WALRUS, ODOBENUS ROSMARUS, FEEDING IN THE BERING SEA: 



A BENTHIC PERSPECTIVE 



JohnS. Oliver, 1 Peter N. Slattery, 1 Edmund F. O'Connor, 1 

 and Lloyd F. Lowry 2 



ABSTRACT 



Walrus, Odobenus rosmarus, feed primarily on benthic bivalves and create a distinct record of their feeding 

 activities on the sea floor. The record consists of furrows, pits, and discarded bivalve shells which were ob- 

 served and sampled with scuba. Documentation of this benthic feeding record suggested that walrus com- 

 monly search for visually conspicuous prey by sight; that, in addition to "rooting" with the snout and 

 vibrissae, walrus excavate bivalve prey by hydraulic jetting; that tusks are not used to excavate prey; and that 

 all prey are excavated before consumption, which generally occurs close to the site of excavation. The 

 mechanism of consumption appears to involve suction from between the shells. Continuous pit-furrow sys- 

 tems indicate the number of prey consumed in single dives, and suggest that a walrus can locate, excavate, 

 and consume more then six clams per minute. The abundance of small infauna that are not walrus prey (e.g., 

 polychaete worms, small bivalves, and crustaceans) was lower inside all excavations, indicating that the 

 structure of bottom communities is highly modified by the extraction of a few large prey. 



Marine mammals are observed primarily at the sea 

 surface. Yet many important activities, especially 

 feeding, occur underwater beyond the view of sur- 

 face observers. While informative glimpses of feed- 

 ing activities are sometimes obtained at the water 

 surface (e.g., Watkins and Schevill 1976, 1979; 

 Wursig and Wursig 1980), and the types of foods con- 

 sumed are indicated by contents of gastrointestinal 

 tracts (e.g., Lowry etal. 1980; Lowry and Frost 1981), 

 knowledge of foraging behavior and the community 

 role of marine mammals is generally poor. Electronic 

 tags, depth recorders, and other instrumentation are 

 improving this limited view (e.g., Watkins et al. 1981; 

 Kooyman 1981). However, the greatest oppor- 

 tunities for studying the feeding ecology of marine 

 mammals may involve species that prey on benthic 

 organisms. 



Bottom-feeding marine mammals often feed in shal- 

 low water, where general feeding grounds usually are 

 known and local feeding areas can be relocated. Be- 

 cause benthic habitats and bottom prey are relatively 

 immobile, prey communities can be sampled with 

 considerable accuracy and precision, and can be ex- 

 perimentally manipulated as well. Largely for these 

 reasons, we understand more about the community 

 role of the sea otter, Enhydra lutris, a bottom feeder, 

 than any other marine mammal (Estes and Palmi- 

 sano 1974; Dayton 1975; Estes et al. 1978, 1982; 

 Simenstad et al. 1978; Duggins 1980). In contrast to 



Moss Landing Marine Laboratories, Moss Landing, CA 95039. 

 Alaska Department of Fish and Game, Fairbanks, AK 99701. 



Manuscript accepted January 1983. 

 FISHERY BULLETIN: VOL. 81, NO. 3, 1983 



the very few species of bottom feeders, most marine 

 mammals feed on mobile prey in the water column 

 where foraging activities are difficult to observe, and 

 no record is made. Nektonic prey are extremely dif- 

 ficult to sample quantitatively. Even if a feeding 

 event were observed, the dynamic nature of prey 

 patches and the pelagic habitat preclude direct 

 measurement of the effect of mammal predation on 

 prey communities. 



Some bottom-feeding marine mammals leave a 

 record of their feeding activities in soft-sediment en- 

 vironments. The record primarily consists of pits and 

 furrows made in the sea floor. For example, the gray 

 whale, Eschrichtius robustus, produces large, bowl- 

 shaped pits while feeding on benthic infauna, es- 

 pecially amphipod crustaceans (Oliver et al. 1983). 

 Gray whales capture and consume invertebrate prey 

 by suction, but also kill, injure, and displace nonprey, 

 modify local habitats, and attract scavenging animals 

 to these excavated bottoms. Walrus; bearded seals, 

 Erignathus barbatus; sea otters; dugongs, Dugong 

 dugon; and manatees, Trichechus spp., also feed in 

 soft-sediment habitats, but only the walrus and gray 

 whale depend primarily on infaunal prey (Anony- 

 mous 1978). 



Walrus are a common and conspicuous element of 

 the marine mammal fauna inhabiting arctic and sub- 

 arctic waters of the Northern Hemisphere. They are 

 particularly abundant in the Bering and Chukchi 

 Seas, where they forage among the bivalve com- 

 munities found on the broad Beringian platform. The 

 contents of numerous gastrointestinal tracts indicate 



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