Chapter 11 — Species of Special Concern 



As noted above, death from interactions with boats 

 is one of two principal threats to Florida manatees. 

 Vessel-related deaths have reached record levels in 

 five of the past seven years and appear to be the result 

 of dramatic increases in vessel traffic. In 1960, the 

 number of registered vessels in Florida was about 

 100,000; in 1990, the number exceeded 700,000. 

 Whereas known vessel-related manatee deaths aver- 

 aged 22 percent of total known mortality from 1978 

 to 1983, they accounted for 27 percent from 1984 

 through 1986. Since 1987, vessel-related deadis have 

 been responsible for 29 percent of the total mortality 

 (31 percent if the unusual cold-related death of 47 

 animals early in 1990 is excluded). 



Increases in perinatal deaths {i.e., stillborn and 

 newborn calves) parallel those of vessel deaths. 

 Previous records have been equaled or exceeded in six 

 of the past seven years. Perinatal deaths averaged 14 

 percent of the total known mortality from 1978 to 



1983, 20 percent from 1984 through 1986, and 24 

 percent since 1987. 



The cause of the increase in perinatal deaths is 

 uncertain and may be due to a combination of factors 

 including contaminant pollution, disease, or environ- 

 mental changes. It also may be related to vessel 

 traffic. That is because some newborn calves may die 

 when their mothers are killed or seriously injure by 

 boat collisions, when they become permanentiy 

 separated from their mothers while dodging intensive 

 boat traffic, or when stress from vessel noise or traffic 

 induces premature births. 



In any case, whereas vessel-related and dependent 

 calf deaUis together accounted for about one-third of 

 the total known mortality in the late 1970s and early 

 1980s, it has accounted for more than 50 percent of 

 total mortality in recent years. Although a reliable 

 measure of population trends has proven elusive, it is 



