A two-month experiment in which piloted commercial ships transiting
Haro Strait, east of Vancouver Island, British Columbia, voluntarily slowed to 11 knots resulted in measurable reductions
in underwater noise. The Port of Vancouver sponsored the research to assess the potential benefits of reducing vessel-generated
underwater noise in Haro Strait to improve foraging among endangered orcas, also known as killer whales.
“Haro Strait is known to be a
key foraging area for the endangered Southern Resident Killer Whale (SRKW) in the summer months. [It] is also a geographically
constrained area with a busy shipping lane, and was identified through other research to be a hotspot for potential loss of
foraging by SRKW as a result of vessel noise,” according to the Port’s report of preliminary findings.
experiment was initiated by the Port’s Enhancing Cetacean Habitat and Observation (ECHO) Program, which identified it
as a priority measure to gain a better understanding of the relationship be-tween vessel speed and noise, along with the potential
benefit to foraging by killer whales. It sought to answer these main questions:
• How does reduced speed change the under-water noise generated by a
specific vessel and by class of vessels?
• How does reduced speed change the total underwater ambient noise received at a specific
location of importance to the killer whales?
• What are the predicted resultant effects on killer whale behavior and foraging, given
the changes in noise?
than 60 percent of the vessels transiting Haro Strait during the trial last summer voluntarily slowed their speed to 11 knots.
The actual reduction in speed depended on an individual vessel’s normal cruising speed. The measured speed reductions
and resulting decreases in noise, by class of vessel, are as follows:
• bulk/general cargo ships, 2.2 knot reduction in speed and 4.9 dB reduction
in source sound level;
container ships: 6.9 knot reduction in speed and 9.4 dB reduction in source sound level;
• passenger ships: 6.2 knot reduction in speed
and 8.1 dB reduction in source sound level;
• vehicle carriers: 5.8 knot reduction in speed and 9.3 dB reduction in source sound level.
Researchers recorded a total of 33
matched pairs of accurate vessel source level measurements for the same vessel at two stations. “When the vessel source
level and vessel speed data relationships for all 33 vessels are plotted on one chart, analysis of the trend line predicts
that slowing speed by 40 percent reduces broadband monopole noise emissions by approximately 10 dB,” the report concludes.
“This relationship is even stronger above 15 kHz, the echolocation frequency range for killer whales.”
While deemed successful in lowering
vessel speed to reduce underwater noise levels in a critical habitat area, the study was not able to document the effect on
orcas because sightings of the mammals were down 70 percent from the previous year. Researchers speculate that “the
poor return of Chinook salmon stocks observed this season is thought to be a significant factor in the reduced inshore presence
results are preliminary, with more details to come. The question remains whether the trade-off between lower sound levels
and longer passage times associated with slower speeds is beneficial to the whales.