Estimated Manatee Population and Documented Deaths

Level of Indicator: 6
Type of Indicator: A


Explanation of Indicator
Given its dependence on a high quality marine habitat, and its status as an endangered species, the manatee is important as an indicator of the environmental health of coastal Florida’s nearshore and transitional habitats.

Manatee population counts indicate the number of manatees frequenting Florida's coastal waters. Shifts in manatee population totals may indicate degradation of the coastal environment, as manatees are grazers that feed on nearshore seagrasses and other s ubmerged vegetation. As the food source becomes degraded or less abundant, manatees will suffer.

Manatee deaths are caused by a variety of factors. The Manatee Salvage Database, maintained by the Florida Marine Research Institute (FMRI), delineates six causes of death: watercraft collision, flood gate/canal lock, other human-related, perinatal, oth er natural, and undetermined. Causes of human-related mortality range from propeller wounds (watercraft collision) to being crushed or asphyxiated in flood gates or canal locks; other human-related causes include strangulation in fishing nets or crab tra ps. Perinatal deaths are manatee carcasses found that are less than 150 centimeters (about five feet) in length. Natural deaths have also been attributed to cold stress or prolonged exposure to cold weather that results in malnutrition and other disease s. The undetermined category includes manatee deaths that were verified by the Marine Patrol but in which carcasses were not recovered; thus the causes of death are unknown.

Data Characteristics
SOURCE
Data on manatee populations can be obtained from Dr. Bruce Ackerman, Florida Marine Research Institute, 100 8th Avenue, S.E., St. Petersburg, Florida, 33701, or at (813) 896-8626. Data on manatee deaths are compiled in the Manatee Salvage Data Base, mana ged by the Florida Marine Research Institute. For a summary report of these data contact Dr. Scott D. Wright, Marine Research Institute, 3700 5th Avenue South, St. Petersburg, Florida 33711, or at (813) 893-2904.

ACQUISITION
Both the population and mortality data can be obtained in hard copy format at no cost.

COLLECTION
Manatee count data are collected for the state at irregular time intervals via aerial survey. Manatee mortality data are compiled annually. Cause of death is determined through necropsies performed by the Florida Marine Research Institute.

Data Limitations
Manatee population counts were done in 1976, 1992, and 1995. This irregular collection pattern limits the comparisons that can be made between the counts. Data on numbers of manatees counted are the actual figures, but they should be considered to be un derestimates due to the limitations inherent in an aerial survey. Manatees move freely year-round which makes obtaining an accurate count difficult as well.

The data pertaining to manatee deaths are dependent upon volunteer reporting, so these numbers could be considered underestimates as well. While the data on manatee populations consider the entire state, the data on manatee mortality are broken down by c ounty. Therefore, the total number of manatee deaths for the state is not represented here.

Data Analysis
The number of manatees counted increased between 1976 and 1995 from 737 to 1,822. It is not known whether this is due to actual increases in the population size or better counting techniques. Between 1992 and 1995 a slight decrease (1.8%) in population was documented. This could be attributed to the sampling technique, and it should not be assumed that the drop in the population counted is representative of an actual population decline.

Manatee Population Estimates for the State of Florida

The numbers of documented manatee deaths have followed a generally increasing trend with periodic declines. The changes in the numbers of documented manatee deaths could be attributed to better reporting, an increased population with normal mortality pat terns, or fluctuations in morbidity.

An analysis of the causes of death indicates that flood gates/canal locks and "other" human-related deaths account for only a small percentage (an average of 6.5% over the twenty-year period of record) of the total deaths. The numbers of perinatal deaths have shown a generally increasing trend, while the numbers of other natural deaths have fluctuated greatly. The numbers of undetermined deaths have demonstrated a somewhat decreasing trend, but undetermined causes still accounted for over 23% of the tot al deaths recorded between 1990 and 1994.

Between 1974 and 1994, the single greatest identifiable cause of death was collisions with watercraft. The trend has been one of increase, with an average of 21% of deaths between 1974 and 1984 and an average of nearly 28% of the total deaths between 198 5 and 1994 attributed to watercraft collisions. In addition, some proportion of the undetermined causes of mortality almost certainly includes unidentified manatee collisions with watercraft.

Manatee Mortality in Coastal Counties by Cause of Death

					Cause of Death
		Year	WC	FG/CL	OHR	PNTL	ON	UD	Total
		1974	3	0	2	0	0	2	7
		1975	6	1	1	7	1	12	28
		1976	10	4	0	14	2	28	58
		1977	13	5	5	9	1	76	109
		1978	20	8	1	9	2	37	77
		1979	24	7	7	9	4	21	72
		1980	14	6	1	12	4	19	56
		1981	22	1	4	12	9	64	112
		1982	19	3	0	14	41	34	111
		1983	13	2	5	17	6	28	71
		1984	29	1	1	25	22	41	119
		1985	32	2	3	21	19	35	112
		1986	33	3	1	26	11	43	117
		1987	39	4	2	30	14	20	109
		1988	42	6	4	29	24	22	127
		1989	46	3	5	38	32	40	164
		1990	46	3	4	43	63	40	199
		1991	52	6	6	49	14	39	166
		1992	37	3	9	43	20	44	156
		1993	33	3	6	39	24	32	137
		1994	46	6	5	45	37	38	177
		Total	579	77	72	491	350	715	2,284
WC=Watercraft collision; FG/CL=Flood gate/canal lock; OHR=Other human-related; PNTL=Perinatal; ON=Other natural; and UD=Undetermined Source: Florida Department of Environmental Protection, Florida Marine Research Institute.
1995. Manatee Salvage Data Base. Summary Report.

Number of Documented Manatee Deaths