MARINE MAMMAL SCIENCE, 10(3):381-383 (July 1994) @ 1994 by the Society for Marine Mammalogy

FIRST ACCOUNT OF STELLER SEA LION (EUMETOPiAS JUBATUS) PREDATION ON A

CALIFORNIA SEA LION (ZALOPHUS CALIFORNIANUS)

On 12 July 1992 we observed a territorial male Steller sea lion (Eumetopias jubatus) attack and eat a small California sea lion (Zalophus californianus) at Ano Nuevo Island, California (37”6’30”N, 122“20’3”W). The prey species was determined by its distinct otariid foreflippers, body shape, and fur color. From its small size and absence of natal coat, we estimated the California sea lion to be a yearling. The attack was observed and photographed between 2005 and 2028.

The Steller sea lion was floating at the surface near his territory. He submerged as a small California sea lion surfaced. The Steller lunged toward it and both animals rolled underwater out of sight. The Steller resurfaced with the California sea lion in his mouth, shook it violently, then dove underwater. Next, he climbed onto his territory with the California sea lion in his mouth, slapped it on the rocks several times, and began eating the entrails. A few minutes later the Steller sea lion dove back into the water with its prey and alternated between flinging it high into the air and diving under water with it. At one point he surfaced with what appeared to be the prey’s head and mouthed it repeatedly before swallowing it. After 23 min the only noticeable remains were a few pieces of flesh and a rib-shaped bone. The other eight territorial males on the breeding area did not participate in the attack.

In prior observations predatory attacks on pinnipeds were brought to the observers’ attention by splashing, gathering of gulls, and the appearance of a surface slick at the kill site (Gentry and Johnson 1981, Klimley et al. 1992). There was little splashing observed during the attack, no slick was visible on the water, and, although Western Gulls (Larus occidentalis) flocked to the area initially, they quickly dispersed. This attack was easily sighted because it occurred less than 75 m from the observation blind and the Steller sea lion involved was under observation at the time.

Steller sea lions are opportunistic feeders known to take a variety of fish and invertebrates (Thorsteinson and Lensink 1962, Fiscus and Baines 1966, Jameson and Kenyon 1977). North of 45”N they have also preyed on northern fur seal pups (Callorhinus ursinus, Gentry and Johnson 1981); ringed seals (Phoca hispida, Tikhomirov 1959); harbor seals (Phoca vitulina, Pitcher 1981); and sea otters (Enhydra lutris, Steller 1742, Tikhomirov 1964). Only subadult and non-territorial adult males are known to consume marine mammals (Gentry and Johnson 198 1, Tikhomirov 1959). Territorial male Steller sea lions typically fast during the breeding season (Fiscus and Baines 1966, Reeves et al. 1992). Although one California sea lion was apparently killed by a territorial male

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Steller sea lion on San Miguel Island (DeLong 1982), the victim was not eaten. This is the first record of a Stellar sea lion consuming a California sea lion.

Stellar sea lions were once the most abundant sea lion in Northern California (Evermann 1921, Rowley 1929). By the 193Os, after decades of exploitation and harassment by humans, their California breeding colonies were limited to protected offshore islands (Rowley 1929). Despite their larger size, Steller sea lions have been progressively displaced along the southern extent of their range by California sea lions. Stellar sea lions have not been recorded on the Channel Islands since 1984 (Reeves et al. 1992). In 1936 California sea lions were first observed on fro Nuevo Island, and both species have coexisted there since without incident (Orr and Poulter 1965, Gisiner 1985).

The attack occurred in conjunction with an unusual abundance of small California sea lions at Aiio Nuevo Island during the 1992 El Nifio event. However, increased numbers of young California sea lions also occurred north of their breeding areas during the 1983 El Niiio (Huber 1991). At the time of the attack some males had already abandoned their territories, but this Steller sea lion was still defending his. We estimate 23% (eight) of the females on the breeding area had not yet copulated at the time of the attack; two days following the attack this male was observed copulating.

ACKNOWLEDGMENTS

We would like to thank Kathryn A. Ono and James ‘I’. Harvey for their advice on the manuscript and Long Marine Laboratory for logistic support. W.H. would also like to thank Big Creek Lumber for donating the lumber for the observation blind and Friends of the Long Marine Laboratory for the William Bay Heald research award.

LITERATURE CITED

DELONG, R. L. 1982. Population biology of northern fur seals at San Miguel Island, California. Ph.D. dissertation, University of California, Berkeley. 185 pp.

EVERMANN, B. W. 1921. The Afio Nuevo Steller sea lion rookery. Journal of Marx- malogy 2:16-19.

FISCIJS, F. H., AND G. A. BAINES. 1966. Food and feeding behavior of Steller and California sea lions. Journal of Mammalogy 47: 195-200.

GENTRY, R. L., AND J. H. JOHNSON. 1981. Predation by sea lions on northern fur seal neonates. Mammalia 45:423-430.

GISINER, R. C. 1985. Male territorial and reproductive behavior in the Steller sea lion, Eumetopias jubatus. Ph.D. dissertation, University of California, Santa Cruz. 146 pp.

HUBER, H. 1991. Changes in the distribution of California sea lions north of the breeding rookeries during the 1982-1983 El Nifio. Pages 129-137 in F. Trillmich and K. A. Ono, eds. Pinnipeds and El Niiio. Springer-Verlag, New York, NY.

JAMESON, R. J., AND K. W. KENYON. 1977. Prey of sea lions in the Rogue River, Oregon. Journal of Mammalogy 58:672.

KLIMLEY, P. A., S. D. ANDERSON, P. POLE AND R. P. HENDERSON. 1992. Spatiotemporal patterns of white shark (Charcharodon carcharias) predation at the South Farallon Islands, California. Copeia 1992:680-690.

ORR, R. T., AND T. C. POULTER. 1965. The Pinniped population of AEio Nuevo Island, California. Proceedings of California Academy of Sciences 32:377-404.

NOTES 383

PITCHER, K. W. 1981. Prey of the Steller sea lion, Eumetopias jubatus, in the Gulf of Alaska. Fishery Bulletin 79:467-472.

REEVES, R. R., B. S. STEWART AND S. LEATHERWOOD. 1992. Sierra club handbook of seals and sirenians. Sierra Club Books, San Francisco, CA.

ROWLEY, J. 1929. Life history of the Steller sea-lions on the California Coast. Journal of Mammalogy lO:l-36.

STELLER, G. W. 1742. Journal notes. Page 21 in J. Rowley 1929. Life history of the sea-lions on the California Coast. Journal of Mammalogy lO:l-36.

THORSTEINSON, F. V., AND C. J. LENSINK. 1962. Biological observations of Steller sea lion taken during an experimental harvest. Journal of Wildlife Management 26: 353-359.

TIKHOMIROV, E. A. 1959. (The feeding of the sea lion on warm-blooded animals.) In Russian. Izvestia TINRO 47: 185.

TIKHOMIROV, E. A. 1964. (Distribution and hunting of the sea lion in the Bering Sea and adjacent parts of the Pacific.) In Russian. Izvestia TINRO 53:281-285.

PAMELA E. BYRNES, Moss Landing Marine Laboratories, San Francisco State University, P.O. Box 450, Moss Landing, California 95039; WENDY R. HOOD, Department of Biology, Boston University, Boston, Massachusetts 022 15. Re- ceived 26 May 1993. Accepted 20 December 1993.

Books

MARINE MAMMAL SCIENCE, 10(3):384-390 (July 1994) 0 1994 by the Society for Marine Mammalogy

REVIEWS

MARINE MAMMAL SENSORY SYSTEMS. J. A. Thomas, R. A. Kastelein and A. Ya. Supin, eds. Plenum Press Publishing Corp., 233 Spring Street, New York, NY 10013, i- xiv, 733 pp. 1992. ISBN o-306-44351-1, hardcover. Price $149 in USA and Canada, $179 outside North America.

This volume contains nearly 50 original research papers presented at the Sympasim on Sensavy Systems and Behavior of Marine Mammals, held in Moscow from 16-25 October 1991. The Preface records that the Russian hosts, particularly Alexander Supin of the Severtsov Institute in Moscow, organized a successful and memorable symposium despite the many difficulties of that period. Structure of the book corresponds to the five symposium sessions: I. Sensory anatomy, morphology and neurology; II. Hearing abilities; III. Echo- location abilities; IV. Acoustic communication and behavior; V. Sensory systems and behavior. These are arbitrary subtitles and while there is considerable overlap, this is unavoidable. The majority of the articles deal with the acoustic sense, but there is an eclectic mix of approaches ranging from immuno-cytological techniques, biophysical ex- periments and comparative morphology to field studies on responses to fishing gear. Some of the work on comparative neurology provides new information pertinent to phylogenetic relationships.

To summarize and critique this large volume in a few pages is impossible, so it seemed mote useful to identify the major questions being addressed and discuss insights and conclusions of the authors. It is imperative that readers new to the field understand that some of these issues have been debated for two or three decades without complete resolution! The following continue to stimulate lively debate: (1) Do dolphins use echolocation as a primary or secondary method of obtaining information about their immediate environment? (2) What is the nature of the images they obtain from echolocation? (3) Are sounds produced and heard in similar ways by different orders of marine mammals? (4) Is there more than one critical site of sound reception in odontocetes? (5) Is there a relationship between ear structure and the type of sounds produced by marine mammals? (6) To what degree has natural selection within specific habitats played a role in channelling particular developments? (7) What functions might be served by nondirectional sound production by some marine mammals? (8) Can we gain insights into the nature and level of intelligence in dolphins by investigating their use of and reaction to acoustic information? (9) Can we obtain evidence about phylogeneric relationships among cetaceans by examining cerebral and auditory morphology and microstructure?

Nikol’skaya and Petrov (p, 377) carried out extensive experiments to determine if bottlenose dolphins used echolocation to locate cues. The dolphins did not, relying instead on listening for characteristic sounds, with echolocation used for verification of detail in the local environment. The definition achieved by echolocating bottlenose dolphins is impressive. Au (p. 357) finds that they can recognize l-dB differences in the amplitude of echoes, distinguish between targets when surface areas differ by lo-20%, identify targets of different thickness or composition, and differentiate between quite subtle geo- metric shapes. Jefferson et al. (p. 663) present data on the ability of some cetacean species to detect fishing gears, while Turl and Thomas (p. 421) discuss significant effects of oceanographic conditions on long-range detection of targets by a false killer whale.

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