Baleen whale responses to a high frequency active pinger: Implications for upper frequency hearing limits

Abstract

EXECUTIVE SUMMARYTo test the possibility of using high-frequency pinger tags to track baleen whales on Navy instrumented ranges, three blue (Balaenoptera musculus) and one humpback (Megaptera novaeangliaea) whales were exposed to two high-frequency pingers from a small boat. The pinger frequencies were 37 and 45 kHz, with source levels of 174 and 163 dB re 1 µPa at 1 m respectively, and repetition rates of one ping per second. Estimated received levels varied between 106 and 134 dB re 1 µPa, with a closest approach distance of 100 m. The whales were monitored for at least an hour prior to the exposure to establish their behavioral state, diving and surface durations, and headings, as well as to acclimate them to the presence of the boat. Two of the blue whales were deep foraging during the exposure, while the third blue whale and the humpback whale were traveling with intermittent bouts of possible surfacing feeding or searching for prey. Each exposure lasted approximately 30-40 minutes while the behavior of the whales continued to be monitored, and the whales were observed for an additional two to three surfacing intervals post-exposure to ensure that their behavior continued as normal. None of the blue whales demonstrated any behavioral response, and continued their normal surface behavior, dive patterns, and dive durations. The humpback whale continued its prior travel heading and speed during the pinger exposure but reduced its dive interval times; however, it had been traveling with a foraging mixed species aggregation of birds and dolphins earlier in the day and rejoined this aggregation at this time and the change in behavior was similar to what had been observed prior. Therefore, it is inconclusive whether the humpback whale responded to the pinger or changed its behavior relative to the other animals in the area, although based on observations, the latter appears more likely. These data begin to provide information on the upper frequency limits of baleen whale hearing. Additional testing during different behavioral states and using different frequencies and source levels will contribute further knowledge to this topic.iv