Jr. Gilbert, Review of previous Pacific walrus surveys to develop improved survey designs, MARINE MAMMAL SURVEY AND ASSESSMENT METHODS, 1999, pp. 75-84
The Pacific walrus (Odobenus rosmarus divergens) occupies the Bering and Ch
ukchi seas between the United States and Russia, and has fluctuated over th
e past 200 years as a result of exploitation. Between 1880 and 1980, annual
harvests were as high as 9000 in some years. High harvests in much of that
time interval were not sustainable. While the present harvest is below his
torical highs, its impact on the population is not known. Efforts to monito
r the population size and trend are necessary to conserve walrus population
s. Surveys of Pacific walruses have been hampered by a variety of problems.
Early efforts were not coordinated between American and Russian researcher
s. The chronology of efforts was coordinated in 1975, 1980, and 1985, but s
urvey designs differed. In 1990, scientists were exchanged and sampling des
igns were shared. However, ice distribution confounded interpretation of re
sults. Walrus numbers often were estimated without estimates of precision.
Variances were calculated only for portions of sea-ice surveys, and land ha
ulouts usually were counted only once. When they were calculated, confidenc
e limits approached the size of the estimate. On sea ice, walrus numbers we
re estimated from counts on one or two days when very large numbers were ob
served. It was assumed that on these days a peak fraction of the population
was hauled out. Problems with past surveys include: walruses could move be
tween land haulouts acid nearby sea ice in the Western Chukchi Sea during t
he course of a survey, walrus groups on sea ice, numbering up to several th
ousands, were difficult to count with accuracy, weather and lack of aircraf
t restricted timing and location of sampling efforts. After participating i
n and reviewing results of past surveys, I recommend consideration of a sur
vey in the spring. This survey should incorporate deployment and monitoring
of satellite transmitters, and replicated counts on both the sea ice and a
t land haulouts. Data for all surveys should be averaged then corrected for
the average fraction of observations of walruses available to be counted.