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

Report No. 6945
BBN Systems and Technologies Corporation
al. 1963; Stirling 1973; Cummings et al. 1984). Sound intensities are only
95-130 dB re 1 pPa at 1 m, which is low in comparison with other Alaskan
marine mammals (Cummings et' a1 . 1984 ; cf . Tables 2.7-2.9 ) .
Walruses produce a stereotyped sequence of sounds consisting of clicks
rasps and a bell-like tone. These sounds are in the frequency range 0.4 to
1.2 kHz with harmonics to 10 kHz (Schevill et al. 1966; Ray and Watkins 1975;
Stirling et al. 1987). Source levels have not been reported.
2.2.4 Seasonal Aspects of Sound Production
In any given location, the contribution to ambient noise made by marine
mammal sounds is strongly dependent on season. Season determines the locations
of most of the marine mammals, and also determines their behavioral
activities and hence the amounts and kinds of vocalizations that they produce.
The seasonal distribution and seasonal influences on sound production of
common Alaskan marine mammals are summarized below.
Spring: Fast ice and dense pack ice covers most of the Chukchi and
Beaufort Seas in the early spring. The ringed seal is the only common
inhabitant of these ice-covered waters. Ringed seal vocalizations become more
common in April at the onset of the breeding season (Stirling et al. 1983;
Cummings et a1 . 1984). Later in spring and during summer the ringed seal
appears to be much less vocal (Stirling et al. 1983).
In spring, walruses, bowheads and white whales are widely distributed
within the moving pack ice in the Bering Sea. Later in spring, bowheads and
white whales aggregate while migrating in the system of opening leads in the
Chukchi Sea. Bearded seals also follow the retreating ice edge into the
Chukchi Sea. Ribbon seals are associated with the ice edge in spring;
however, when the ice edge retreats, they remain in the Bering Sea. At the
springtime ice edge in Lancaster Sound, N.W.T., a somewhat analogous situation,
ambient noise was dominated by bearded seal, white whale and narwhal
sounds (Finley et al. 1983, 1984). Walruses produce their stereotyped songs
during the mating season in March and April (Stirling et al. 1983). However,
it is not known if they vocalize during the remainder of the year as well. In
the high arctic, bearded seal vocalization rates increased from late winter to
early summer; however, it was not known if this was due to an increase in call
rate or an increase in the numbers of seals present-(Stirling et al. 1983).
Spring migrating bowheads sometimes produce a stereotyped song in addition to
the more common moans and other calls (Cummings and Holliday 1987; C.W. Clark
pers. comm.). Thus, in spring, marine mammal sounds would probably contribute
significantly to ambient noise levels near the ice edge in the northern Bering
Sea and in the system of leads in the Chukchi Sea. This has been confirmed in
the Barrow region during recent acoustic studies in spring.
In late spring, northern fur seals and Steller sea lion males come ashore
to establish breeding territories. The in-air vocalizations associated with
agonism among males have been documented. However, it is not known if similar
vocalizations also occur at sea prior to hauling out. Spotted seals winter
along the Bering Sea ice edge. At breakup, they migrate to nearshore areas in
the northern Bering Sea and Chukchi Sea. Harbor seals use nearshore areas