Wild blacktip sharks turn away from low-frequency sound up to 74 meters from the source

I would defend the control tone before any other part of this design. It sits at 12.5 times the upper edge of the highest test band ^[20], and it came out of the same speaker on the same rig as the test tones ^[3]. A shark reacting to the equipment, or to the act of playback, should have reacted to the control as well. None did ^[7].

The distance figure needs one correction. The farthest response, 74 m, is 243 feet ^[8]. The release opens with "nearly 250 feet (76 meters)" ^[10], 2 m beyond the measured maximum ^[19].

Distance is also a property of this speaker at this volume. The sounds were played at high intensity to startle the sharks ^[4]. Calibrated hydrophones mapped sound levels at different distances so the team could calculate what each shark heard at the moment it responded ^[5]. That received level is the number another researcher could carry to a different sound source. The release reports the far-field share of responses ^[9] but does not give the received levels, or the count of sharks and responses behind the percentage.

The far-field responses are what interest the researchers. "What makes this finding particularly interesting is that the sharks were responding to sounds beyond the acoustic near field, where the sound behaves differently than it does close to the source," said Stephen Kajiura, the senior author and a professor of biological sciences at Florida Atlantic University ^[11][18]. Sharks lack the gas-filled swim bladder that helps many bony fish detect sound pressure. They are thought to rely on inner-ear structures, including the macula neglecta, that sense movement and vibration ^[13]. Kajiura said the result "suggests that they are detecting the particle motion associated with sound even at considerable distances from the source" ^[12].

The open-water setting made the measurement possible. Blacktips gather in large numbers along the Palm Beach County coast each winter ^[16]. "Their abundance and accessibility made it possible for us to observe them from above without disturbing their natural behavior, while also presenting controlled underwater sounds," Kajiura said ^[15]. A drone filmed the sharks from 40 to 50 m up, and frame-by-frame analysis measured each response distance and heading change ^[6]. The speaker drifted up to 19 m from the anchored boat to limit the boat's influence ^[2]. According to the release, working in the wild avoids the wall reflections and other limitations of laboratory tanks ^[14].

The release calls the work the first quantified evidence that free-swimming sharks detect and orient away from sound in the acoustic far field ^[17]. On the method described, that claim about hearing holds. A loud, low-frequency source changed the heading of blacktips at up to 74 m ^[8]. Saying that human-made noise changes shark behavior at such ranges is a larger claim. It would need received-level thresholds and a stimulus closer to that noise, and this playback was designed to startle ^[4]. The behavior the release describes measuring is response distance and change in swimming direction ^[6].