Decreasing annual nest counts in a globally important loggerhead sea turtle population

Abstract

The loggerhead sea turtle (Caretta caretta) nests on sand beaches, has both oceanic and neritic life stages, and migrates internationally. We analyzed an 18-year time series of Index Nesting Beach Survey (Index) nest-count data to describe spatial and temporal trends in loggerhead nesting on Florida (USA) beaches. The Index data were highly resolved: 368 fixed zones (mean length 0.88 km) were surveyed daily during annual 109-day survey seasons. Spatial and seasonal coverage averaged 69% of estimated total nesting by loggerheads in the state. We carried out trend analyses on both annual survey-region nest-count totals (N = 18) and annual zone-level nest densities (N = 18 x 368 = 6624). In both analyses, negative binomial regression models were used to fit restricted cubic spline curves to aggregated nest counts. Between 1989 and 2006, loggerhead nest counts on Florida Index beaches increased and then declined, with a net decrease over the 18-year period. This pattern was evident in both a trend model of annual survey-region nest-count totals and a mixed-effect, "single-region" trend model of annual zone-level nest densities that took into account both spatial and temporal correlation between counts. We also saw this pattern in a zone-level model that allowed trend line shapes to vary between six coastal subregions. Annual mean zone-level nest density declined significantly (-28%; 95% CI: -34% to -21%) between 1989 and 2006 and declined steeply (-43%; 95% CI: -48% to -39%) during 1998-2006. Rates of change in annual mean nest density varied more between coastal subregions during the "mostly increasing" period prior to 1998 than during the "steeply declining" period after 1998. The excellent fits (observed vs. expected count R2 > 0.91) of the mixed-effect zone-level models confirmed the presence of strong, positive, within-zone autocorrelation (R > 0.93) between annual counts, indicating a remarkable year-to-year consistency in the longshore spatial distribution of nests over the survey region. We argue that the decline in annual loggerhead nest counts in peninsular Florida can best be explained by a decline in the number of adult female loggerheads in the population. Causes of this decline are explored.

Bahamas connection: residence areas selected by breeding female loggerheads tagged in Dry Tortugas National Park, USA

“…Endangered Species Act. Consistent interactions with fisheries and nest number declines at major rookeries throughout Florida instigated a proposal to raise the level of protection for loggerheads in the USA [24,25]. Recently, nine distinct population segments of loggerheads were designated as endangered or threatened [26], with proposals following for critical habitat [27,28].…”

Modeling and mapping isotopic patterns in the Northwest Atlantic derived from loggerhead sea turtles

“…Individual females appear to show fidelity to both nesting and feeding areas throughout their adult life (Miller et al 2003, Broderick et al 2007, Tucker et al 2014. NWA loggerheads are well studied at nesting beaches (Ehrhart et al 2003, Witherington et al 2009) and on some neritic foraging grounds used by adults and juveniles (e.g., Ehrhart et al 2007, Epperly et al 2007, BraunMcNeill et al 2008, Eaton et al 2008. NWA juveniles generally mimic adult female migratory behavior, encompass the same geographic areas (i.e., Read 2007, Mansfield et al 2009), and exhibit similar fidelity to foraging grounds (Avens et al 2003, McClellan andRead 2007).…”

Assignment of nesting loggerhead turtles to their foraging areas in the Northwest Atlantic using stable isotopes

“…Concern was raised when nesting activity in one of these NWA populations declined markedly from 1998 to 2007 (Witherington et al 2009); however, an increase in nesting numbers has been reported in recent years (Van Houtan and Halley 2011). Anthropogenic threats (Jackson et al 2001, Witherington et al 2009, Finkbeiner et al 2011) and changing oceanographic conditions (Chaloupka et al 2008, Saba et al 2008, Van Houtan and Halley 2011 have been proposed as the main drivers of fluctuations in sea turtle abundance. Because these factors may change depending on geographic location (Kot et al 2010 and references therein), efforts to identify foraging grounds of sea turtles are vital to understand spatial and temporal fluctuations in nesting numbers.…”