268 



Coastal & Marine Ecosyslfiiis — Our Livinf> Rt'\(iunes 



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76 78 80 82 84 86 88 90 92 

 Year 



Fig. 2. Number of manatee deaths 

 from watercraft collisions and 

 number of Florida registered ves- 

 sels from 1976-93 (data from 

 National Biological Service and 

 Florida Department of 

 Environmental Protection). 



Estimates of manatee reproductive traits are 

 similar across study sites (Table), despite large 

 habitat differences among study areas. There is 

 also agreement in reproductive estimates 

 obtained from salvaged carcasses (Marmontel 

 in press), indicating that Florida manatees have 

 probably achieved a maximum level of repro- 

 duction (O'Shea and Hartley in press). 



Aerial Surveys 



The population of Florida manatees cannot 

 be directly estimated because they are often dif- 

 ficult to .see. They occupy waters that may be 

 turbid or obscured by overhanging branches; 

 they can move long distances between counting 

 areas over a short time; and many environmen- 

 tal factors, particularly temperature, influence 

 their distribution and behavior (Lefebvre et al. 

 in pi'css). 



Three statewide aerial surveys, coinciding 

 with maximum manatee use of winter aggrega- 

 tion sites, resulted in counts of 1.268 (January 

 1991), 1,465 (February 1991). and 1.856 

 (January 1992; Ackerman in press). The differ- 

 ences in these counts are thought to retlect the 

 influence of different environmental conditions, 

 not changes in population size. Manatee pres- 

 ence at winter aggregation sites varies within 

 and between winters, depending upon the pat- 

 tern and severity of winter cold fronts. 



Garrott et al. (1994) developed a population 

 index by using a temperature co\ariate to model 

 a simple linear trend in annual aerial survey 

 data from the winters of 1977-78 through 1991- 

 92. Their analyses showed an increasing trend 

 m the temperature-adjusted counts of 7%-12% 

 annually on the Atlantic coast, but the degree to 

 which these increases are related to true popula- 

 tion growth is unknown. No pronounced tempo- 

 ral trend was detected at the largest aggregation 

 site on the southwest coast. 



While this result seems promising because it 

 shows no evidence for major declines, it is tem- 

 pered by other factors. The number of human- 

 related manatee deaths on the Atlantic coast is 

 more than twice as high as on the gulf coast 

 (Ackerman et al. in press). This fact is reflected 

 in the lower survival rate of adult manatees on 

 the Atlantic coast than at Crystal River and Blue 

 Spring (O'Shea and Langtimm in press). 

 Reynolds and Wilcox (1994) found that the 

 number of calves sighted at winter aggregation 

 sites has decreased since 1982. and that in three 

 recent winters, the percentage of manatees 

 sighted that are calves has also decreased. They 

 note that mortality of calves at or near time of 

 birth is the fastest-growing type of manatee 

 mortality, thus the downward trend in aerial sur- 

 vey calf counts is a cause for concern and fur- 

 ther investigation. 



Recovery Criteria 



Species recovery criteria for the Florida 

 manatee are three-fold: the population trend 

 must be stable or increasing; moilality must be 

 stable or declining; and threats to manatee habi- 

 tat must be under control (USFWS 1989). 

 Better population and life-history data suggest a 

 greater potential for increase and higher num- 

 bers than previously recognized, and strong 

 steps taken by local, state, and federal govern- 

 ments are increasing the number and area of 

 sanctuaries and slow boat-speed zones. These 

 steps may reduce mortality if they are continued 

 and expanded, allowing the population to recov- 

 er more quickly. 



Management has focused on ways to reduce 

 human-related mortality. Of greatest concern 

 has been an increase over the years in the num- 

 ber of human-caused deaths, particularly those 

 caused by collisions with boats (Fig. 2). Boat 

 strikes account for 78% of human-related man- 

 atee mortality and 23% of all documented 

 deaths (Wright et al. in press). A moderate 

 reduction in the number of boat-related deaths 

 in the last 2 years caused optimism; however, 

 watercraft collisions accounted for 49 manatee 

 deaths in 1994, almost matching the record 

 number of 51 in 1991 (Fig. 2). 



Habitat Threats 



Habitat threats are far from under control, 

 however. Florida has one of the fastest-growing 

 human populations in the nation, with an esti- 

 mated net gain of close to 1 ,000 people per day 

 (Fernald et al. 1992). Much growth has 

 occurred along the coast, with inevitable conse- 

 quences for coastal habitats. For example, about 

 a third of the 600,000 ha ( 1.5 million acres) of 

 seagrass meadows present in coastal Florida in 

 the 1940"s no longer exist (Lewis 1987). One of 

 the most important regions for manatees on the 

 Atlantic coast is the Indian River Lagoon. Over 

 the past 20 years, losses of submerged aquatic 

 vegetation in some areas of the lagoon have 

 exceeded 95% (Busby and Vimstein 1993). 

 Submerged freshwater plants have also been 

 affected adversely by increases in turbidity and 

 nutrients. 



Debris, particularly monofilament line, plas- 

 tics, and unattended fishing nets and ropes, 

 directly threatens manatees, who may ingest or 

 become entangled in these materials (Beck and 

 Barros 1991). Manatees are also vulnerable to 

 natural and human-caused catastrophes, such as 

 disease and oil spills, particularly when the ani- 

 mals are concentrated at winter aggregation 

 sites. 



