Aquatic Mammals 2010, 36(2), 129-137, DOI 10.1578/AM.36.2.2010.129 Observation of Steller Sea Lion (Eumetopias jubatus) Predation on a Harbor Seal (Phoca vitulina richardii) in the Glacier Bay Region of Southeastern Alaska Jamie N. Womble 1 and Suzanne Conlon 2* 1 National Park Service, Glacier Bay Field Station, 3100 National Park Road, Juneau, AK 99801, USA; E-mail: jamie.womble@oregonstate.edu 2 Alaska SeaLife Center, P.O. Box 1329, Seward, AK 99664, USA * Current Address: 993A Kupulau Drive, Kihei, HI 96753, USA Abstract Pinnipeds prey primarily on fish and invertebrates; however, several species are known to prey upon other pinniped species. Herein, we document an observation of a Steller sea lion (Eumetopias juba- tus) attacking and partially consuming a juvenile harbor seal (Phoca vitulina richardii) in Johns Hopkins Inlet in Glacier Bay National Park, Alaska. Population trends for Steller sea lions and harbor seals contrast dramatically in the Glacier Bay region. Although other marine predators are known to attack harbor seals, it is possible that Steller sea lions could potentially have both a direct and indi- rect influence on harbor seals in the Glacier Bay region. Key Words: Steller sea lion, Eumetopias jubatus, harbor seal, Phoca vitulina richardii, pinniped, predation, feeding, Glacier Bay National Park, southeastern Alaska Introduction Predation may have direct and indirect effects on prey populations and can influence the dis- tribution, abundance, and evolution of a species (Polis et al., 1989). Interspecific predation occurs in many foraging guilds (Burger, 1979; Creel & Creel, 1996); however, the ultimate mechanisms may vary and may be a result of competition for space or food, overt aggression, or predation as a source of food. Interspecific killing among mam- malian carnivores is relatively common and may account for a substantial proportion of known mortalities in some species (Palomares & Caro, 1999). Although pinnipeds prey primarily on fish and invertebrates, several species, including southern sea lions (Otaria byronia) (Majluf, 1987; Harcourt, 1993), New Zealand sea lions (Phocarctos hook- eri) (Bradshaw et al., 1998; Robinson et al., 1999), leopard seals (Hydrurga leptonyx) (Siniff & Bengtson, 1977; Shaughnessy & Goldsworthy, 1990; Boveng et al., 1998; Walker et al. 1998; Hiruki et al., 1999), walrus (Odobenus rosmarus) (Fay, 1960; Lowry & Fay, 1984; Fay et al., 1990), and Steller sea lions (Eumetopias jubatus) (Gentry & Johnson, 1981; Pitcher & Fay, 1982; Byrnes & Hood, 1994; Calkins, 1998; Mathews & Pendleton, 2006) are known to prey upon other pinniped spe- cies. Naïve pups or smaller individuals are often targeted; and in some cases, interspecific predation by pinnipeds may limit population growth of other pinniped species (Boveng et al., 1998). Although it can be difficult to quantify predator- prey interactions in diving mammals (Bertilsson- Friedman, 2006), the influence of predation on the behavior of individual prey (Frid et al., 2007) and on population trajectories of prey species has recently received substantial attention (Springer et al., 2003; Williams et al., 2004). In the North Pacific Ocean, Steller sea lions and harbor seals (Phoca vitulina richardii) are sympatric throughout much of their ranges, and both species have undergone substantial declines in some areas (Merrick et al., 1987; Pitcher, 1990; Small et al., 2008; Mathews & Adkison, 2010). In Alaska, Steller sea lions are primarily piscivorous (Pitcher, 1981; Sinclair & Zeppelin, 2002; Womble & Sigler, 2006; Trites et al., 2007; McKenzie & Wynne, 2008; Sigler et al., 2009); however, there have been obser- vations and diet studies that have documented Steller sea lions attacking and feeding upon other pinniped species. Pinniped species that have been attacked or consumed by Steller sea lions include northern fur seal neonates (Callorhinus ursinus) (Gentry & Johnson, 1981), California sea lions (Zalophus californianus) (Byrnes & Hood, 1994), ringed seals (Phoca hispida) (Tikhimriov, 1959), harbor seals (Pitcher & Fay, 1982; Mathews & Adkison, 2010), spotted seals (Phoca largha), and bearded seals (Erignathus barbatus) (Calkins et al., 1999).