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

Report No. 6945
BBN Systems and Technologies Corporation
The vessels of the fishing industry are widely distributed sources with a
medium to low sound level output. As a result, the potential noise impact of
individual fishing vessels on marine mammals is typically lower than that of
large ships and many petroleum industry sources. However, when many trawlers
are operating in a concentrated area, as occurs when the seasons first open
for some specific species, the composite local noise level may be increased
considerably. Sound levels have not been reported for these composite fishing
operations but if 3 to 6 vessels are operating in close proximity, a 10 to
15 dB increase in local noise l'evel over that expected from a single trawler
is possible. Based on the source level data shown in Table 3.2 for trawling
operations, this would increase the received levels in the immediate area of
the concentrated fishing activity to values found near drillships and dredges.
The other major Alaskan industries, lumbering and mining, contribute
noise to the marine environment primarily through their use of shipping for
movement of materials. This is covered under the category of transportation.
Some mining activities near coastal regions contribute indirectly to local
noise levels by movement of materials across beaches using aircraft and
landing barges. The recent movement of gold dredging activities offshore,
primarily in the Nome area, is likely to increase local underwater noise
levels. No specific acoustic source level data are available for gold dredges
but data for several types of offshore dredges are presented in Table 3.2.
The gold dredge operating off Nome is a large bucket type of dredge. It is
possible that the noise levels of this dredge are more closely related to
those of the transfer type of dredge than the clamshell dredge since the
dredging operation is continuous rather than periodic.
Source level spectra for selected sources from Table 3.2 are shown in
Fig. 3.13. The spectra for the seismic sources are seen to be similar in
level and shape. The icebreaker spectrum has a large amount of energy at high
frequencies which is typical of cavitation noise. The dredge noise output
level can be seen to be higher than that of the drillship (Explorer II),
particularly above 63 Hz. The dredge spectrum shown here is the loudest of
the three available dredge examples. The trawler spectrum is representative
of large trawlers (30 to 50 m) operating at 5 kts.
3.3.2 Transportation sources
Table 3.3 presents a compilation of relevant source information for the
transportation industry. The general category of transportation sources has
been subdivided into ships and boats, aircraft, and helicopters as shown in
parts A., B., and C. of the table. The items selected are intended to be
representative of the ships and aircraft used in Alaska. The format is
identical to that used for Table 3.2.
Boats and Ships
Large oil tankers and cargo carriers of the type serving the Alaska
pipeline at Valdez and passing through Alaskan waters on the route to Japan
are the loudest water transportation source, often having dominant bandwidth
source levels exceeding 185 dB when operating at full speed. In restricted
waters when their speed is reduced to 10 kt or less, the source levels