Assessment of Sea-Turtle Status and Trends Integrating Demography and Abundance (2010) / Chapter Skim
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4 Abundance and Trends
4 Abundance and Trends
Pages 55-72
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From page 55... ...
Data of those kinds are seldom published except when they are the only estimates early in a time series (Addison, 1997; Seminoff, 2002)
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From page 56... ...
. Counts Assessment of population abundance on nesting beaches may be based on counts of eggs, tracks, nests, and nesting females.
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From page 57... ...
have been counted as representing reproductive effort and nesting females with assumptions of constant nesting success (nests and crawls) and constant clutch frequency (the number of clutches deposited by an individual turtle in a nesting season; Godley et al., 2001)
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From page 58... ...
. That method has been used to provide counts of turtles within a nesting season and to estimate total nesting females in multiple years.
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From page 59... ...
and have provided abundance estimates based on nestingfemale counts over multiple nesting seasons. Modeling abundance on the basis of the identification of nesting females requires minimal additional data on reproductive rates because these rates can be measured as part of the method.
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From page 60... ...
methods in these studies. Types of Sampling Onetime sampling has been used to detect the presence and absence and to approximate the population density of sea turtles in an area, usu ally when there is a potential for harm from human activities, such as channel dredging or explosions (National Marine Fisheries Service, 1991; Clarke and Norman, 2005)
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From page 61... ...
and depend on sampling locations and times chosen by vessel operators. Many sampling efforts to count sea turtles take place at standardized index locations with periodic or haphazard scheduling.
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From page 62... ...
and generally are presented as a combined function of relative abundance and relative mortality (or morbidity)
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From page 63... ...
Aerial surveys have the largest geographic scope, but there are presumed tradeoffs in low detectability and misidentification of species, especially when flight speeds and altitudes favor marinemammal target species rather than sea turtles (Marsh and Sinclair, 1989)
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From page 64... ...
Novel imaging methods developed for other fields of study have the potential for use in aerial and vessel surveys of sea turtles. They would allow both an increase in the proportion of turtles available to be counted and an increase in the recording of observed turtles in a way that would reduce detection bias.
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From page 65... ...
. Modeling Counts and Abundance Estimates Data representing observed turtles are applied most often to mea sures of relative abundance or density by using pointcount methods, striptransect methods (Marsh and Saalfeld, 1989)
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From page 66... ...
However, nonrandom sam pling, as at individual index sites, can be valuable in assessing qualitative annual trends. The problems with the reliability of CPUE to represent relative population abundance are likely to be reduced as multiple cap ture projects are used within a regional metaanalysis.
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From page 67... ...
As with CPUE from individualcapture project locations, CMR can estimate regional population abundance more powerfully if it uses mul tiple capture sites. CMR data collection coordinated within a networked array of sites, including nesting beaches, would provide one of the most detailed and powerful datasets possible for assessments of seaturtle abundance and for measurement of many important demographic rates (Chaloupka and Limpus, 2001; Bjorndal et al., 2005)
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From page 68... ...
• Multiannual nearsaturation tagging of nesting females on the nesting beach provides a straightforward way to count turtles, mea sure recruitment, and estimate survival and reproductive rates, but the required effort is extensive and would be difficult and expensive to main tain throughout a population's range and nesting season for a statistically powerful time series. • Seasonal nest counts require less effort per spatiotemporal unit.
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From page 69... ...
standard ized populationwide track or nest counts with spatiotemporal sampling that could detect biologically significant spatial trends; (2) nest counts in representative index locations and seasons with spatiotemporal sampling over a time series long enough to detect biologically significant spatial and annual trends (e.g., a change of 1% per year)
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From page 70... ...
are not a substitute for abundance measures in index sites. However, broadscale surveys can fit into an integrated network of sampling projects by calibrating counts between wellsampled index sites and poorly sampled sites, by identifying spatial overlap with fisheries and other human activities, and by providing the only possible measure of relative abundance in inaccessible areas.
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From page 71... ...
Assigning abundance to a con ceptual model implies that turtles are identified by their genetic stock and that abundance measures apply to specific life stages. Secondarily recommended for multiple index sites are measures of relative abundance with quantified effort and estimated values for detectability, having rela tiveabundance measures that can be assigned to specific ageclasses of a population's conceptual model.
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