Inter-annual variability in breeding census data across species and regions

Abstract

There is an intense interest in long-term trends of species abundance that may reflect, for example, climate change or conservation actions. Less well studied are patterns in the magnitude of inter-annual variability in abundance across large spatial scales. We collated abundance time-series for 133 nesting sites across the globe of the seven sea turtle species. Inter-annual variability in nest numbers was lowest in loggerhead turtles (Caretta caretta), Kemp’s ridley (Lepidochelys kempii) and flatback turtle (Natator depressus) and highest in green turtles (Chelonia mydas), likely reflecting their lower trophic position compared to other species and hence tighter coupling of food availability to environmental conditions each year. The annual number of nests in green turtles could vary by 60-fold between successive years. We identified regional patterns in the magnitude of inter-annual variability in green turtle nest numbers, variability being highest for nesting beaches around Australia and lowest in the western Indian Ocean and equatorial Atlantic. These regional patterns are likely linked to corresponding patterns of environmental variability with, for example, areas subjected environmental extremes as part of the El Nino Southern Oscillation (ENSO) showing high inter-annual variability in nest numbers.

Hidden demographic impacts of fishing and environmental drivers of fecundity in a sea turtle population

“…In our models, environmentally linked fecundity parameters explained increasing nesting trends in the context of high bycatch estimates better than our current scenario model (Figure 4). Nest counts for most sea turtle populations worldwide show major interannual variability, and species foraging at lower trophic positions could be more tightly coupled to environmental variation (Hays et al., 2022). Loggerhead turtles nesting in Cabo Verde are primarily oceanic foragers foraging at a low trophic position (Cameron et al., 2019; Cardona et al., 2017; Eder et al., 2012; Hawkes et al., 2006) and probably experience major changes in food availability coupled to variable upwelling rates (Benazzouz et al., 2014).…”

“…In fact, individual simulations from this model scenario (Appendix S5) suggest that nest numbers could increase to 100,000 per year with annual variability in the short term while the adult population is still in long‐term decline. Estimating female population size from variable nest numbers alone is therefore problematic and has the potential to be misleading in the short term because it has very little power to detect long‐term trends (Hays et al., 2022). A deeper understanding of the environmental dynamics driving nesting variability (e.g., Neeman et al., 2015) might allow calibration of observed nest numbers with environmental data to infer adult population sizes.…”

Effect of body size on the long-term reproductive output of eastern Atlantic loggerhead turtles Caretta caretta

“…Nest counts have increased recently across the archipelago, and during the 2018 nesting season, the number of nesting loggerhead turtles at Cabo Verde tripled in comparison to the previous year (2017), which was a record year for the NN registered (Marco et al 2012a, 2018b, Laloë et al 2020, Patino-Martinez et al 2022). This could be interpreted as evidence of an increased population size, but caution is needed because increased NN might also result from changes in the RI and the total number clutches laid per season, even if population size remains unmodified (Hays et al 2022). Actually, the loggerhead turtles nesting on Boa Vista forage in the Canary Current upwelling region (Hawkes et al 2006, Pikesley et al 2015, one of the most productive ecosystems in the world (Benazzouz et al 2014, Gómez-Letona et al 2017) but also subject to major interannual cycles of primary productivity (Gómez-Letona et al 2017).…”

Nesting Ecology of Lepidochelys olivacea in Lobito, Angola

“…Currently, the widely used conservation measures are limited to protecting nesting sites, reducing pollution, and reducing bycatch and poaching [ 10 ]. One major obstacle many projects face is the lack of published data concerning nesting and feeding areas; for example, despite significant efforts in recent years, the Atlantic African coast still poses a major challenge to the conservation of sea turtle species [ 9 , 11 ], and finding scientific articles or reports is difficult.…”