The effect of the El Niño Southern Oscillation on the reproductive frequency of eastern Pacific leatherback turtles

Abstract

Summary1. Pacific leatherback turtle Dermochelys coriacea populations have been declining precipitously. It has been suggested that fishery-associated mortality is the leading factor causing the decline; however, the sensitivity of leatherbacks to climate variability relative to their population ecology is unknown. 2. We investigated the effects of interannual climate variability, as governed by the El Niño Southern Oscillation (ENSO), on leatherback nesting ecology. We used equatorial Pacific sea surface temperature (SST) anomaly data over various time scales derived from both moored buoys and remote satellites as signals of ENSO. We then incorporated these data into a remigration probability model for the largest nesting population of eastern Pacific leatherbacks at Parque Nacional Marino Las Baulas (PNMB), Costa Rica. 3. Our results showed that nesting females of PNMB exhibited a strong sensitivity to ENSO, as reflected in their nesting remigration probabilities. Cool La Niña events corresponded with a higher remigration probability and warm El Niño events corresponded with a lower remigration probability. 4. We suggest that productivity transitions at leatherback foraging areas in the eastern equatorial and south-eastern Pacific in response to El Niño/La Niña events result in variable remigration intervals and thus variable annual egg production. This phenomenon may render the eastern Pacific leatherback population more vulnerable to anthropogenic mortality than other populations. 5. Synthesis and applications . Physical indices of environmental variation can be used to estimate the probability of leatherbacks remigrating to nest at PNMB. This type of modelling approach can be extremely useful for understanding the effects of climatic variation on the population dynamics of sea turtles. Our remigration probability model can be applied to any monitored sea turtle nesting population where nesting site fidelity and beach monitoring coverage remains high. This modelling approach can help nesting beach monitoring programmes forecast remigrant numbers based on prior climate data, and can further quantify anthropogenic mortality by validating survival estimates.

Multidecadal trends in the nesting phenology of Pacific and Atlantic leatherback turtles are associated with population demography

“…Specifically, CT ns at Playa Grande was negatively correlated with the MEI during the migration period, and CT ns at Sandy Point was negatively correlated with the NAO during the departure period. Positive values of the MEI are generally associated with lower food availability for leatherback turtles in the Pacific Ocean (Saba et al 2007, Reina et al 2009), and similar patterns might also be true for the NAO in the Atlantic Ocean (Attrill et al 2007). When less food is available to pre-nesting turtles, they might not be able to brood as many eggs and thus lay fewer clutches on arrival at the nesting grounds, leading to a shorter nesting season.…”

“…As a result, the broad-scale distribution and abundance of gelatinous zooplankton are often tied to large oceanographic phenomena, such as the El Niño-Southern Oscillation (ENSO; Raskoff 2001), North Atlantic Oscillation (NAO; Lynam et al 2004), and the At lantic Multidecadal Oscillation (AMO; Edwards et al 2013). Such oceanographic phenomena are therefore also linked to foraging success in leatherback turtles (Saba et al 2007, Reina et al 2009). Moreover, the relationships observed between oceanographic conditions and the onset of migration towards the nesting grounds has led to the hypothesis that turtles foraging in more productive areas are able to acquire resources more rapidly, allowing them to migrate to their nesting grounds earlier (Saba et al 2007, Sherrill-Mix et al 2008).…”

“…Such oceanographic phenomena are therefore also linked to foraging success in leatherback turtles (Saba et al 2007, Reina et al 2009). Moreover, the relationships observed between oceanographic conditions and the onset of migration towards the nesting grounds has led to the hypothesis that turtles foraging in more productive areas are able to acquire resources more rapidly, allowing them to migrate to their nesting grounds earlier (Saba et al 2007, Sherrill-Mix et al 2008). In addition, as leatherback turtles are expected to forage while migrating (Lambardi et al 2008), a decrease in food availability en route could result in increased time spent searching for food and a longer migration duration.…”

Environmental and anthropogenic impacts on intra-specific variation in leatherback turtles: opportunities for targeted research and conservation

“…Although leatherbacks feed on gelatinous zooplankton and not on the lowest trophic levels represented by NPP measurements, high NPP levels are commonly used as proxies for high consumer biomass and complex trophic dynamics (Saba et al 2008a,b). When the sea surface temperature (SST) of the tropical EP is cooler than usual (La Niña phase of ENSO), NPP is typically enhanced and the reproductive frequency of EP leatherbacks increases (Saba et al 2007); the opposite holds true during the warm El Niño phase. The influence of ENSO has been observed among several other populations of sea turtle species in the Pacific Ocean (Limpus & Nicholls 1988, Chaloupka 2001, Chaloupka et al 2008b).…”

“…Furthermore, fluctuating oceanographic conditions due to large-scale climate phenomena such as the El Niño-Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO) can have observable impacts on primary productivity and resource abundance and/or distribution in the oceans (Chavez et al 1999, Stenseth et al 2002. Strong ENSO episodes in the Pacific Ocean influence reproduction of sea turtles (Limpus & Nicholls 1988, Saba et al 2007, drive foraging movements of large predators like tuna (Lehodey et al 1997), and can cause population declines in pinnipeds (Trillmich & Limberger 1985) and seabirds (Schreiber & Schreiber 1984). Thus, for species whose geographic distributions include areas with high rates of climate variability, and/or variation in biophysical and biogeochemical characteristics that influence resource availability, differences in life history traits and population dynamics are possible.…”