Abstract
Long-term data are critical for assessing the status, trends, abundance, and distributions of wildlife populations. However, such data streams are often lacking for protected species, especially highly mobile marine vertebrates. Using five decades of aerial surveys, we assessed changes in marine megafauna on the insular coral reef ecosystem of Guam (Marianas Archipelago in Micronesia). The data allowed estimates of relative abundance, trends, and geographic distributions for several important taxa: sea turtles, sharks, manta rays, small delphinids, and large delphinids. These surveys occurred in 32 years from 1963-2012 amounting to 632 flights lasting 809 hours over a 70.16 km2 area. Over this span, surveyors recorded 10,622 turtle, 1,026 shark, 60 manta ray, 7,515 small delphinid, and 95 large delphinid observations. Since the 1960s, sea turtles increased an order of magnitude (r = 0.07) and sharks decreased five-fold (r = -0.03). Turtle increases were largely restricted to one geographical area, where optimal habitat coincides with low human density and a marine protected area. Shark observations declined proximate to human population centers. Trends for the other taxa were less informative, but each taxon had geographic foci. Protections in the region may be working to recover turtle populations, but failing (or have not yet had sufficient time) to recover overfished shark populations. Long-term analyses of vulnerable marine megafauna in this data-limited region are uncommon, and should be used to guide more focused studies that inform regional management and conservation of these species.
Highlights
Long-term data are important for assessing the conservation status of populations, but they are lacking for many marine species, large vertebrates (Pimm, 1991; Brown et al, 2001; Willis and Birks, 2006; Magurran et al, 2010)
The aerial survey results displayed a variety of patterns in megafauna temporal trends, trend variability, abundance, and spatial distribution over time
Turtle observations increased from 1963 to 2012 (Figure 1A) and varied spatially, with the highest densities occurring along the south, east, and north coasts, in areas having low human density, reefs with coral cover, and either seagrass beds or an marine protected areas (MPAs)
Summary
Long-term data are important for assessing the conservation status of populations, but they are lacking for many marine species, large vertebrates (Pimm, 1991; Brown et al, 2001; Willis and Birks, 2006; Magurran et al, 2010). In the recent green turtle (Chelonia mydas) status review under the U.S Endangered Species Act, for example, only 32 of 462 (7%) global nesting sites had sufficient data to analyze abundance trends (Seminoff et al, 2015). 45 of 87 (52%) species on the IUCN Red List are classified as data deficient (lacking robust data on abundance and distribution) and cannot. Besides assessing status and past trends, such data streams may allow scientists and managers to evaluate and target recovery potentials of historically over-exploited species (Lotze et al, 2011)
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