TRENDS IN COUNTS OF FLORIDA MANATEES AT WINTER AGGREGATION SITES

The Florida manatee (Trichechus manatus latirostris) is one of the most endangered marine mammals in U.S. waters. Growth in the human population and consequent coastal development in Florida have led to concerns that the manatee population may be declining. Therefore, we assessed temporal trends in counts of Florida manatees obtained during aerial surveys conducted at winter aggregation sites along the east coast of Florida and near Fort Myers in southwestern Florida. We conducted 3-11 surveys each winter from 1977-78 to 1991-92. We used log-linear models to identify temperature covariates that explained variability in the counts, then adjusted counts for these covariates and determined if there was any temporal pattern in year coefficients. Covariates we considered for inclusion into the model included short-term air and water temperatures and a series of time-lagged temperature variables from National Oceanic and Atmospheric Administration (NOAA) degree-day summations for periods 5-40 days prior to survey date. Although most temperature variables contributed to the model, the most parsimonious general model we could construct for the east coast and Fort Myers counts incorporated a 10-day summation of degree-days. Approximately 25-35% of the variation in the winter counts could be explained using this simple model. An analysis for temporal trends in the temperature-adjusted counts indicated that east coast counts increased steadily from 1977-78 to 1991-92. Analyses of these data for the Fort Myers population were inconclusive with no pronounced temporal trend evident. Because we cannot be certain that counts at winter aggregation sites are indicative of population trends, we caution against accepting the results of the trend analysis unless independent datasets provide reasonable evidence to corroborate this analysis.