ABSTRACT: Abundance estimates corrected for changes in detection are needed to assess population trends. We used transect-count surveys and N-mixture models to estimate green turtle Chelonia mydas and hawksbill turtle Eretmochelys imbricata detection and total abundance at foraging grounds in Bonaire during 2003−2018, and we used these total abundance estimates to fit a Bayesian state–space logistic model and make abundance predictions for 2019−2030. During 2019−2022, we also recorded distance categories to estimate detection and total abundance using distance sampling and N-mixture models. In the present study, we focus on distance sampling to estimate observer detectability and total abundance, and to determine if total abundance increased, declined, or did not change during 2019−2022 and when compared with 2003−2018 estimates and 2019−2030 predictions. Detectability averaged 0.53 (SE = 0.02) for green turtles and 0.51 (SE = 0.06) for hawksbill turtles. Density (ind. km−2) and population size (individuals in the 4 km2 survey region) averaged 72.1 (SE = 17.3) and 288 (SE = 69) for green turtles and 21.8 (SE = 4.6) and 87 (SE = 18) for hawksbill turtles. Green turtle total abundance did not change during 2019−2022 (p > 0.05) but remained low when compared with 2003−2018 estimates and 2019−2030 predictions. Hawksbill turtle total abundance declined between 2020 and 2021 (z = 2.15, p = 0.03) and increased between 2021 and 2022 (z = −3.04, p = 0.002), but 2019−2022 estimates were similar to 2003−2018 estimates and 2019−2030 predictions. Our methodology can be used to monitor sea turtle populations at coastal foraging grounds in the Caribbean.
Endangered Species Research
Green turtles Chelonia mydas and hawksbill turtles Eretmochelys imbricata are neg- atively impacted by natural and anthropogenic disturbances. Unknown numbers of turtles are killed annually in the coastal waters of Bonaire and Klein Bonaire, Caribbean Netherlands. We used N-mixture models, conventional distance sampling and the multiple Lincoln-Petersen method to estimate abundance from transect-count and net-capture surveys. Maximum likelihood and Bayes- ian generalised linear models were used to assess trends in annual abundance in 2003−2018, and a Bayesian state-space logistic model was developed to generate the posterior distributions of pop- ulation parameters and make abundance predictions for 2019−2030. Mean ± SE annual abun- dance was 555 ± 149 green turtles (2.5th and 97.5th percentiles = 337, 943) and 70 ± 13 hawksbill turtles (49, 101), and there were no trends in western Bonaire and Klein Bonaire in 2003−2018. Mean annual abundance was 348 ± 135 green turtles (171, 731) and there was a positive trend inside Lac Bay, southeast Bonaire, 2003−2018. Green turtles have higher population growth rate and carrying capacity, and therefore can sustain higher human-induced mortality than hawksbill turtles. However, under low mortality rates (<0.100), both species can fluctuate stably between the lower and upper limits of the carrying capacity. The methodology implemented can be adapted to estimate sea turtle abundance, monitor and model their population dynamics, and assess the neg- ative impact of human-induced mortality in other Caribbean islands.
KEY WORDS: Chelonia mydas · Eretmochelys imbricata · Abundance · N-mixture model · Distance sampling · Multiple Lincoln-Petersen method · Generalised linear model · Bayesian state-space logistic model
The management of small rookeries is key to conserving the regional genetic diversity of marine turtle populations and requires knowledge on population connectivity between breeding and foraging areas. To elucidate the geographic scope of the populations of marine turtles breeding at Bonaire and Klein Bonaire (Caribbean Netherlands) we examined the post-breeding migratory behavior of 5 female loggerheads Caretta caretta, 4 female green turtles Chelonia mydas, and 2 male and 13 female hawksbill turtles Eretmochelys imbricata during the years 2004-2013. After leaving Bonaire, the 24 tracked turtles frequented foraging grounds in 10 countries. The distances swum from Bonaire to the foraging areas ranged from 608 to 1766 km for loggerhead turtles, 198 to 3135 km for green turtles, and 197 to 3135 km for hawksbill turtles, together crossing the waters of 19 countries. Males represented the minority in this study, but we made 2 key observations that require further research: males remained in the vicinity of the breeding area for 3-5 mo, which is 2-5 times longer than females, and males migrated greater distances than previously recorded. Although the turtles dispersed widely across the Caribbean, there appeared to be 2 benthic foraging areas of particular importance to all 3 species of marine turtles breeding at Bonaire, namely the shallow banks east of Nicaragua and Honduras (n = 8 tracked turtles) and Los Roques, Venezuela (n = 3). Marine turtles breeding at Bonaire face threats from legal turtle harvesting, illegal take, and bycatch in the waters that they traverse across the Caribbean.
In 2005 to 2006 we assessed the status of the Caribbean coot Fulica caribaea in the Netherlands Antilles, largely semi-arid islands in the South Caribbean, with small numbers of permanently available fresh water bodies. The Caribbean coot is a freshwater bird which is dependent on the seasonal availability of freshwater ponds for breeding; it breeds on 4 of the 6 islands of the Netherlands Antilles, viz. Curaçao (first recorded in 1956), Bonaire (1974), Aruba (1977), and St. Maarten (1981). Compared to the period up to and including 1979, group sizes in 1980 to 2006 were larger on Curaçao, and it appears more abundant in the latter period on all islands. We report on 49 sites (>5 ha) in the Caribbean where the species has been recorded, or where we would expect it to occur on the basis of available habitat. Threats to the Caribbean coot include drainage or reclamation of habitat, hunting, and pollution. Few sites receive protection. The coot has a restricted range of occupancy of some 1000 km2, spread out over 13 islands, representing 10 countries. Based on its restricted range, coupled with high levels of threat and the limited amount of protection, we recomend that the species be included as ‘Vulnerable’ in the IUCN (International Union for the Conservation of Nature and Natural Resources, World Conservation Union) Red List. An increase in the level of (legal) protection, in addition to an increase in the amount of habitat included in the regional protected areas network and heightening the awareness of the needs of this Caribbean endemic are overdue. The establishment of permanent freshwater ponds, especially in the arid parts of its range, appears favourable for the species, and may aid conservation.
Lionfish (Pterois volitans and P. miles) have spread rapidly throughout the Caribbean Sea since 1985, where they negatively impact native fish communities and therefore are considered by some as the most damaging invasive species in the Caribbean to date. To combat further population growth and spread of lionfish and to protect native fish communities, various Caribbean islands have started control efforts. On Bonaire, a removal program based on volunteers using spear guns was started immediately after the first lionfish was sighted in 2009, and a similar program was started on neighboring Curaçao 2 yr later. To determine the effectiveness of these removal efforts, differences in the density and biomass of lionfish were compared between areas in which lionfish were directly targeted during removal efforts (i.e. ‘fished’ areas) on Bonaire and areas where they were not (i.e. ‘unfished areas’) on both Bonaire and Curaçao. Lion- fish biomass in fished locations on Bonaire was 2.76-fold lower than in unfished areas on the same island and 4.14-fold lower than on unfished Curaçao. While removal efforts are effective at reducing the local number of lionfish, recruitment from unfished locations, such as those too deep for recreational diving and at dive sites that are difficult to access, will continuously offset the effects of removal efforts. Nevertheless, our results show that the immediate start and subsequent contin- uation of local removal efforts using volunteers is successful at significantly reducing the local density and biomass of invasive lionfish on small Caribbean islands.
The use of satellite tracking for the fundamental and applied study of marine turtles began in the 1980s but has undergone rapid growth in recent years. To provide a background against which to judge the past success and future directions of these research efforts we carried out a comprehensive review of over 130 scientific papers on the use of this technique in this taxon. We show how satellite tracking has changed over time as well as outlining biases in spatial, species and lifestage coverage. Descriptions of migration routes and other habitats have offered novel insights into the basic life history patterns of some species, highlighted focal areas for conservation and reinforced the multi-national nature of the stakeholders of many populations. In foraging areas, knowledge is growing as to how animals move within dynamic seascapes, thus facilitating our understanding of 3-dimensional habitat use and seasonal patterns of behaviour. More experimental approaches have elucidated navigational capabilities and post-release survival following fisheries interaction and long-term captivity. In addition, through the Internet and other media, satellite tracking appears to have been effective in engaging public attention in many countries. Finally, we discuss why the use of the technique has increased so markedly over time and point out key areas of concern that we feel should be addressed by the community of researchers and donors who focus on sea turtles