News

Science Sector

By Beth Brady, PhD, Director of Science and Conservation

About 20-25% of manatee deaths are caused by watercraft collisions, and many others bear scars from these encounters. A manatee’s long, flat lungs run along either side of its spine and are surrounded by rib bones. This lung and rib structure makes manatees vulnerable to boat strikes, as broken ribs can puncture their lungs and cause serious internal injuries. Some manatees have been hit multiple times, so much so that researchers identify individual manatees by their scars. So why do manatees keep getting hit by watercraft?

One possible explanation is that manatees may not always hear boats approaching, especially in shallow water where sound behaves differently. Although manatees have excellent hearing—especially in the 6-32 kHz range¹ (human hearing ranges from 0.02-20 kHz)—most boat engines emit lower-frequency noise, around 2 kHz. In shallow areas, these low-frequency sounds can bounce off the surface and seafloor, sometimes canceling each other out through a phenomenon known as Lloyd’s Mirror effect. This interference can make boats harder to detect near the surface—right where manatees often swim, feed, and surface to breathe.

Still, research indicates that manatees do perceive watercraft as a threat and alter their behavior to avoid them. In controlled experiments, manatees moved to deeper water when exposed to underwater boat sounds.² Similarly, tagged wild manatees would change their direction of travel and seek deeper areas when boats approached.³

Some researchers have suggested that manatees may have trouble detecting slow-moving boats because their engines are quieter than engines in fast boats. This idea would suggest that manatees are less likely to react—or react more slowly—to slower boats. However, studies show that manatees respond to both slow and fast boats with about the same frequency. Manatees often reacted a bit later to slow boats because they’re harder to hear, but the slower speed still gives them time to respond once detected.³

Another challenge is that manatees may have difficulty detecting boats coming from behind them. In one study, two manatees were tested on their ability to pinpoint where a sound was coming from.⁴ When sounds came from ahead of them, their guesses were more consistent. But when sounds came from behind, their responses were more scattered, suggesting it was harder for them to tell the direction. This may be partly due to their anatomy: unlike most mammals, manatees have only six neck vertebrae (instead of the usual seven), limiting how much they can turn their heads. Like humans and other animals, manatees are better at localizing sounds in front of them than behind—unless they physically turn their bodies to help figure it out.

Further, background noise can affect a manatee’s ability to hear approaching boats, and this noise varies by location—busier areas tend to be louder. Recent research has found that under normal conditions, a fast-moving boat might be heard 27 seconds before reaching a manatee, but with elevated background noise, that warning time can drop to just 5 seconds. Slower boats typically allow more time to respond—at least 30 seconds—when background noise is low. However, when noise levels are high, that reaction time is sharply reduced, making some situations significantly more dangerous for manatees.⁵

Slowing down in manatee speed zones gives manatees the best chance to avoid boat collisions. Reduced speeds allow both the boater and the manatee more time to see or hear each other and react in time to avoid a collision. In addition to slowing down, boaters can further reduce the risk by wearing polarized sunglasses to better spot manatees, following posted speed zone signs, and staying alert for any signs of manatee presence.

References:

¹Gaspard III, J. C., Bauer, G. B., Reep, R. L., Dziuk, K., Cardwell, A., Read, L., & Mann, D. A. (2012). Audiogram and auditory critical ratios of two Florida manatees (Trichechus manatus latirostris). Journal of Experimental Biology, 215(9), 1442-1447.

²Miksis‐Olds, J. L., Donaghay, P. L., Miller, J. H., Tyack, P. L., & Reynolds III, J. E. (2007). Simulated vessel approaches elicit differential responses from manatees. Marine Mammal Science, 23(3), 629-649.

³Rycyk, A. M., Deutsch, C. J., Barlas, M. E., Hardy, S. K., Frisch, K., Leone, E. H., & Nowacek, D. P. (2018). Manatee behavioral response to boats. Marine Mammal Science, 34(4), 924-962.

⁴Colbert-Luke, D. E., Gaspard, J. C., Reep, R. L., Bauer, G. B., Dziuk, K., Cardwell, A., & Mann, D. A. (2015). Eight-choice sound localization by manatees: performance abilities and head related transfer functions. Journal of Comparative Physiology A, 201, 249-259.

⁵Rycyk, A. M., Bauer, G. B., Wells, R. S., Gaspard III, J. C., & Mann, D. A. (2022). The influence of variations in background noise on Florida manatee (Trichechus manatus latirostris) detection of boat noise and vocalizations. Plos one, 17(5), e0268513.