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

Report No. 6945
BBN Systems and Technologies Corporation
Reaction distances of harbor seals hauled out on land seem to be at least as
great for quiet unpowered vessels (kayaks, canoes) as for motorboats (Allen et
al. 1984; Osborn 1985), suggesting that these seals may react more to the
sight than to the sound of small vessels. Reactions of pinnipeds in the water
to approaching vessels have rarely been reported. Fay et al. (1986) indicated
that walruses tolerated closer approaches when they were in the water than
when they were hauled out on ice pans.
Toothed Whales--Toothed whales show considerable tolerance of vessel
traffic in many circumstances. However, they sometimes react at considerable
distances when confined by ice or shallow water, or when they have learned to
associate that vessel with harassment. Although received sound levels at
which toothed whales do and do not react have not been reported, the threshold
of responsiveness is likely to vary widely in parallel with the widely varying
distance thresholds.
Dolphins often approach vessels and swim in their bow wakes, apparently
unaffected by the high noise levels to which they must be exposed when within
a few meters of the vessels. However, dolphins subject to harassment by tuna
seining operations actively avoid tuna seiners and other vessels at distances
of several kilometers (e.g. Norris et al. 1978; Au and Perryman 1982; Hewitt
1985). The avoidance reaction is suspected to be in response to underwater
sound, in which case the animals must be reacting strongly to received noise
levels far lowe'r than those tolerated by dolphins that ride the bow waves of
various vessels.
Similarly, white whales exhibit highly variable sensitivity to vessel
noise. For example, in Bristol Bay, Alaska, white whales occur regularly
amidst large fleets of fishing vessels. However, when these white whales move
up a river they appear to be more sensitive to approaching outboard-powered
boats (Stewart et al. 1982). Reactions of white whales to oil industry
vessels operating in shallow coastal waters have been studied in the Mackenzie
Delta area of the Canadian Beaufort Sea. There white whales sometimes avoid
tugboats and similar vessels at distances as great as 2.4 km, but at other
times occur within 0.2 km from such vessels (Fraker 1977a, b, 1978) .
Observations in that same area when ice was present in spring suggested that
white whales are more sensitive to boats when ice restricts the animals to
confined areas (Norton Fraker and Fraker 1982). White whales in leads
consistently swam away from supply ships that were in motion at distances of 1
to several kilometers. White whales in the eastern Canadian high arctic have
consistently shown very great sensitivity to noise from ships and from
icebreaking. Strong avoidance reactions have been demonstrated repeatedly
when the ship was several tens of kilometers away and when the ship noise was
barely above the background ambient noise (LGL and Greeneridge 1986). Thus,
no single noise threshold applies to all situations in which white whales
occur. Their sensitivity varies widely with the circumstances.
Baleen Whales--There have been specific studies of the reactions of gray,
humpback,'and bowhead whales to vessels, and limited information, largely
anecdotal, is available for some other species ( Richardson et a1 . 1984 ) .
Watkins (1986) summarized some of the reactions of whales to boats based on
his extensive experience near Cape Cod. Most low-amplitude vessel sounds