2012 COURSE DATES: AUGUST 4 – 17, 2012 - Sirenian International

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of the individual factors or covariates had a significant influence on the number of
manatees sighted per scan (Table 1).
There was a negative effect between number of volunteers and number of manatees
spotted (Wald Chi-Square = 7.343 df =1, p < 0.007, Table 2). The greater number of
volunteers present on the boat, the fewer number of manatees spotted.
3.2 Trends of manatee counts in the Drowned Cayes
We conducted 960 20-minute scans during 42 survey periods from 2001 - 2007. The
number of manatees sighted per scan ranged from 0 to 6. Overall mean number of
manatees sighted per scan was 0.54 (0.89 SD) (Figure 3). Mean number of manatees per
survey period ranged from 0.21 to 1.08 manatees per scan. Yearly means per scans ranged
from 0.33 manatees in 2002 to 0.71 manatees in 2001 (Figure 4). There was no detectable
change in the mean number of manatees sighted per scan from the first survey period in
2001 to the last survey period in 2007 (Likelihood Ratio Chi-Square = 1.56, df = 1, p =
0.212, B = -0.003, Figure 4).
3.3 Power Analysis
Using a 20-minute scan duration, 28 points per survey period, and 6 periods, we had 4%
power to detect a 10% decline in number of manatees over 15 years within 7 years – our
study duration, 9% power to detect a 25% decline and 22% power to detect a 50% decline.
Extending this study duration to 15 years would give us a 11% power to detect a 10%
decline, 51% power to detect a 25% decline and 100% power to detect a 50% decline
(Table 3; Figure 5a, b, c).
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