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OMI Completes a Five Year
Experimental
Vessel Impact Study on Humpback Whales
The Ocean Mammal Institute recently completed the first phase of a long term experimental study on the impact of vessel traffic on the Hawaiian humpback whales. This is the first experimental study done to assess the effects of boat traffic on endangered whales.
Method
We first documented the behaviors of a pod of whales when there were no boats around. Then we radioed one of our five experimental vessels to approach the pod at 10 knots up to the 100 yard approach limit. The vessel approached and then left the area while our shore-based team recorded the movements and behaviors of the whales. We could then statistically compare the whales' behaviors before and after the boat approached them. We also compared the whales' behaviors in response to the recorded engine decibel levels of the five boats we used to approach the whales. We had five experimental impact trials with each vessel.
Results
Data analysis showed that the loudness of the boat's engine and the rate of change in noise level significantly affected the whales' swimming speed. Whales swam three times faster around the loudest boat which had two 200 hp outboard engines which had an intensity level slightly above 120 decibels when recorded at 100 yards. They swam twice as fast around a second boat with two 200 hp inboard diesel engines which peaked at slightly over 120 decibels at 2000 Hz. Perhaps the sudden, 120 decibel whine at 2000 Hz resulted in the more rapid swimming around this boat. Other researchers have noted that whales appear to respond to rate of change in noise level.
Our results indicate that if the boat engine noise gets above 120 decibels at any frequency, the whales swim significantly faster. This result is consistent with other studies showing that whales tend to avoid sounds of 120 decibels. For example, gray whales began to avoid playback sounds at 110-120 decibels and 50% of the animals were showing avoidance at 117-123 decibels (Malme et al., 1988).
The whales spent a significantly greater percentage of time underwater when the boat with the quietest engine approached them. The whales' longest down time was also around this boat. If the whales are less aroused around quiet boats, they wouldn't have to breathe as frequently as they do around louder, more stressful engine noise.
In another part of this study, we approached pods of humpback whales, put a hydrophone in the water and attempted to record their vocalizations or social sounds. These social sounds are different from the complex songs produced by male humpback whales. Our findings indicate that the type of boat and engine used to approach the whales to record social sounds had an impact on the number of pods producing social sounds. When we approached in a 15 ft. inflatable with a 25 hp engine, we recorded social sounds from 24% of the pods. When we approached in a 20 ft inflatable with two 75 hp engines, we recorded social sounds in 3.5% of the pods and when we approached in a 24 ft. boat with a 225 hp engine, 4.2% of the pods produced social sounds.
These studies show that whales' swimming speed and amount of time underwater is affected by the noise level of boats that approach them. In addition, the probability of social sound emission is affected by the type of boat and engine used to approach the pod.
References
Malme, C. L., Wursig, B., Bird, J. E. and Tyack, P. (1988). Observations of feeding gray whaleresponses to controlled industrial noise exposure. In: Sackinger, W. M. et al (Eds) Port and Ocean Engineering Under Arctic Conditions. Volume II. University of Alaska, Fairbanks, AK Geophys. Inst