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

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3.4 Power Analysis of sampling design
The highest power, i.e. 16%, 71% and 100% after 15 years, is achieved for all three decline
rates when 28 points with a high (>30%) probability of sighting a manatee are surveyed 8
times per year (Table 3). This survey design has a 90% power to detect a 50% decline by
the 11 th year (Figure 5c). The design with the least power, i.e. 8%, 34% and 99% after 15
years, surveys just 21 points, regardless of the probability of sighting a manatee, 6 times a
year (Table 3). Overall, survey protocol changes had little impact on our ability to detect
slight declines over 15 years, but did impact our ability to detect moderate and precipitous
declines (Figure 5a, b, c) over that time horizon. Increasing the number of scan points from
21 to 28 had about the same effect as increasing the scan duration from 20 to 30 minutes
(about a 10% increase in power to detect moderate declines). Increasing the number of
survey periods from 6 to 8 resulted in an increase in power from 57% to 71% in the
moderate decline scenario (Table 3). Differences in power across designs are not observed
before 10 years of survey in the case of slight declines, 8 years for moderate declines, and 6
years for precipitous declines (Figure 5a, b, c). All designs show the same basic result:
only precipitous declines can be detected with a 100% certainty on a 15 year time horizon
(Figure 5a, b, c).
4. Discussion
Using 30-minute point scans from a small boat platform is an effective and repeatable
method for monitoring manatees in typical manatee habitats around the world. Several
authors debate the validity of population indices in comparison to population estimates - the
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