the Ross Sea (
77°S). These are arguably the two such species best adapted to polar existence, and are the only pair of diving predators that co-exist at high polar latitudes throughout the year. That emperor penguins are exquisitely adapted to their high polar existence can be best illustrated by the fact that they brood their egg and chick throughout the Antarctic winter, when most other vertebrates have left the region (Stonehouse, 1964
; Croxall, 1997). Similarly, Weddell seals are able to hunt underneath the thick ice of the Ross Sea throughout the dark and cold winter months, when all other marine mammals have left the region (Castellini et al., 1992). Yet, despite a long history of study, we know surprisingly little about the foraging ecology or habitat use of these two species outside of the summer research season.
The question of how these two species co-exist within the Ross Sea is of particular interest because the Ross Sea is home to large and overlapping distributions of both predators (Fig. 1). The Ross Sea is home to six emperor penguin colonies (at Cape Crozier, Beaufort Island, Franklin Island, Cape Washington, Coulman Island, and Cape Roget) that house approximately 180,000 birds (Ainley et al., 1984; Kooyman and Mullins, 1990; Kooyman, 1993), and to an estimated 50,000 adult Weddell seals, which are dispersed across a wide area (Stirling, 1969). While the seal colonies within McMurdo Sound are well studied (Testa and Siniff, 1987; Castellini et al., 1992; Burns et al., 1999), little is known about the smaller colonies that exist along the western Victoria Land coast. The overlapping distribution of these two apex predators, and their year-round presence within the Ross Sea creates the potential for competition.
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When considering potential competition between Weddell seals and emperor penguins, it is crucial to remember that these are both air-breathing, warm-blooded species that overlap not in their utilization of above-ice resources, but rather underwater prey. Thus, it is first essential to examine whether the two species have similar diving abilities. Over the past 30 years, many studies have shown that the ability of diving vertebrates to remain submerged depends upon the amount of oxygen that can be stored within tissues and made available during dives, and the rate at which that stored oxygen is used (Butler and Jones, 1997
; Kooyman and Ponganis, 1998). The ratio of these two parameters is the aerobic dive limit (ADL), or the maximum dive duration that can be achieved by relying primarily on oxidative metabolism. Because most free-ranging divers do not rely extensively on anaerobic processes to extend dive time, the ADL can be used as an index of an animal's dive capacity (Kooyman et al., 1980; Butler and Jones, 1997).
Weddell seals and emperor penguins are both champion divers, with similar modifications to both oxygen stores and use rates. Both species have increased oxygen stores 3–4 fold over typical terrestrial values (87 ml O2·kg–1 in Weddell seals, 62 ml O2·kg–1 in penguins, vs. 20 ml O2·kg–1 in humans; Ponganis et al., 1993; Kooyman et al., 1999). In addition, both species have shifted the bulk of their oxygen stores from the lungs to their muscle and blood. This reduces problems inherent in breath-hold diving with large, freely exchanging gas volumes (Kooyman and Ponganis, 1998). Penguins and seals also use several techniques to reduce oxygen consumption. For example, both significantly reduce heart rates during long dives, and reduce transport costs by utilizing burst-and-glide swimming patterns (Hill et al., 1987; Kooyman et al., 1992; Davis et al., 1999; T. Williams, personal communication). Therefore, it appears that emperor penguins and Weddell seals possess similar adaptations to exploit the underwater prey resources available to them within the Ross Sea. In addition, studies of diving behavior have indicated that both use their underwater habitat in similar ways (Fig. 2; Testa, 1994; Kooyman and Kooyman, 1995).
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Thus, prior studies have demonstrated that Weddell seals and emperor penguins live in the same area, have similar diving capacities and can use the same foraging depths. These results suggest that the potential for foraging competition between these two species is high, should resources become limiting. Furthermore, studies linking Weddell seal foraging patterns and reproductive success to environmental conditions suggest that food availability does have the potential to impact predator population dynamics (Burns and Schreer, 2000
; Testa et al., 1991). Unfortunately, there is no equivalent data available for emperor penguins. Even if resources are not always limiting, when top predators co-exist within a stable environment, they generally do so through niche separation. Since the Antarctic environment is relatively stable within the long lifetimes of seals and penguins, these results demonstrate the potential for foraging competition between the two species. It is this potential for trophic overlap between emperor penguins and Weddell seals within the Ross Sea that we propose to address here. |
DISCUSSION |
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Given the extant data, it is possible to evaluate three main strategies that emperor penguins and Weddell seals might use to maximize niche separation and so reduce the potential for interspecific competition. These strategies include differential prey selection, utilization of different foraging depths, and seasonal or geographic separation of habitat use. Each of these hypotheses will be considered in turn.
Prey selection
There have been numerous studies of Weddell seal and emperor penguin diets, both within and outside of the Ross Sea (Plötz, 1986; Green and Burton, 1987; Castellini et al., 1992; Putz, 1995; Kirkwood and Robertson, 1997a; Burns et al., 1998; Cherel and Kooyman, 1998). In almost all studies, fish make up the bulk of the diet, with cephalopods and crustaceans generally accounting for less than one-third of the prey consumed. Within the Ross Sea, Cherel and Kooyman (1998) showed that fish made up 89–95% (by mass) of the emperor penguin's diet, and Green and Burton (1987) demonstrated that Weddell seals were even more specialized fish consumers (99.3% diet by mass). In addition Pleuragramma antarcticum, the Antarctic silverfish, is the primary fish prey for both predators (88.6% by numbers for penguins, 95.3% for seals; Castellini et al., 1992; Cherel and Kooyman, 1998). This is not surprising, given that P. antarcticum is probably a large component of the fish biomass throughout the Ross Sea (>90% of the biomass in McMurdo Sound; Eastman, 1985; Everson, 1985).
More remarkably, despite differences in body and mouth size, seals and penguins do not appear to be selecting different size classes of P. antarcticum (Fig. 3). Although seals eat slightly larger fish than do penguins, individuals of each species consume fish of many size classes. Juvenile fish (those <100 mm SL) dominate the diet of both predators (Castellini et al., 1992; Burns et al., 1998; Cherel and Kooyman, 1998). Differences in the mean size of ingested prey are likely due to the different locations, seasons, and years at which prey samples were collected (seal scats were collected throughout 1981 from Erebus Bay and White Island; penguin stomach samples were collected periodically at Coulman Island, Cape Washington, and Cape Roget between October and December, 1986–1993). While approximately 10% of the fish selected by seals were larger than any selected by penguins in the Ross Sea, the absence of large (>140 mm) P. antarcticum in the penguin diet may be due to the fact that diet samples were collected only during the chick rearing period. If adult birds return to the colonies with smaller fish for their chicks, but take larger fishes for themselves early in each foraging trip, then diet samples would be biased toward smaller fish sizes. Observed changes in penguin dive patterns towards the end of foraging trips support this hypothesis (Kooyman and Kooyman, 1995; Cherel and Kooyman, 1998). In addition, emperor penguins foraging along the Mawson coast eat slightly larger fish during the winter than when rearing chicks in the summer (Kirkwood and Robertson, 1997a, b). This pattern may hold in the Ross Sea as well. Thus, it seems that seals and penguins forage predominantly on the same species and take fish of similar sizes. Trophic separation, if present, must arise through other mechanisms.
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Diving behavior
While early studies of diving behavior demonstrated that both penguins and seals were capable of reaching similar depths (Kooyman et al., 1971
; Castellini et al., 1992; Ancel et al., 1992; Testa, 1994; Kooyman and Kooyman, 1995; Fig. 2), they did not reveal whether seals and penguins preferentially foraged at similar depths. While predators must forage at depths where prey are abundant, to maximize prey intake, they must also increase the proportion of the dive cycle (dive time + surface recovery time) spent at depth. Therefore, preferred foraging depths (i.e., the maximum depth of foraging dives) generally integrate both predator diving capacity and prey behavior (Kooyman et al., 1980; Butler and Jones, 1997; Kooyman and Ponganis, 1998).
While we know little about the under-ice movements of P. antarcticum, the diving capacity of both Weddell seals and emperor penguins has been well characterized, and can be compared to foraging behavior as measured by dive recorders (Fig. 4). The ADL of adult emperor penguins (<5 min, determined from post-dive lactate levels) is similar to that of Weddell seal juveniles, as is their modal dive duration (juvenile seals 5.8 min, adult penguins 5 min; Kooyman and Kooyman, 1995; Burns and Castellini, 1996; Burns, 1999). These values are considerably shorter than those of the larger adult seals (ADL <20 min, modal dive duration <13 min; Kooyman et al., 1980; Burns, 1999). That a much greater proportion of free ranging dives made by juvenile seals and emperor penguins exceed their ADL suggests that both are commonly operating at close to a physiological limit (Burns, 1999).
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Since greater breath-hold ability provides access to deeper depths, the fact that Weddell seal adults make a larger proportion of deep dives than either juvenile seals or adult penguins is expected (Fig. 4B; dives limited to those >50 m). However, there is considerable overlap between the foraging dive depths of all three age classes (Kooyman and Kooyman, 1995
; Schreer and Testa, 1996; Burns and Schreer, 2000). This is particularly true at the shallow depths for adult penguins and juvenile seals, which suggests that the largest potential for trophic overlap exists between these two groups. Remarkably, very deep dives (>350 m) make up a similar proportion (<10%) of the behavioral repertoire of both adult seals and penguins. Thus, despite differences in physiological capacity, there is little indication that depth separation is the main mechanism by which the potential for foraging competition is reduced.
Seasonal patterns of habitat use
Direct competition for the same prey resources can be minimized if predators forage in different geographic areas or at different times of the year. For emperor penguins and Weddell seals, satellite telemetry and population studies have provided data on seasonal patterns of habitat use that can be used to address this hypothesis (Ancel et al., 1992; Testa, 1994; Kooyman et al., 1996; Burns et al., 1999). However, differences in the basic life history of these species must be considered first.
The life history pattern of emperor penguins closely ties them to their colonies. In the fall, adults gather in the colonies to form pair bonds and mate. After egg laying, females forage to replenish energy reserves, but males remain in the colonies and incubate the egg until it hatches in mid-winter. Then, both sexes provision the chick throughout the spring by undertaking 1–3 wk long foraging trips. In late December, chicks fledge, and adults leave the colonies to feed and molt. Three to four months later, adults return to the breeding colonies and the cycle starts again (Stonehouse, 1964; Kooyman and Kooyman, 1995; Croxall, 1997; Kirkwood and Robertson, 1997a).
Within the constraints of this life history pattern, there are three periods when adult foraging effort is likely elevated: after egg-laying when females are replenishing energy stores, during the chick provisioning period, and after the chicks are fledged, when adults are building energy reserves for the coming molt. Satellite telemetry has revealed that habitat use patterns differ between these periods. During the chick provisioning trips adults generally remain within 200 km of the colonies and concentrate their foraging activities in the daylight hours (Ancel et al., 1992; Kooyman and Kooyman, 1995; Croxall, 1997; Cherel and Kooyman, 1998; Kooyman et al., 1998). Once chicks are fledged, adults travel west for approximately one month until they reach the eastern Ross Sea. There, in the drifting pack ice more than 1,200 km from their colonies, they undergo the annual molt, and shed their tracking devices (Kooyman et al., 1998). In contrast, newly fledged juvenile penguins leave the colonies in January and travel north of the pack ice zone, beyond 60°S, and out of the Antarctic treaty area (Kooyman et al., 1996). While adults return to their colonies roughly three months later, we do not know when juveniles return.
In contrast, Weddell seals have greater freedom of movement throughout the year. During the Antarctic spring (October–December) adults congregate in nearshore colonies, where females give birth to a single pup and males vie for underwater mating territories (Hill, 1987; Castellini et al., 1992). During this period, breeding adults and nursing pups remain inshore, and foraging effort is low (Testa et al., 1985; Hill, 1987). However, by the end of the breeding season in January, adults and newly weaned juveniles begin to disperse from the area and often do not return until the following spring. Thus, seals are tied to their fast-ice colonies for a much shorter period than are emperor penguins.
However, like penguins, satellite telemetry studies conducted within McMurdo Sound has indicated that habitat use patterns differ between adult and juvenile seals. Juveniles begin to disperse away from their natal colonies as the fast ice breaks up in early summer. They generally travel no more than 500 km north of their natal colonies and remain within 120 km of the Victoria Land coast (Burns et al., 1999). In contrast, adult seals remain near the colonies throughout the summer, during which time they complete their annual molt. When adults leave McMurdo Sound, they tend to move into the central Ross Sea rather than along the coast (Testa, 1994). Consequently, adults use the deep pack ice habitat of the greater Ross Sea to a larger extent than do juveniles.
Seasonal separation
With this understanding of seasonal movements for both Weddell seals and emperor penguins, it is possible to address the question of trophic separation due to differences in foraging locations or seasonal cycles. During the spring (Fig. 5A), there is little potential for overlap because seals are in their breeding colonies and foraging little, if at all (Testa et al., 1985; Hill, 1987). From the perspective of the emperor penguins, this minimizes the potential for interspecific competition during a period when adults are foraging to provision themselves and their rapidly growing chicks (Ancel et al., 1992). In early summer (Fig. 5B) as juvenile seals begin to move out of the breeding colonies, both adult and juvenile penguins leave the Ross Sea (Kooyman et al., 1996; Kooyman et al., 1998; Burns et al., 1999). This exodus minimizes spatial overlap between penguins and juvenile seals, which have similar dive capacities and behavior.
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In the fall (April through June), habitat use patterns are effectively the reverse of the spring pattern, and there is little potential for interspecific competition (Fig. 5C). During this period, adult seals are actively foraging throughout the Ross Sea to recover mass lost during the previous breeding season, and juvenile seals are learning to forage along the coastal margins (Testa, 1994
; Burns et al., 1999). At the same time, emperor penguins are largely confined to their colonies to breed and incubate their eggs (Stonehouse, 1964; Croxall, 1997; Kirkwood and Robertson, 1997b). While there is little overlap between seals and penguins for most of the season, competition likely increases in June when female penguins leave the colonies to foraging after egg laying. We know little about fall movement patterns in the Ross Sea, but Kirkwood and Robertson (1997b) found that female penguins along the Mawson Coast foraged primarily within <100 km of their colony. If penguins in the Ross Sea behave similarly, then trophic overlap in the fall is likely limited to the coastal zone.
During winter months the potential for interspecific competition is high (Fig. 5D). From July through September, when adult penguins are foraging over wide areas to gather sufficient prey for themselves and their chicks, juvenile seals are foraging in the same areas, at the same depths, and likely for the same prey items. In addition, adult seals are moving back towards their inshore colonies and foraging to gain sufficient reserves to last the coming breeding season (Ancel et al., 1992; Testa, 1994; Kooyman and Kooyman, 1995; Kirkwood and Robertson, 1997a; Burns et al., 1999). If juvenile penguins are still far to the north at this time, it may be so they can avoid this period of high inter- and intra-specific competition. Unfortunately, data on winter foraging ecology and diving behavior of seals and penguins is scarce, so it is impossible to assess whether other factors, such as shifts in prey selection, play a larger role in reducing potential competition at this time.
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CONCLUSIONS |
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SYNOPSISINTRODUCTIONDISCUSSIONCONCLUSIONSReferences |
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Initial studies on the depth, foraging location, and diet of emperor penguins and Weddell seals suggested that there was a high potential for competition for prey resources. Closer examination of the extant data indicate that despite similarities in foraging depths and prey selection, trophic overlap is largely minimized by temporal and geographic differences in habitat use. Yet, while temporal shifts in breeding and molt schedules minimize the trophic overlap between adult penguins and seals, during winter months both juvenile seals and adult penguins forage in the coastal margins of the Ross Sea. Their similar size, aerobic dive capacity, and modal foraging depths further enhance the potential for competition between these two groups. While in most years the consequences to such overlap are probably small, during periods of food shortage juvenile seal survival will likely be reduced. This is because juvenile seals are already working at the edge of their physiological capabilities, and can neither increase dive effort and exploit deeper foraging areas, as can adult seals, nor reduce their intake requirements by abandoning chicks, as can adult penguins. In summary, trophic separation among these year-round residents of the Ross Sea is maintained largely by geographic and seasonal differences in habitat use, but is not complete. Consequently, competition in times of food shortage has the potential to differentially affect juveniles and adults. These findings emphasize the need to incorporate the year-round behaviors of all age classes when considering species interactions.
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ACKNOWLEDGMENTS |
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This research was funded by grants from the Office of Polar Programs of the National Science Foundation to J. W. Testa (adult seal behavioral studies), M. A. Castellini and J.W. Testa (juvenile seal behavioral studies), and G. L. Kooyman (penguin behavioral studies). A Postdoctoral Fellowship from the Institute of Marine Science at the University of California Santa Cruz provided additional financial support (J.M.B). Logistic support for all research was provided by Antarctic Support Associates, the U.S. Navy, Air National Guard, and Coast Guard. Many people assisted in the field components of these studies, and we are grateful for their help and support. We would also like to thank the two anonymous reviewers whose comments significantly improved the final product. Research on Weddell seals was carried out under permit No. 801 of the Marine Mammal Protection Act. Experimental protocols were approved by the University of Alaska Fairbanks (seals) and the University of California San Diego (penguins) Institutional Animal Care and Use Committees and complied with the "Principles of Animal Care," publication No. 86–23, revised 1985, of the National Institutes of Health, and all current laws of the United States of America.
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FOOTNOTES |
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1 From the Symposium on Antarctic Marine Biology presented at the Annual Meeting of the Society for Comparative and Integrative Biology, 4–8 January 2000, at Atlanta, Georgia.
2 E-mail: afjmb4{at}uaa.alaska.edu
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