DURATION OF STEREOTYPED UNDERWATER VOCAL DISPLAYS BY MALE ATLANTIC WALRUSES IN RELATION TO AEROBIC DIVE LIMIT

During the breeding season from January to mid-April, adult male Atlantic walruses (Odobenus rosmarus rosmarus) dive repeatedly for an average duration of 4–6 min and give stereotyped underwater vocal displays. Between dives, they surface for 1–2 min, take 4–6 breaths, and give stereotyped vocalizations between breaths. Male walruses vocalize in the presence of groups of females and calves, young adult males, or by themselves as lone singers. This pattern is repeated throughout the breeding season and can be maintained for extended periods, sometimes exceeding 48 h. The prolonged underwater vocal displays of male walruses seem possible because the animals do not exceed the aerobic dive limit (ADL), estimated to be 9.8 min for a 1,100-kg animal, nor do they exceed the behavioral ADL of 7.9 min, determined from the histogram of dives for males singing alone. The number of breaths taken after dives and the postdive surface times remained fairly constant despite dive duration, suggesting that the walruses remained within their aerobic dive limits. The duration of most dives made by displaying males vocalizing alone during the breeding season, and dive duration of walruses feeding for protracted periods outside the breeding season, are both roughly half the estimated ADL.     

FEMALE REPRODUCTIVE PATTERNS AND FETAL GROWTH OF ATLANTIC WALRUSES (ODOBENUS ROSMARUS ROSMARUS) IN FOXE BASIN,NORTHWEST TERRITORIES,CANADA

We examined the reproductive tracts of 79 female Arlantic walruses (Odohenus rosmarus rosmarus) caught in July and August by Inuit hunters in northern Foxe Basin, NWT, Canada. We found no evidence of past or present ovulation in animals less than 5 yr old, but the poor representation of juvenile and adolescent animals precluded statistical analysis of the average age of maturity. The ovulation frequency of 64 mature females (0.52) indicated biennial ovulation, while the pregnancy rate (0.33) and birth rate (0.30) suggested that females gave birth once every three years. The sex ratio of 17 fetuses was not significantly different from 1:1. Fetuses increased 2.9 mm d^-1 in crown-rump length and 0.12 g^1/3 d^-1 in mass over the range of collection dates. Based on the inverse regressions of fetal growth, implantation occurred in late June early July.     

MANAGING THE EXPLOITATION OF PACIFIC WALRUSES: A TRAGEDY OF DELAYED RESPONSE AND POOR COMMUNICATION1

The Pacific walrus population has been depleted and subsequently allowed to recover three times in the past 150 yr. As we see it, the population has been made to fluctuate like an r-selected species, rather than being maintained at a high, stable level, as befits a K-selected species. The latest depletion began in the 1930s but was not recognized until 25 yr later, by which time the population had been reduced by at least half. Without benefit of communication, the U.S.S.R. and the State of Alaska put similar protective measures into place by 1960, and in the next two decades the walrus population recovered again, at least doubling in size. By 1980, it already was showing density-dependent signs of having approached or reached the carrying capacity of its environment. As productivity and calf survival declined sharply in the late 1970s and early 1980s the catches more than doubled. We believe that the combined effects of natural curtailment and human intervention may be bringing the population down again rather rapidly. With the present, crude monitoring methods, delayed management responses, and poor international communications, however, the downward trend may not be acknowledged for at least another decade, by which time the unilateral Soviet and American corrective measures are likely to be too much, too late. Walrus management needs to be based less on response to immediate crisis and more on long term prediction than it has been in the past. Because the U.S.A. and U.S.S.R. are trying to manage the same walrus population, without sufficient communication or consensus and sometimes to opposite ends, an international joint management program needs to be implemented.     

AERIAL CENSUS OF PACIFIC WALRUSES IN THE CHUKCHI SEA, 1985

The U.S. Fish and Wildlife Service conducted a survey of the walruses in the pack ice of the Chukchi Sea between 16 September and 2 October 1985, as part of a joint effort with the Soviet Union to estimate the size of the Pacific walrus population. American observers conducted censuses from two aircraft along randomly selected north–south lines over the pack ice. The observers counted walruses within a constant viewing angle that corresponded to a total strip width of 1.38 km at an altitude of 152 m.
In nine days of flying, 15,312 walruses were observed, of which 10,140 were on 5,764 km² of census strips. Few walruses were observed east of 161°W longitude or west of 170°W longitude, hence the census effort was stratified. Walrus concentrations between 161° and 170° shifted slightly westward during the 2-wk duration of the censuses. The differences among days in observed walrus density were due to changes in the numbers of walruses on the ice within 37 km of the ice edge. The number of observable walruses in pack ice of the eastern Chukchi Sea was estimated to be 62,177 (SD = 19,480), based on censuses conducted on 29 and 30 September and 1 October. At that time there were also at least 15,238 in Bristol Bay, Bering Sea. The Soviets counted 39,572 on the shores of the western Chukchi and Bering seas and estimated 115,531 in pack ice of the western Chukchi Sea. Summing U.S. and Soviet estimates, the total population of Pacific walruses in 1985 was 232,518. This number was comparable with earlier estimates from censuses conducted jointly by the U.S. and the Soviets. However, information on fraction hauled out, segregation, and movements is needed for more precise estimates.     

IMMOBILIZATION OF WALRUS WITH ETORPHINE HYDROCHLORIDE AND ZOLETIL®

Abstract: The paper describes the use of the drugs Zoletil® and etorphine for the immobilization of walrus ( Odobenus rosmarus rosmarus ) for attachment of satellite telemetry equipment. Three animals weighing approximately 1,500 kg each were injected intramuscularly with Zoletil® at a dose between 1.4 and 2.2 mg/kg. One walrus died while the two others were adequately restrained. The induction time was between 14 and 29 minutes and the effect lasted for 75-220 min.
Thirty-eight animals of weight 900-1,500 kg were darted intramuscularly with etorphine at a dose between 3.3 and 8 mg/kg. Thirty-six were immobilized although one died later. The induction time for etorphine was two to ten minutes. The action of this drug was terminated with an injection of the antidote di-prenorphine.
The use of etorphine was accompanied by convulsive movements and apnoea while Zoletil® produced a gradual and smooth entry into and withdrawal from immobilization.     

STATUS OF THE PACIFIC WALRUS POPULATION, 1950–19891

The Pacific walrus (Odobenus rosmarus divergens) population is an important ecological and economic resource of the Bering Sea region. We describe population change, beginning with a low in 1950, through a high in about 1980, and ending in 1989. Estimates of abundance for the years after 1989 were not attempted due to the lack of harvest data and other population parameters. Selective hunting practices resulted in biased data regarding population composition and reproductive performance. Rates of reproduction had to be estimated from ovarian data, which indicated a dramatic drop in the 1980s. High harvests in the 1980s likely contributed to a decline in the population, but uncertainties as to accuracy of population estimates and other data raise reasonable doubts, especially with respect to the number of males, for which the most recent (1985) population estimate suggests a sharp decline. Past population estimates were revised upwards to compensate for walruses underwater and not seen in aerial surveys. The weaknesses in the available data make it clear that effective management of the population will require many improvements in collection of data regarding harvests, population structure, reproduction, and population trend.