Tidal Cycle Effects on the Occurrence of the Florida Manatee (Trichechus manatus latirostris) at the Port Everglades Power Plant

Abstract

The seasonal distribution of the Florida manatee (Trichechus manatus latirostris) is influenced predominantly by feeding locations in the summer and proximity to warm-water refuges during colder months. The tidal cycle may further influence distribution through its impact on manatee movement and foraging. Although the importance of tide on distribution and habitat selection has been acknowledged, it has yet to be studied quantitatively with respect to the manatee population in southeast Florida. The purpose of this study was to examine the effect of the tidal cycle on manatee occurrence at the Florida Power & Light (FPL) Port Everglades Power Plant during the winter. Walking surveys were conducted in Port Everglades during manatee season, 15 November through 31 March 2004 to 2009. The number of manatees in four established locations was noted, and the animals were categorized as calf, juvenile, or adult. Water temperature data were also collected at the sample locations. Because many surveys yielded zero manatees observed, data were analyzed using the zeroinflated negative binomial model. Although the results show no correlation between tidal state and total manatee occurrence, they do suggest that the probability of observing a cow/calf pair is greater during high tide when compared to low and midtides (p < 0.05). Total manatee occurrence and the presence of cow/calf pairs were both significantly correlated with water temperature (p < 0.05). These results are in contrast to those from other locations in Florida and are relevant to the optimal timing of manatee surveys to ensure that all animals using a warm-water refuge are observed and included in population estimates.