Fishers' Ecological Knowledge and Stable Isotope Analysis Reveal Mangrove Estuaries as Key Developmental Habitats for Critically Endangered Sea Turtles

Abstract

Successful conservation of endangered, migratory species requires an understanding of habitat use throughout life stages. When dedicated scientific studies are difficult to conduct, local expert knowledge can provide crucial baseline data to guide study design and aid data interpretation. In 2008, fishers in El Salvador demonstrated that eastern Pacific hawksbill sea turtles (Eretmochelys imbricata)—a population conservation biologists considered virtually extirpated—use mangrove estuaries as nesting habitat rather than open-coast beaches used by hawksbills in other regions. We confirmed and amplified this observation by using fishers' ecological knowledge to guide biological sampling for stable isotope analysis to assess if eastern Pacific hawskbills use mangrove-dominated estuaries as developmental habitats. We found that immature hawksbills experience a pelagic stage and then recruit to estuaries at ~37 cm curved carapace length, where they increase reliance on estuarine resources until they approach adult sizes. This life history strategy makes them especially vulnerable to in-water nearshore threats, and necessitates targeted expansion of conservation efforts throughout the eastern Pacific. Our analysis also provides a model for integrating traditional scientific approaches with local knowledge—a model that could yield crucial advances in other understudied regions.

Tracking movements and growth of post-hatchling to adult hawksbill sea turtles using skeleto+iso

“…However, findings here strongly suggest that this is not the case-that hawksbills in this population do not immediately settle into the mangrove estuary, but instead spend at least a few years in an offshore, if not oceanic, habitat during their post-hatchling juvenile stage. These results are also supported by the SIA of hawksbill scute layers by Wedemeyer-Strombel et al (2021) which also found higher δ 13 C values in the earliest scute layers. This offshore movement pattern is further supported when the growth rates estimated in the current study are related to the length-frequency distribution of turtles found in both the mangrove estuary and the rocky reef habitats.…”

“…To isotopically distinguish two separate habitats, we referenced hawksbill skin stable isotope values previously published and from the current study to help characterize the mangrove estuary (Bahía de Jiquilisco, Wedemeyer-Strombel et al, 2021) and the rocky reef / ocean (Punta Amapala, current study) as two isotopically distinct locations (inside or outside estuary). In Wedemeyer-Strombel et al (2021), skin from juvenile (<65 cm CCL) hawksbills captured in Bahía de Jiquilisco had stable carbon isotope values that ranged from ca.…”

“…This unique population of hawksbills in the eastern Pacific (EP) had sought refuge in dense mangrove estuaries, nesting in the sand banks dispersed among mangrove roots, an atypical behavior for sea turtles, in addition to more typical scattered nesting at contiguous, open-coast sandy beach sites (Bolten, 2003;Gaos et al, 2010Gaos et al, , 2012a. This population remains at high risk of bycatch during fishing efforts in the estuaries (blast fishing; Liles et al, 2011;Wedemeyer-Strombel et al, 2021) and adjacent coastal rocky reefs (bottom-set gillnets; Liles et al, 2017), and is further impacted by habitat degradation from coastal development and climate change. The conservation and population recovery of hawksbills in the EP is highly dependent on management actions (i.e., nest relocation, habitat protection, bycatch mitigation), and a better understanding of how hawksbills use and move between distinct habitats that will help prioritize conservation efforts (Liles et al, 2015a(Liles et al, ,b, 2019.…”

“…In El Salvador, the two high-use habitats for hawksbills, mangrove estuaries and nearshore rocky reefs, are known to be isotopically distinct because of the uniquely low δ 13 C values that characterize the mangrove-based foodweb (Seminoff et al, 2021;Wedemeyer-Strombel et al, 2021). Like in other mangrove-based foodwebs, the low δ 13 C values are a result of the carbon cycle used by these marine angiosperms, which differ from other marine flora such as macroalgae and seagrasses that have higher δ 13 C values (Marshall et al, 2007;Bouillon et al, 2008).…”

“…Like in other mangrove-based foodwebs, the low δ 13 C values are a result of the carbon cycle used by these marine angiosperms, which differ from other marine flora such as macroalgae and seagrasses that have higher δ 13 C values (Marshall et al, 2007;Bouillon et al, 2008). As a result, these mangrove habitats are isotopically distinct from open ocean and coastal reef areas (Fry and Ewel, 2003;McMahon et al, 2011;Wedemeyer-Strombel et al, 2021). These habitats in El Salvador-both estuaries and rocky reefs-are under increasing pressure from local fishing efforts, which have resulted in high levels of fishery-related bycatch and, subsequently, offer the opportunity to collect humeri samples from turtles stranded in each of these locations.…”