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July 2004 Mass Stranding of Melon-Headed Whales in Hawai'i

melon headed whales swimming in tight circles
Melon-headed Whales in Hanalei Bay, July 3, 2004
(Peponocephala electra)
Photo: Gretchen Johnson

Executive Summary
On July 3-4, 2004, between 150-200 melon-headed whales (Peponocephala electra) occupied the shallow waters of Hanalei Bay, Kaua'i, Hawai'i for over 28 hours. Attendees of a canoe blessing observed the animals entering the Bay in a single wave formation at 0700 hrs* (local time) on July 3, 2004. The animals were observed moving back into shore from the mouth of the Bay at 0900 hrs. The usually pelagic animals milled in the shallow confined bay and were returned to deeper water with human assistance. The animals were herded out of the Bay with the help of members of the community, the Hanalei Canoe Club, local and Federal employees, and volunteers/staff with the Hawaiian Islands Stranding Response Group beginning at 0930 on July 4, 2004 and were out of visual sight by 1030 hrs.

Only one animal, a calf, was known to have died (on July 5, 2004) following this event. The animal was noted alive and alone in the Bay on the afternoon of July 4, 2004 and was found dead in the Bay the morning of July 5, 2004. On July 7, 2004, a full necropsy, magnetic resonance imaging, and computerized tomography examination were performed on the calf to determine the manner and cause of death. The combination of imaging, necropsy and histological analyses found no evidence of infectious, internal traumatic, congenital, or toxic factors. Although cause of death could not be definitively determined, it is likely that maternal separation, poor nutritional condition, and dehydration contributed to the final demise of the animal. Although we do not know when the calf was separated from the female, the movement into the Bay, the milling and re-grouping may have contributed to the separation or lack of nursing especially if the maternal bond was weak or this was a primiparous calf.

Environmental factors, abiotic and biotic, were analyzed for any anomalous occurrences that would have contributed to the animals entering and remaining in Hanalei Bay. The bathymetry is similar to many other sites within the Hawaiian Island chain and dissimilar to that which has been associated with mass strandings in other parts of the U.S. The weather conditions appear to be normal for this time of year with no fronts or other significant features noted. There was no evidence for unusual distribution or occurrence of predator or prey species, or unusual harmful algal blooms. Weather patterns and bathymetry that have been associated with mass strandings elsewhere were not found to occur in this instance.

This event was spatially and temporally correlated with Rim of the Pacific Exercises (RIMPAC) which is a biennial, sea control/power projection fleet exercise that has been conducted since 1968 and involves U.S. forces and forces from various Rim-of-the-Pacific nations. Official sonar training and tracking exercises in the Pacific Missile Range Facility (PMRF) warning area did not commence until approximately 0800 hrs (local time) on July 3 and were thus ruled out as a possible trigger for the initial movement into the Bay.

However, the six naval surface vessels transiting to the operational area on July 2 intermittently transmitted active sonar [for ~9 hours total from 1315 to 0030] as they approached from the south. The potential for these transmissions to have triggered the whales' movement into Hanalei Bay was investigated. Analyses with the information available indicated that animals to the south and east of Kaua'i could likely have detected active sonar transmissions on July 2, and reached Hanalei Bay on or before 0700 on July 3, 2004. However, data limitations regarding the position of the whales prior to their arrival in the Bay, the magnitude of sonar exposure, behavioral responses of melon-headed whales to acoustic stimuli, and other possible relevant factors preclude a conclusive finding regarding the role of sonar in triggering this event. Propagation modeling suggest that transmissions from sonar use during the July 3 exercise in the PMRF warning area may have been detectable at the mouth of the Bay. If the animals responded negatively to these signals, it may have contributed to their continued presence in the Bay. The U.S. Navy ceased all active sonar transmissions during exercises in this range on the afternoon of July 3, 2004. Subsequent to the cessation of sonar use, the animals were herded out of the Bay.

While causation of this stranding event may never be unequivocally determined, we consider the active sonar transmissions of July 2-3, 2004, a plausible, if not likely, contributing factor in what may have been a confluence of events. This conclusion is based on: (1) the evidently anomalous nature of the stranding; (2) its close spatiotemporal correlation with wide-scale, sustained use of sonar systems previously associated with stranding of deep-diving marine mammals; (3) the directed movement of two groups of transmitting vessels toward the southeast and southwest coast of Kaua'i; (4) the results of acoustic propagation modeling and an analysis of possible animal transit times to the Bay; and (5) the absence of any other compelling causative explanation. The initiation and persistence of this event may have resulted from an interaction of biological and physical factors. The biological factors may have included the presence of an apparently uncommon, deep-diving cetacean species (and possibly an offshore, non-resident group), social interactions among the animals before or after they entered the Bay, and/or unknown predator or prey conditions. The physical factors may have included the presence of nearby deep water, multiple vessels transiting in a directed manner while transmitting active sonar over a sustained period, the presence of surface sound ducting conditions, and/or intermittent and random human interactions while the animals were in the Bay.

*This time is set by reliable observations and a canoe blessing ceremony.

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