Subject: Manatee:Florida Manatee Population Status (fwd)

Mike Williamson (
Mon, 2 Jun 1997 10:53:01 -0400 (EDT)

---------- Forwarded message ----------
Date: Fri, 30 May 1997 23:45:29 -0700
From: MARMAM Editors <marmamed@UVic.CA>
Reply-To: Marine Mammals Research and Conservation Discussion
Subject: Florida Manatee Population Status (fwd)

Forwarded message:
From: Sirenia Project <>


Submitted by: Lynn Lefebvre, U.S. Geological Survey-Biological
Resources Division, Florida Caribbean Science Center,
Gainesville, Florida

     The total deaths of Florida manatees in 1996 was indeed
staggering: 415, compared with 201 in 1995 (Department of
Environmental Protection, Florida Marine Research Institute).
While the large increase in manatee mortality in 1996 is
certainly cause for concern, I would urge both researchers and
managers to avoid any hasty conclusions based on the mortality
patterns of one year.  A large portion of the added mortality was
clearly associated with the extensive and severe red tide event
that occurred in southwest Florida during late winter and spring
(151 deaths were attributed to the red tide epizootic).  An
unknown portion of the recovered carcasses (82) in the
"undetermined" cause-of-death category could also have been
victims of the epizootic; these animals are generally too
decomposed for cause of death to be identified.  In addition to
the red tide, severe cold during the 1995-96 winter accounted for
17 mortalities, compared to none in the previous, mild winter.

     Red tides, which are caused by dense blooms of the
dinoflagellate Gymnodinium breve, are a recurring phenomena in
southwestern Florida.  Karen Steidinger with the Florida Marine
Research Institute has found anecdotal records that suggest red
tide was noted in Tampa Bay hundreds of years ago.  The first
well-documented manatee mortality event associated with a red
tide was in 1982 (O'Shea et al. 1991), when 39 manatee deaths
were attributed to exposure to G. breve toxin.  Researchers
hypothesized that the toxin was concentrated by tunicates, which
were incidentally ingested by seagrass-grazing manatees.

     Perhaps the most troubling aspect of the 1996 manatee
mortality statistics is that 60 deaths were caused by watercraft
collisions, the highest number yet in a single year.  This was a
substantial increase over watercraft-related deaths in 1995 (42);
moreover, it reversed a promising trend toward declining
watercraft-related mortalities since 1991, when 51 such deaths
were recorded.  Collisions with watercraft are still by far the
leading cause of human-related manatee mortality (approximately
78%), and account for about 25% of all documented deaths (Wright
et al. 1995).  Unless, and until, cold weather and red tides are
under our control, watercraft-craft related mortality is the
primary target for mortality reduction.

     In their recently published paper on manatee population
viability analysis, Marmontel et al. (1997) projected a slightly
negative population growth rate and low probability of
persistence over 1000 years if adult mortality trends are not
reduced.  They recommended that management efforts include
reduction of boating activities and speed in areas occupied by
manatees.  This common-sense approach has met with considerable
controversy in some of the counties charged with the development
of manatee protection plans.  Managers must meet the challenge of
finding innovative, constructive ways to involve the boating
public in the manatee recovery process.  Without public
cooperation and support, the seemingly unavoidable increase in
Florida's human population and numbers of boaters will crush any
hope of achieving a sustainable manatee population in Florida.

     As if cold weather, red tides, and increasing numbers of
watercraft weren't enough to worry about, manatees may soon be
facing another problem: the disappearance or sporadic
availability of warm water at sites north of their natural winter
range.  The proposed deregulation of Florida's power plant
industry may have profound effects on manatee mortality,
distribution, and habitat use.  However, the power-generating
industry has been very responsive to the needs of manatees, and
careful planning should prevent severe impacts on the manatee
population even if deregulation occurs.

     Assessing the outlook for manatees as pessimistic or
optimistic probably has more to do with the personality of the
assessor than the outlook.  I find some cause for optimism
because manatees (1) are one of the most loved species in the
world, (2) are highly adaptable to human-altered environments,
and (3) seem to like us despite all we've done to them and their
habitat.  Helene Marsh (James Cook University, Queensland,
Australia) presented a seminar at the University of Florida last
February, in which she compared and contrasted the life history
strategies of manatees and dugongs, and assessed their chances
for survival in the 21st century.  Noting the manatee's higher
reproductive rates and greater adaptability to urban
environments, she placed her bets on manatees doing better than
dugongs when in close proximity to humans, but dugongs having the
better chance for long-term survival in some of the more remote
regions where they occur.  I will stick with the assessment that
Tom O'Shea and I made (Lefebvre and O'Shea 1995):

     "Population and life-history information suggests that the
     potential long-term viability of the Florida manatee
     population is good, provided that strong efforts continue to
     curtail mortality, habitat quality is maintained or
     improved, and steps are taken to offset potential
     catastrophes [such as oil spills or disease]."

                        LITERATURE CITED

Lefebvre, L.W., and T.J. O'Shea.  1995.  Florida manatees.  Pp.
     267-269 in Laroe, E.T., G.S. Farris, C.E. Puckett, P.D.
     Doran, and M.J. Mac, eds.  Our Living Resources: a report to
     the nation on the distribution, abundance, and health of
     U.S. plants, animals, and ecosystems- Coastal & Marine
     Ecosystems.  U.S. Department of the Interior, National
     Biological Service, Washington, D.C.  530pp.

Marmontel, M., S.R. Humphrey, and T.J. O'Shea.  1997.  Population
     viability analysis of the Florida manatee (Trichechus
     manatus latirostris), 1976-1991.  Conservation Biology
     11(2): 467-481).

O'Shea, T.J., G.B. Rathbun, R.K. Bonde, C.D. Buergelt, and D.K.
     Odell.  1991.  An epizootic of Florida manatees associated
     with a dinoflagellate bloom.  Marine Mammal Science 7(2):

Wright, S.D., B.B. Ackerman, R.K. Bonde, C.A. Beck, and D.J.
     Banowetz.  1995.  Analysis of watercraft-related mortality
     of manatees in Florida, 1979-1991.  Pp. 259-268 in T.J.
     O'Shea, B.B. Ackerman, and H.F. Percival, eds.  Population
     biology of the Florida manatee (Trichechus manatus
     latirostris).  Information and Technology Report 1.  U.S.
     Department of the Interior, National Biological Service,
     Washington, DC.  289pp.

-To submit a message to MARMAM, send it to:
-Please include your name and e-mail address in the body of the
text of all submissions, and ensure your message has a subject
-To subscribe to MARMAM, send a message to:
saying: subscribe marmam Yourfirstname Yourlastname
-To contact the MARMAM editors, write to: