ECOLOGY AND BIOLOGY OF THE PACIFIC WALRUS 



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these animals consumed about 280 to 530 kcal/kg3/4 per day (Table 24). The 

 mean intake in these terms, 380 kcal/kg3/4 per day, was similar to that for 

 maximal intakes by domestic animals reported by Kleiber (1961). 



Based on these gross caloric intakes by walruses in captivity, I estimated the 

 rates of consumption of food (kg per day) for free-living walruses of different 

 body weights, assuming that their intake would be mainly bivalve mollusks (as 

 indicated by Fay et al. 1977). The results (Table 25) suggest that the mean rates 

 of consumption of benthic invertebrates by free-living walruses may be at least 5 

 to 7% of their TBW per day. One should bear in mind, however, that these are 

 net rates of predation, for the walrus eats only the exposed soft parts of shelled 

 mollusks, not the whole organism. Since those soft parts (foot, siphon) amount to 

 no more than one-fourth to one-third of the total weight of the mollusks, the 

 biomass of gross rate of predation will be three to four times that of the parts 

 actually ingested. Thus, the 1,000-kg walrus that consumes 60 kg of molluscan 

 feet and siphons per day actually will destroy about 180 to 240 kg (wet weight 

 biomass) per day of those mollusks. 



Assuming that the Pacific walrus population currently comprises about 

 200,000 animals and that its sex and age compositions are about as estimated for 

 the 1972 population (see chapter on Population), then the average TBW per 

 individual is about 720 kg, and the average net rate of intake of food is about 

 6.2% of TBW per day. For this population as a whole, about 8,900 tons of food 

 will be consumed daily and about 3.25 x 10^ tons per year. If 96 % of that intake 

 is molluscan soft parts (mainly feet and siphons) and 4% is other organisms, 

 swallowed whole (Fay et al. 1977), then the gross rate of predation on benthic 

 invertebrates may amount to a biomass of between 9.5 and 12.6 x 10^ tons per 

 year, or about 2.5 to 3% of the estimated standing stock of benthos on the 

 Bering-Chukchi shelf. This would be a small proportion if the walrus were the 

 only user of the benthic fauna and were nonselective in its feeding. Being a 

 predator on certain kinds of mollusks, mainly in shallow waters, the walrus can 

 utilize less than one-third of the total shelf biomass, and this is fed on by a wide 

 variety of other organisms, including fishes, crabs, sea stars, cephalopods, birds, 

 seals, and cetaceans. Furthermore, because walruses do not distribute themselves 

 uniformly over the Bering-Chukchi shelf, the population probably has a much 

 greater impact on the resources of some areas than of others (Fay et al. 1977). 

 The greatest impact appears to be on the wintering areas, and I believe it is there 

 that food may exert its greatest influence as a limiting factor on population 

 growth. 



In the wintering areas, the intake of food by the breeding adult males appears 

 to be small, but this probably is more than compensated by the increased intake 

 by pregnant and lactating females, whose daily rate of consumption is likely to 

 be two to three times that of nonpregnant, nonlactating animals. During two 

 expeditions on icebreaking ships in the St. Lawrence concentration area in late 

 February to mid- April, I observed that nearly all of the late-term pregnant 

 females sighted were in the water, apparently feeding, whereas more than half 

 of the other females were sleeping or just swimming when sighted. 



During the northward migration in spring (April-June), I observed that the 

 animals taken in the harvest at St. Lawrence Island showed a reversal of that 

 tendency for different feeding rates. Those most often having food in the stomach 



