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

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monitoring program requires two key components: a reliable population index and
powerful statistical analysis. Indices are a proxy for population abundance (Caughley
1977; Gibbs 2000; Kindberga et al. 2009) and are based on the premise that systematic
surveys will detect the same proportion of the population over time. Thus, changes in the
number of animals detected reflect changes in population size (Gibbs 2000). To be a good
surrogate to population size, an index must have a positive, linear relationship with actual
abundance (Gibbs 2000; Gibbs et al. 1998; Williams and Thomas 2009), and a constant
detection probability over habitat, sighting conditions and time (Anderson 2001; Gibbs
2000; Gibbs et al. 1998; Thompson 2004; Williams and Thomas 2009).
Detecting trends in abundance depends on statistical power (Gerrodette 1987; Gibbs
2000; Gibbs et al. 1998; Taylor and Gerrodette 1993) and an effective monitoring program
must generate data that can be statistically analyzed to detect trends (Gibbs 2000; Gibbs et
al. 1998). Both the accuracy of the population index and sampling structure – number of
plots, survey frequency, scan duration, number of years – will affect the ability to detect
changes in population abundance. Power analysis, through simulation, is the process used
to determine the statistical power of an index and sampling regime (Gerrodette 1987; Gibbs
2000; Gibbs et al. 1998; Taylor and Gerrodette 1993).
Most sirenian populations are found in developing nations where monitoring funds
are scarce. To be valuable, monitoring methods must be sound, repeatable and inexpensive
(Aragones et al. in press; Aragones et al. 1997; Dick and Hines 2011; Hines et al. 2005;
Williams and Thomas 2009). This paper evaluates a low cost and repeatable boat-based
method for monitoring sirenians. We used this method on West Indian manatees
(Trichechus manatus manatus) in the Drowned Cayes area of Belize, Central America and
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