using strip-transect aerial surveys to estimate manatee abundance in the ten thousand islands region of southwest florida
Using Strip-transect Aerial Surveys to Estimate Manatee Abundance in the Ten Thousand Islands Region of Southwest Florida
Poster presented April 2003, at the Greater Everglades Ecosystem Restoration Conference
1U.S. Geological Survey, Center for Aquatic Resource Studies, Sirenia Project, 412 NE 16th Ave., Room 250, Gainesville, FL 32601
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Strip-transect aerial surveys have been used extensively in Australia to estimate trends in offshore dugong populations (Marsh and Sinclair 1989; Marsh 1995). 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 ultimate objective is to determine if manatee density and distribution in the nearshore waters of the Ten Thousand Islands and farther south into the Everglades National Park change in response to restoration of natural hydrologic patterns in southwestern Florida. The northern 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 will be a successful tool for determining manatee population trends 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.
Transects flown during a pilot study in 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 prior to the 2001 surveys. Eight surveys were conducted between 15 July and 30 August 2001 and eight surveys 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. Transect lengths ranged from 3.4 to 8.4 km; water area surveyed ranged from 0.79 to 1.53 km_ per transect (Table 1). Two of the transects, number 31 and 32, were completely over water (Fig. 1B). During the 2002 surveys, a subsample of transects were reflown in an attempt to estimate detectability bias and identify plumes sighted on initial flights.
Manatee locations were plotted on navigational charts, and flight paths were recorded on a Garmin GPS III in 2002. 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).
Manatee group locations for all survey dates are shown in Figure 3. The corrected number of manatee groups (a group = 1 or more individuals in the same location) sighted on transects ranged from 12.9 to 27.1 and 15.0 to 20.4 per survey during 2001 and 2002, respectively (Table 1). The corrected number of individuals counted ranged from 15.1 to 61.7 in 2001, and from 26.5 to 54.1 per survey in 2002 (Table 1). Mean group size per survey ranged from 1.1 to 2.3 during 2001 and 1.4 to 2.7 during 2002. Survey-specific population estimates in this study ranged from 58 to 237, or 1.62 to 6.64 per km_ during 2001 and 102 to 208, or 2.85 to 5.81 per km_ in 2002. Annual population estimates from these surveys were 126 + 19.5 (0 + S.E.) in 2001 and 137 + 11.3 in 2002 (Table 2). Reflights of transects on 7 surveys in 2002 yielded higher number of groups on six of the seven surveys. Of the 51 observations made during reflights:
The Faka Union Canal is known to attract large numbers of manatees (Fig. 4), 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 59 to 247 in 2001 (0 =132+20) and 108 to 207 in 2002 (0 =141+11.2). High-density stratum counts accounted for 28 and 30 percent of the total estimated population in 2001 and 2002, respectively.
To assess the potential for detecting statistically significant trends in the Ten Thousand Islands population, we used TRENDS software (Gerrodette 1993). We used a CV of 0.09 based on results from the surveys in 2001 and 2002 (Table 2).
The following parameters were also selected: _ = 0.05; 1-tailed test; linear model of rate of change; CV proportional to the square root of the abundance estimate; standard normal distribution.
If number of sampling periods is 6 per year, sampling is continued for a minimum of 4 years, and CV = 0.09, then power = 0.99.
If number of sampling periods is 8 per year, sampling is continued for a minimum of 4 years, and CV = 0.09, then power = 1.0.
Thus, we should be able to adequately detect an annual rate of change of 10% per year with a minimum of 6 surveys per year if weather and logistics prevent the completion of 8 surveys during the summer survey window (June-September).
The number of sediment plumes observed during the Ten Thousand Island surveys (72 in 2001 and 102 in 2002; Table 2 & Fig. 5) suggests that many manatees may not be directly observable. Observations from transect reflights suggest that more than 25 percent of plumes may represent an actual manatee group. Reflights of selected transects may be useful to develop survey-specific correction factors and for incorporating observed plumes in a total population estimate.
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 and glare. Nevertheless, the strip-transect approach shows promise for monitoring the manatee population using this region during the warm season, if environmental variability can be minimized by selecting survey dates with optimal environmental conditions.
U.S. Fish and Wildlife Service and the U.S. Geological Survey, through the Resource Partnership Program, Big Cypress National Preserve, Cincinnati Zoo, Columbus Zoo, and Save the Manatee Club supported this research. We thank Skip Snow, Everglades National Park, for his guidance and enthusiasm, which helped to launch this study. Completing these surveys was truly a team effort. Backseat observers were Margie Barlas and Lucy Keith of the Florida Fish and Wildlife Conservation Commission, Steve Schulze of Big Cypress National Preserve, Doug Suitor of Collier County Natural Resources Department, and Khabira Al-Muhyee Ettaji of Everglades National Park. Special thanks to Deborah Jansen of Big Cypress National Preserve for securing funding for several flights, loaning us a GPS to record flight lines, and allowing Steve Schulze to help with the flights. Last but not least, thanks to Emilio Echeverria, Gary Roam, Ken Poteet, and Alex Mendez of Speed Aviation for piloting us safely through the flights.
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. 1995. Fixed-width aerial transects for determining dugong population sizes and distribution patterns. Pages 56-62 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.
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SOFIA Project: Predicting Effects of Hydrologic Restoration on Manatees along the Southwest Coast of Florida
U.S. Department of the Interior, U.S. Geological Survey
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