Manatees win some and lose some.

New estimates of long-term survival suggest that some populations of the endangered Florida manatee are growing. However, manatees on the state's populous southeastern coast are not increasing, and their survival appears precarious (SN: 3/29/97, p. 191).

Differing patterns of boat traffic and development may explain the variation between regions, report scientists in the April Ecology. Florida's Crystal River and Blue Spring, areas with low speed limits for boats and little development, have growing manatee populations. Regions with weaker conservation measures have seen essentially no growth in manatee populations. "There's a lot of hope for these animals," says coauthor Thomas J. O'Shea, now of the U.S. Geological Survey in Fort Collins, Colo. Manatee-conscious management efforts like those in Crystal River and Blue Spring have paid off, he says, "but these numbers show that it's very easy to tip the balance in a negative way."

The researchers gleaned data from an ongoing project that uses photographs to identify individual manatees. Initially, scientists had used the photos only to learn how often manatee females had calves and how far the adults roamed.

By the mid-1990s, scientists had 20 years' of data documenting manatee births and individuals' survival or demise. Those data, analyzed with recently developed statistical techniques, allowed researchers at the USGS in Gainesville, Fla., and the National Center for Scientific Research in Montepellier, France, to calculate with greater accuracy the probability that an adult manatee would live from one year to the next.

Estimating adult survival rates is essential to calculating population growth rates for large, long-lived mammals, says O'Shea. "We haven't been able to do that very well for manatees up to this point."