Manatee Diving Patterns and Watercraft Collision Risk

http://www.savethemanatee.org/mort.htm

 

It is well known that manatees are threatened by watercraft collisions, but could their diving patterns influence their risk of colliding with a watercraft? Manatees are at a higher risk when they are closer to the surface where the watercrafts are located or when they cannot move away when a watercraft is nearing them.  

            Nine manatees were tagged with GPS and time-depth recorders to study their diving patterns in Tampa Bay, Florida to see how these diving patterns may influence their risk of watercraft collision. The manatees were studied over four winters from December to March of the years 2002 to 2005. Manatees do not do well in the cold so they tend to travel to locations where there is warm water, such as near Tampa Electric’s Big Bend Power Station. Surveys taken in this area have shown that more than 600 manatees congregate in this area to stay warm during the winter months, so this was a prime location to conduct this study. The warm waters in this area range between a no-boat zone and a zone where boating is permitted.

            The researchers found that the manatees spent 49% of their time outside of the no-boat zone of the canal. The mean dive depth of the nine study manatees was 1.09 meters and the deepest dive was 16.2 meters. A dive profile was created that showed that the manatees will take multiple deep dives in a row, but TDR records showed that the manatees spent most of their time near the surface. As stated earlier, manatees are at greater risk of watercraft collision near the surface. They also found that the shorter dives took place most often at night, while the deep dives most often took place during the day.

            Interestingly, the researchers discovered that the manatees were at greater risk of being struck by a watercraft when they were in an area with a seagrass bed. During faster travel the manatees spent less time in the projected striking depth area where they would be more susceptible to being struck by a watercraft. They spent more time in the striking depth are during the night and less time during the day.

            Overall, it was concluded that the manatees spent about 78% of their time in the striking range where they are at the highest risk of a watercraft collision. Since the manatees spend most of their time in shallower water, they are less likely to be able to escape when a watercraft is coming near them. Seagrass beds are often found in shallow waters which is why the manatees are at a higher risk of being struck by a watercraft when they are in the seagrass bed area.

            Manatees seem to occupy shallow areas the most, so when watercrafts are driving through these areas the manatees are at a higher risk of being hit and aren’t able to escape easily. When manatees rest during the day they tend to dive to deeper depths, decreasing their collision risk. Manatee diving patterns do influence their risk of being struck by a watercraft, but the environment in which they occupy also influences their risk.

Edwards, H. H., Martin, J., Deutsch, C. J., Muller, R. G., Koslovsky, S. M., Smith, A. J., & Barlas, M. E. (2016). Influence
of Manatees' Diving on Their Risk of Collision with Watercraft. Plos ONE, 11(4), 1-15. doi:10.1371/journal.pone.0151450