Investigating differences in population recovery rates of two sympatrically nesting sea turtle species

Abstract

Estimating life-history traits and understanding their variation underpins the management of long-lived, migratory animals, while knowledge of recovery dynamics can inform the management of conservation-dependent species. Using a combination of nest counts and individual-based life-history data collected since 1993, we explore the drivers underlying contrasting population recovery rates of sympatrically nesting loggerhead (Caretta caretta) and green (Chelonia mydas) turtles in North Cyprus. We found that nest counts of loggerhead and green turtles from 28 beaches across the island increased by 46% and 162%, respectively over the past 27 years. A Bayesian state-space model revealed that, at our individual-based monitoring site, nesting of green turtles increased annually at four times the rate of that of loggerhead turtles. Furthermore, we found that loggerhead turtles nesting at the individual-based monitoring site had stable reproductive parameters and average adult survival for the species and are the smallest breeding adults globally. Based on results from multiple matrix model scenarios, we propose that higher mortality rates of individuals in all age classes (likely driven by differences in life history and interaction with fisheries), rather than low reproductive output, are impeding the recovery of this species. While the increase in green turtles is encouraging, the Mediterranean population is estimated to have around 3,400 adults and is restricted to the Eastern Basin. The recovery of loggerhead turtles is likely to be compromised until mortality rates in the region are adequately quantified and mitigated. As survival of immature individuals is a powerful driver for sea turtle population numbers, additional efforts should target management at pelagic and neritic foraging areas. Understanding threats faced by immature life stages is crucial to accurately parameterise population models and to target conservation actions for long-lived marine vertebrates.

Missing Data in Sea Turtle Population Monitoring: A Bayesian Statistical Framework Accounting for Incomplete Sampling

“…Status assessments are fundamental to species conservation, however, they are often hindered by a lack of long-term spatial and temporal data (Nichols and Williams, 2006;Jackson et al, 2008;Sims et al, 2008;Piacenza et al, 2019;Whiting et al, 2020). In the case of sea turtles, ground-derived counts of the number of clutches laid in a nesting season are most often used to derive population trends and abundance (Witherington et al, 2009;Stewart et al, 2011;Stokes et al, 2014;Omeyer et al, 2021), although such estimates are prone to variability in sampling effort within and among years and sites. Here, we combine data from several sources to present the first overview of long-term trends in sea turtle nesting effort in the Republic of the Congo using a Bayesian statistical modelling framework that accounts for sampling uncertainties within sites and seasons in a robust probabilistic manner.…”

A standard protocol to report discrete stage-structured demographic information

“…Consequently, recruitment estimates are often supplemented into MPMs from controlled conditions; examples include the laboratory (Jouvet et al, 2018), greenhouse (Gontijo & Carvalho, 2020), zoo (Clubb et al, 2009) and botanic garden (Jiménez-Valdés, Godínez-Alvarez, Caballero, & Lira, 2010). Since some MPM methods require a full life cycle to obtain key metrics (e.g., 𝜆: Caswell, 2001, p. 106;transient metrics: Stott et al, 2011), external study sites or literature sources are often used to parameterize components of the MPM to "close the loop" in incomplete life cycles (Omeyer et al, 2021). However, captive populations may not represent wild population dynamics (Clubb & Mason, 2003;Johnsson, Brockmark, & Näslund, 2014), particularly in regards to survival (Jule, Leaver, & Lea, 2008;Che-Castaldo et al, 2021) or reproduction (Clubb et al, 2009).…”

“…Since some MPM methods require a full life cycle to obtain key metrics ( e.g. , λ : Caswell, 2001, p. 106; transient metrics: Stott et al, 2011), external study sites or literature sources are often used to parameterize components of the MPM to “close the loop” in incomplete life cycles (Omeyer et al, 2021). However, captive populations may not represent wild population dynamics (Clubb & Mason, 2003; Johnsson, Brockmark, & Näslund, 2014), particularly in regards to survival (Jule, Leaver, & Lea, 2008; Che-Castaldo et al, 2021) or reproduction (Clubb et al, 2009).…”

“…Indeed, time steps can vary from 12-24h as in nematode worms Caenorhabditis elegans and aphids Myzus periscae (Li, Ju, Liao, & Liao, 2014; Bruijning, Jongejans, & Turcotte, 2019), to monthly/annually periods in mammals and plants (Coulson et al, 2001; Schödelbauerová, Tremblay, & Kindlmann, 2010; Ferreira, Kajin, Cerqueira, & Vieira, 2016), all the way to 50 years in slow-growing red woods (Namkoong & Roberds, 1974). From these data, researchers estimate losses through mortality, transition probabilities among st/ages and their per-capita a/sexual contributions via reproduction (Mandujano & Escobedo-Morales, 2008; Nordstrom, Dykstra, & Wagenius, 2021; Omeyer et al, 2021).…”

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

“…The relative abundance of small turtles nesting at JBR increased throughout the study period and resulted in a drop of the average carapace length. Neophytes are usually smaller than remigrants in marine turtles (Lamont et al 2014, Stokes et al 2015; hence, increasing numbers of small nesting females might suggest improved recruitment to the adult population (Arendt et al 2013, Omeyer et al 2021. 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).…”