Longevity & Causes of Death

Manatee

Longevity & Causes of Death

Longevity

  1. Manatees probably live a maximum of about 50 to 60 years.

Manatees probably live a maximum of about 50 to 60 years.

Aging

  1. A consequence of manatee tooth replacement is that scientists cannot use the teeth to age individual manatees, a procedure commonly used for many other species of marine mammals. Scientists instead use a method of aging manatees based on studying growth patterns that develop on ear bones.

Predators

  1. Although predation has not been documented, scientists believe sharks, alligators, or crocodiles may occasionally attack West Indian manatees.
  2. Crocodiles and sharks sometimes prey on West African manatees.
  3. Jaguars, caimans, and sharks prey on Amazonian manatees.

Causes of Death

  1. Natural causes.
    • Manatees are very susceptible to cold weather. It is not unusual for many to die during extremely cold weather. In 1996, at least 17 died due to cold related illnesses. During an exceptionally harsh winter in 1990, 46 manatees succumbed to the bad weather.
    • Power plants create warm-water discharges that are
      sought by manatees.

    • Amazonian manatees become concentrated in confined areas when lakes dry up. They are susceptible to starvation and predation.
    • As in any animal population, a variety of diseases and parasites can be responsible for manatee deaths.
    • In early 1996, about 150 manatees died in southwest Florida. Scientists determined the mass mortality was caused by a red tide organism called Gymnodinium breve which produces toxins. The manatees died after ingesting or breathing in the toxins which attacked their organs (Luoma, 1996).
  2. Human-related causes.
    • Watercraft accidents.
      • More than 770,000 boats were registered in Florida for the 1995-96 season.
      • Manatees are sometimes difficult for boaters to see in the water.

      • Due to their dark color and because they are slow swimmers, manatees are difficult to see in the water. At least 20% to 22% of manatee deaths in Florida occur from watercraft accidents yearly. Collisions with boats and barges is the largest human-related factor causing manatee deaths in Florida. (U. S. Fish and Wildlife Service, 1995).
      • Although the engine propellers are dangerous, collisions with boats cause the most watercraft related deaths. In 1995, 12 manatees expired from propeller injuries while 25 died from impact wounds. Collisions with boats cause life-threatening internal injuries that are often hard to locate and treat.
      • Only 5% of 400 manatees surveyed had head injuries from boating accidents while more than 70% had wounds on their upper backs. This suggests that manatees try to dive away from boats but cannot escape in time to avoid injuries (Bloch, 1993).
    • Manatees become entangled in fishing and crab trap lines,
      sometime resulting in serious wounds.

      This manatee's flipper became entangled in a crab
      trap and will require medical treatment.

    • Flood gates and navigation locks.
      • As manatees swim upstream, they may encounter flood gates and navigation locks. When flood gates are left partially open, manatees sometimes swim through them. They may become trapped in the water current from the upstream side and drown. The remote-controlled flood gates have also crushed manatees.
      • Entrapment in flood control gates and navigation locks regularly kill manatees and is the second leading human factor for manatee deaths. A record 16 manatees died from these devices in 1994. From 1976 to 2000, 166 manatees were confirmed dead from flood gates and navigational locks in the state of Florida.
        • To help the problem, engineers are installing pressure-sensitive mechanisms on flood gates known to cause the most manatee deaths. The cost of each sensor is about $50,000 (U. S. Fish and Wildlife Service, 1995).
      • Poaching and vandalism.
        • In spite of the fact that manatees are legally protected, poachers and vandals are also responsible for manatee deaths. In some areas manatees are hunted for food. Others are victims of harassment.
      • Accidental entanglement and pollution.
        • Florida manatees have been found with fishing lines or crab trap lines wrapped around them. These materials can wrap tightly around the manatee's flippers, causing serious infections, amputations, or death.
        • In a survey of 439 manatee carcasses, 14.4% had debris in their stomach.
      • Habitat destruction.
        • Since the 1950s destruction of the natural habitats of many coastal animals is accelerating at an alarming rate. Much of the suitable manatee habitat in Florida (seagrass beds, salt marshes, and mangrove areas) has been destroyed.
        • The human population in Florida increases by nearly 1,000 people daily.
        • About 250,000 acres of forests are lost every year in Florida and roughly one-fourth of the state remains semi-natural.
  3. Death increases.
    • All total, a record 415 manatees died in 1996. Even when the 150 manatees that died from the red tide outbreak were subtracted, it was still the worst year for manatee deaths ever recorded in Florida. The total mortality rate for Florida manatees has risen an average of 5.3% every year since 1976.