REVIEW: Slower boat speeds reduce risks to manatees

Abstract

Collisions with boats account for approximately 25% of all documented manatee Trichechus manatus latirostris deaths in Florida. The state of Florida, as well as the US Fish and Wildlife Service and various local governments, have established boat speed limits in areas frequented by manatees as a means of reducing risks. Boat speed limits are believed to reduce risks in 3 primary ways: (1) greater reaction time for the boat operator; (2) greater reaction time for the manatee; and (3) reduced severity of injuries in the event that a manatee is hit by a boat. We review the current research and other available information that forms the conceptual basis for this risk reduction method and also examine the primary objections that have been raised against requiring slower boat speeds. The available information supports the position that reducing boat speeds in specific areas is an appropriate, reasonable, and defensible management action. Additional research on how manatees detect and respond to boats would be very useful, as would further empirical studies assessing the effectiveness of existing boat speed limits.

Audiogram and auditory critical ratios of two Florida manatees (Trichechus manatus latirostris)

“…Gerstein (Gerstein, 2002) presents data from recordings of two boat passes at different speeds and argues that manatees may be less able to detect slow-moving boats compared with fast-moving boats, and suggests that boat impacts may be caused to slow-moving boats. However, most documented boat strikes on manatees are due to fast-moving boats (Calleson and Frohlich, 2007). The audiogram of the manatee supports the idea that in many coastal areas where there is elevated background noise because of biological sound sources, such as snapping shrimp, manatee hearing is limited by background noise.…”

“…Observational studies of behavioral responses to boat approaches show that if a manatee is in shallow water, and a deep-water channel is nearby, the manatee is likely to head towards deeper water (Nowacek et al, 2004). This response may lead to the manatee crossing a boat path, and is supportive of boat slow speed zones in manatee habitat, so that if there is a collision, it is less likely to result in death (Calleson and Frohlich, 2007). (Terhune and Ronald, 1975), bottlenose dolphin (plus signs) (Johnson, 1968), false killer whale (open triangles) (Thomas et al, 1990) and harbor porpoise (asterisks) (Kastelein et al, 2009).…”

Serum Amyloid A (SAA) as a Biomarker of Chronic Infection Due to Boat Strike Trauma in a Free-ranging Florida Manatee (Trichechus manatus latirostris) with Incidental Polycystic Kidneys

“…Up to 97% of manatees in a USGS photo identification database for manatees have multiple scar patterns that indicate nonlethal encounters with propellers and boat skegs (O'Shea et al, 2001). Of the 5,251 necropsy records at the Marine Mammal Pathology Laboratory (from 1974-September 2004), 1,294 (25%) list direct effects of watercraft-related trauma as cause of death (Calleson and Frohlich, 2007).…”

Prevalence and Impacts of Motorized Vessels on Bottlenose Dolphins (<I>Tursiops truncatus</I>) in the Indian River Lagoon, Florida

“…The creation, enforcement, and compliance of slow speed zones within critical manatee habitats have reduced the number of deaths of Florida manatees dramatically (Laist & Shaw, 2006;Calleson & Frohlich, 2007). A similar decline in injuries would be expected for dolphins if critical habitats are defined and protected.…”