MARINE MAMMAL COMMISSION - Annual Report for 1995 



mortality for Antillean manatees are poaching for food 

 and entanglement in gillnets, for Florida manatees 

 most human-related mortalities are caused by colli- 

 sions with boats or entrapment in flood gates and 

 navigation locks. For both subspecies, however, the 

 habitat loss due to coastal development may pose the 

 greatest long-term threat. 



As shown in Table 2, the number of manatee 

 deaths recorded annually in the southeastern United 

 States increased substantially between the late 1970s 

 and 1990, when a record 214 dead manatees were 

 found. Although a large number of cold-related 

 deaths (about 45 animals) contributed to the record 

 level in 1990, most of the increasing mortality trend 

 during the 1980s was attributed to increases in the 

 number of vessel-related deaths (i.e., propeller 

 wounds, hull impacts, or crushing by the weight of 

 watercraft hulls) and perinatal deaths (i.e., stillborn 

 and newborn calf deaths for which the cause usually 

 is undetermined). 



During the 1980s vessel-related deaths increased 

 from about 20 to 50 animals per year. Almost all 

 manatee deaths in the southeastern United States occur 

 in Florida and the increase in watercraft deaths 

 paralleled an increase in the number of boats regis- 

 tered in Florida. In response the Florida Department 

 of Natural Resources (now the Florida Department of 

 Environmental Protection), in cooperation with the 

 Fish and Wildlife Service, began a major initiative in 

 1989 to increase boater awareness and develop water- 

 way regulations aimed at reducing collisions between 

 manatees and boats. As described below, work on 

 this initiative has continued. 



Early in the 1990s total annual mortality declined, 

 but in 1994 and 1995 it again increased to near-record 

 levels. In most of these years the number of vessel- 

 related and perinatal deaths continued to be significant 

 factors determining total annual mortality. Other 

 factors, however, including deaths due to natural 

 causes, drowning and crushing in flood gates and 

 navigation locks, and deaths due to undetermined 

 causes, became increasingly important in some of 

 these years. For example, in 1995 total mortality 

 exceeded 200 animals for only the second time, but 

 vessel-related deaths declined from 51 animals in 1994 

 to 43 animals, representing only 21 percent of the 



total mortality. This is the lowest percentage of total 

 annual mortality for the category since 1983. With no 

 cold-related deaths recorded in 1995 and with a 

 decrease in vessel-related deaths, the increase in total 

 mortality between 1994 and 1995 was due mainly to 

 increases in perinatal deaths, which reached a record 

 high, and in deaths from natural causes. 



Since beginning the new efforts to reduce vessel- 

 related deaths in 1989, deaths due to this cause have 

 ranged between 35 and 53 animals per year. In 

 contrast to increases in this mortality category through 

 the 1980s, the overall trend since 1989 has been 

 relatively stable. Considering that boat registrations 

 in Florida have continued to increase, the lack of a 

 further increase in vessel-related deaths may be an 

 early sign that measures being taken by State and 

 Federal agencies are beginning to successfully address 

 this source of mortality. It is too soon, however, to 

 draw definitive conclusions. 



With a record high 56 perinatal deaths in 1995, the 

 steady increase seen before 1989 in this mortality 

 category appears to be continuing. The causes of 

 perinatal deaths are not clearly understood. They 

 could be related to physiological stress due to the 

 species' location at the northern limit of its range, 

 disease and bacterial infections, disruption of physio- 

 logical or biochemical processes by pollution, stress 

 among pregnant and nursing females due to vessel 

 traffic or other human activity, and the inexperience 

 of young females raising their first calves. Other 

 possible factors could be increasing levels of pollution 

 or human-related stress. It also is possible that the 

 perinatal death rate is constant, but that the size of the 

 manatee population has increased or that reporting of 

 carcasses has improved giving the appearance of an 

 increased rate. 



Although small when compared to perinatal and 

 vessel-related deaths, the numbers of manatees killed 

 in flood gates and navigation locks increased in the 

 1990s and reached a record high of 16 animals in 

 1994. In 1995 the number of deaths in this category 

 fell to eight. As discussed below, the increase in the 

 1990s has prompted efforts led by the South Florida 

 Water Management District and the Army Corps of 

 Engineers to reduce this source of mortality, and there 



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