Three decades of stranding data reveal insights into endangered hawksbill sea turtles in Hawai‘i

Abstract

Hawksbill sea turtles Eretmochelys imbricata inhabiting the Hawaiian Islands are extremely rare and listed as endangered under the US Endangered Species Act. The paucity of data on basic hawksbill ecology continues to hinder effective management of the species. We analyzed stranding data collected between 1984 and 2018 to gain insights into the distribution, demography, and conservation challenges facing hawksbills in Hawai‘i. In doing so, we present a comprehensive description of the population across developmental stages and rank threats that may be impeding their successful recovery. Over the >30 yr data set, we recorded a total of only 111 juvenile and adult hawksbill stranding events. Interactions with nearshore recreational fishing gear were documented for a large proportion (48.6%) of stranding events in the Hawaiian Islands, identifying this as the primary management challenge for the species. Stranding events were biased towards females (female to male sex ratio of 4.8:1.0), which may be indicative of the population as a whole. Even though the majority of hawksbills nest on the islands of Hawai‘i Moloka‘i, and Maui, the greatest number of juvenile to adult strandings was found to be on the island of Oahu (n = 47). Temporal distribution of the majority of adult hawksbill strandings (72.2%) occurred during a 4 mo period between June and September. We discuss these and other findings that help identify future research and conservation efforts to mitigate anthropogenic threats in Hawai‘i for this enigmatic population.

Non-random mating between nesting sites of Hawaiian hawksbill turtles: demographic discontinuity within a small isolated population

“…This is because assortative mating within breeding groups can allow lineages to purge deleterious mutations more efficiently, in a similar fashion as subdivided populations (Muralidhar et al 2022). In other parts of the world, hawksbill breeding sex ratios can be heavily female biased, presumably due to a limited supply of males in small populations (Gaos et al 2018), but the Hawaiian rookeries have even fewer breeding individuals than elsewhere and a nearly 1:1 sex ratio, notwithstanding that females may outnumber males 4:1 overall in the archipelago (Brunson et al 2022). Therefore, it cannot be ruled out that Hawaiian hawksbills are highly selective in their mate choices, even within nesting complexes (Fig.…”

“…In the Hawaiian archipelago, hawksbill turtles are described as rare and their resident population is thought to include less than 100 adult females, with only 5-26 nesting annually (van Houtan et al 2012; van Houtan et al 2016; Gaos et al 2021). The population may also be predominantly female, as only 1 in 5 strandings in Hawaii are male (Brunson et al 2022). Field surveys of nesting activity have been conducted since the 1980s with varying levels of effort, and predictive modeling based on these data suggests a recent uptick in the number of nesting females (Gaos et al 2021).…”

Non-random mating within an Island rookery of Hawaiian hawksbill turtles: demographic discontinuity at a small coastline scale

“…In the Hawaiian archipelago, hawksbill turtles are described as rare and their resident population is thought to include less than 100 adult females, with only 5-26 nesting annually [27][28][29]. The population may also be predominantly female, as only 1 in 5 strandings in Hawaii are male [30]. Field surveys of nesting activity have been conducted since the 1980s with varying levels of effort, and predictive modelling based on these data suggests a recent uptick in the number of nesting females [29].…”

Insights obtained from a multidecadal citizen science scheme: grey seal (Halichoerus grypus) strandings in Cornwall and Isles of Scilly (2000–2020)

“…A long time series of strandings data can also set a comprehensive baseline which can be used to monitor deviations in mortality numbers (Prado et al ., 2022). The importance of stranded animals for biological information and monitoring has long been demonstrated (Peltier and Ridoux, 2015), generating important data for birds (Ortiz-Alvarez et al ., 2022), turtles (Tomás et al ., 2008; Brunson et al ., 2022), cetaceans (Coombs et al ., 2019), sharks (Wosnick et al ., 2022), sirenians (Adimey et al ., 2014), seals (Baker, 1984) and invertebrates (Sheehan et al ., 2017; Canepa et al ., 2020). Carcasses of stranded individuals present an excellent opportunity to gain knowledge and understanding of rarely observed species (Chan et al ., 2017), and offer insights into causes of mortality and incidence of disease (Hart et al ., 2006; ten Doeschate et al ., 2017; Brunson et al ., 2022; Ortiz-Alvarez et al ., 2022).…”

“…The importance of stranded animals for biological information and monitoring has long been demonstrated (Peltier and Ridoux, 2015), generating important data for birds (Ortiz-Alvarez et al ., 2022), turtles (Tomás et al ., 2008; Brunson et al ., 2022), cetaceans (Coombs et al ., 2019), sharks (Wosnick et al ., 2022), sirenians (Adimey et al ., 2014), seals (Baker, 1984) and invertebrates (Sheehan et al ., 2017; Canepa et al ., 2020). Carcasses of stranded individuals present an excellent opportunity to gain knowledge and understanding of rarely observed species (Chan et al ., 2017), and offer insights into causes of mortality and incidence of disease (Hart et al ., 2006; ten Doeschate et al ., 2017; Brunson et al ., 2022; Ortiz-Alvarez et al ., 2022). Strandings can also act as early indicators of environmental change by documenting species occurrence and revealing shifts in distribution due to changing oceanic conditions (Truchon et al ., 2013; Byrd et al ., 2014; Prado et al ., 2016; Sepúlveda et al ., 2020; Warlick et al ., 2022).…”