Strip-transect aerial surveys have been used extensively in Australia to estimate trends in offshore dugong populations (Marsh and Sinclair, 1989). The use of strip-transect methods in estimating manatee population size and trend, however, has been limited (Miller et al. 1998). Manatee surveys have typically not been designed to sample quantified survey areas, or to produce estimates of abundance. While useful in obtaining minimum manatee counts and distribution information, the latter surveys do not permit statistical comparison of survey results over time (Lefebvre et al., 1995). Our objective in this study is to determine if manatee density and distribution in the nearshore waters of the Ten Thousand Islands and the Everglades National Park change in response to restoration of natural hydrologic patterns in southwestern Florida. The Ten Thousand Islands region is of particular interest because of proposed changes to the Southern Golden Gate Estates and Faka Union Canal drainage. We want to statistically compare pre- and post-restoration indices of manatee abundance. We also believe that strip-transect methods are likely to be successful in the Ten Thousand Islands region, unlike many other regions of Florida, in which manatees may be highly aggregated at winter sites, or their density may be too low and distribution too linear to permit this approach.

Six surveys were conducted between 25 July and 22 October 2000, eight were conducted between 15 July and 30 August 2001, and eight were conducted between 20 June and 17 September 2002. We established parallel transects, 1 km apart, with a survey strip width of approximately 250 m. Transects flown during July-October 2000 were oriented perpendicular to shore, between Palm Bay and the Ferguson River (fig. 1A). Based on results from these surveys, we omitted 5 transects (26-30; fig. 1A) and established 5 new transects (31-35; fig. 1B) near Cape Romano for the 2001 and 2002 surveys. Transect lengths ranged from 6.6 to 8.4 km in 2000 and 3.4 to 8.4 km in 2001 and 2002, respectively; water area surveyed ranged from 0.79 to 1.53 km² per transect in 2000 and 0.83 to 1.53 km² per transect in 2001-2002. Manatee locations were plotted on topographic maps, and flight paths were recorded on a Trimble Basic Plus GPS. Surveys were conducted from a Cessna 172 at an altitude of 153 m, traveling at approximately 120-140 km per hour. Perception bias, which occurs when some of the manatees visible within a strip transect are missed by an observer, was estimated by applying a Petersen mark-recapture model to counts made by two observers (Pollock and Kendall 1987; Marsh and Sinclair 1989). We did not attempt to develop a correction factor for manatees that were not visible within the transects (availability bias), thus our results are underestimates of actual manatee numbers and densities.

Figure 1. Spatial arrangement of 30 manatee aerial survey strip-transect polygons in the Ten Thousand Islands during flights July-October 2000 (A) and July 2001-September 2002 (B). [click on images above for larger version]

The corrected number of manatee groups (a group = 1 or more individuals in the same location) sighted on transects ranged from 7.0 to 25.7, 12.9 to 27 and 15.0 to 20.4 per survey during 2000, 2001, and 2002, respectively. The corrected number of individuals counted ranged from 10.0 to 39.8 in 2000, 15.1 to 61.7 in 2001, and 24.6 to 61.2 per survey in 2002. Mean group size per survey ranged from 1.0 to 2.0, 1.1 to 2.3, and 1.4 to 3.0 during 2000, 2001, and 2002 respectively. Survey-specific population estimates in this study were 1.09 to 4.57 per km² in 2000, 1.62 to 6.64 per km² in 2001, and 2.65 to 6.58 per km² in 2002. Excluding the Cape Romano transects, the overall distribution of sightings was somewhat bimodal, with average (mean = 0.44 groups per transect) or higher than average number of groups sighted on transects 1-9 and 17-21 during 2000. Transect 6 starts near the mouth of the Barron River, and Transects 19 and 20 start near the mouth of the Faka Union Canal. Virtually no manatee sightings were made on transects 25-30, at the western end of the study area during 2000. The replacement of these five transects with five transects near Cape Romano (31-35) in 2001 produced a somewhat higher estimate of manatee abundance in the region. The Faka Union Canal is known to attract large numbers of manatees, particularly in the winter, presumably because of the availability of freshwater at its head and thermal buffering provided by its depth. In this study, we considered the canal to be a separate, high-density stratum, analogous to the "hot spots" described by Miller et al. (1998). When manatee counts from this stratum were added to the transect-based estimates, estimates for the whole study area on all dates ranged from 39 to 187 in 2000, 59 to 247 in 2001, and 95 to 235 in 2002.

Population estimates and densities in this study were similar to those for the Banana River, an important area for manatees on the Atlantic coast in the warm season. The latter estimates ranged from 112 to 209, or approximately 0.67 to 1.26 per km² (Miller et al. 1998). Mean group size per survey in Ten Thousand Islands (1.62) was lower than in Banana River surveys (2.19). Group size was 2.00 in 18 of the 22 Ten Thousand Island surveys and 2.00 in 13 of 15 Banana River surveys (Miller et al. 1998). These findings suggest that poorer water clarity in the Ten Thousand Islands than in the Banana River, where the bottom can be seen in most of the survey area, may contribute to greater variability and smaller observed group size in our surveys.

To assess the potential for detecting statistically significant trends in the Ten Thousand Islands population, we used the TRENDS software (Gerrodette, 1993) with estimated CVs of 0.30 and 0.15, based on observed survey results. We determined that we would need a minimum of eight surveys per year for a minimum of 4 years to detect an annual rate of change of 10 percent per year. Variation in group size and population estimates is a reflection of the challenging survey conditions presented by the Ten Thousand Islands, as well as additional variability caused by weather. Nevertheless, the strip-transect approach shows promise for monitoring the manatee population using this region during the warm season, if weather-related variability can be minimized.

REFERENCES

Gerrodette, T. 1993, TRENDS: software for a power analysis of linear regression. Wildlife Society Bulletin 21:515-516.

Lefebvre, L.W., B.B. Ackerman, K.M. Porter, and K.H. Pollock, 1995, Aerial survey as a technique for estimating trends in manatee population size-problems and prospects. Pages 63-74 in T.J. O'Shea, B.B. Ackerman, and H.F. Percival, editors. Population biology of the Florida manatee: U.S. Department of the Interior, National Biological Service Information and Technology Report 1.

Marsh, H., and D.F. Sinclair, 1989, Correcting for visibility bias in strip transect aerial surveys of aquatic fauna: Journal of Wildlife Management 53:1017-1024.

Miller, K.E., B.B. Ackerman, L.W. Lefebvre, and K.B. Clifton, 1998, An evaluation of strip-transect aerial survey methods for monitoring manatee populations in Florida: Wildlife Society Bulletin 26(3):561-570.

Pollock, K.H., and W.L. Kendall, 1987, Visibility bias in aerial surveys: a review of estimation procedures: Journal of Wildlife Management 51:502-510.

Contact: Easton, Dean. U.S. Geological Survey, Center for Aquatic Resources Studies, Sirenia Project; 412 NE 16^th Ave., Rm. 250; Gainesville, FL., Phone: (352)372-2571, FAX: (352)374-8080, Dean_Easton@usgs.gov

(This abstract was taken from the Greater Everglades Ecosystem Restoration (GEER) Open File Report 03-54)

Back to Project Homepage