Analysis and Ranking of the Acoustic Disturbance Potential of ...

Report No. 6945
BBN Systems and Te~h~ologies Corporation
Table 2.3.
~ensities' of Marine Mamals (~o./km~) observed2 on Aerial Survey
Transects Along the Nearshore Zone of the North Aleutian Shelf,
Including the Port Moller Area (Troy and Johnson 1987).
Spr in6 Summer Fall Winter
Mar-May June-Aug Sept-Nov Dec-Feb
Sea otter 0.52 0.57 0.97 0.57
Steller sea lion 0.17 0.21 0.01 0.22
Harbor seal 0.20 0.44 0.01 0.02
Harbor porpoise 0.02 0.00 0.00 0.02
Dall's porpoise 0.00 0.02 0.00 0.00
Gray whale 0.02 0.02 0.01 0.00
l~i~hest monthly densities observed during each three-month period are shown.
'~ctual densities may be considerably higher. During aerial surveys some
marine mammals are submerged and therefore invisible to observers; others are
present at the surface but not seen by the observers. Shipboard behavioral
observations of gray whales in the Chirikof Basin indicated that in order to
correct for submerged gray whales, raw density estimates derived from aerial
surveys should be divided by 0.280 (July surveys from a Grumman Goose) and
0.358 (September surveys from a Twin Otter). Thus, the raw density estimates
were 2.8 to 3.6 times too low. Also, these correction factors do not take
into account animals present at the surface but not seen by the observers
(Miller 1986). Davis et al. (1982) developed a correction factor for
bowheads at the surface that were missed by the observers. They estimated
that only 68.5% of the bowheads at the surface in their study were detected
by the primary observers. Comparable correction factors are not available
for the other species listed here.
Although there are records of up to 4000 walruses hauling out in the Port
Moller area, their use of this area is apparently irregular, and may be
declining. Frost et al. (1982) reported that walruses hauled-out in this area
in 1968, 1969, 1979 and 1980. None were reported to be there in 1981 and only
four in 1982. Records of hauled out walruses are more frequent from Amak
Island and Cape Seniavin. Troy and Johnson (1987) recorded peak numbers of
walruses in April, in the coastal zone of their ~orth Aleutian Shelf study
area.
Chirikof Basin
Several species of marine mammals occur in the Chirikof Basin of the
northern Bering Sea. This area is one of the main feeding areas of gray
whales, whose densities in the area are summarized in Table 2.4.