On April 1 - 4, 2002, an invited panel of population biologists, manatee scientists, and other interested parties gathered in Gainesville, Florida to, among other things, review and assess manatee research efforts to date and shed light on the status of the state’s manatee population. Sponsored by the U.S. Geological Survey’s Florida Caribbean Science Center, the U.S. Fish & Wildlife Service, the Florida Fish & Wildlife Conservation Commission, the U.S. Marine Mammal Commission, and others, the workshop was especially timely as state managers are presently re-evaluating the endangered status of the manatee as a result of a petition filed by the Coastal Conservation Association, a saltwater angler’s group. The U.S. Fish and Wildlife Service could also consider downlisting manatees in the near future.
Rather than answering the question, “Is the manatee population growing, stable, or declining?,” the workshop raised more questions and uncovered serious gaps in research efforts to date.
For example, researchers don’t yet know what the reproductive health of the overall population is. Humans have drastically altered manatee habitat in Florida and the manatee’s reproductive rate may react very quickly to habitat changes. Studies of dugongs, another sirenian found in the Indian and Pacific Oceans, show a dramatic decrease in fecundity (a measure of reproductive ability) after habitat alteration (loss of seagrass).
Also discussed at the workshop was the lack of data on the age structure and sex ratio of the living population, including how many calves are born each year and what percentage they are of the entire population. Panelists emphasized that full life histories, including information on different segments of the population, need to be obtained. Most importantly, panelists stated the goal of recovery should be a sustainable, viable population of manatees enduring for the long term. In addition, they felt that while evaluating the manatees’ past status is important, there was a need to emphasize the continuing and growing threats to the manatees’ long-term survival and how these threats could be controlled.
Further, Meghan Pitchford of the Florida Fish and Wildlife Conservation Commission reported finding that of the female manatee carcasses that have been aged, 73% did not live long enough to produce more than one calf, and only 20% lived long enough to produce more than two calves. Excluding the 0-1 year age class, the average age of recovered carcasses is 7.7 years. Although scientists cautioned that age and sex structure of dead manatees may not indicate the age and sex structure of the living population, it is apparent many manatees have died well before the age researchers believe they are biologically capable of living.
The distinguished panel of scientists did not recommend using annual synoptic surveys or power plant aerial surveys for any population assessment studies. Instead, the panel suggested significantly reworking the aerial survey design or using a multitude of other methods to answer complex population trend questions. This is very important because manatee protection opponents have stated the manatee population has grown, basing their opinion largely on the higher number of manatees counted during the 2001 statewide synoptic survey, and they continue to use synoptic survey data inappropriately as a method to assess population trends!
We came away from the workshop with an overriding conviction that it is just too early for federal and state agencies to consider changing the manatees’ endangered status. Save the Manatee Club’s position is unchanged: as long as threats to the manatees’ long-term survival and manatee mortality from human causes continue to climb, we have not solved the problem. Presently, manatees continue to face risks that are likely to only increase, necessitating the implementation of additional protection measures now.
Special thanks to Dr. Daryl Domning, Dr. Daniel Odell, Patti Thompson and Suzanne Tarr, who helped with this article.
