Green turtle

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In their 2019 paper, Sea Turtle Conservation Bonaire (STCB) used sea turtle data collected between 2003 and 2018 to estimate abundance and predict future population trends for green and hawksbill turtles on Bonaire’s west coast. The non-profit organization has now expanded that research in their latest paper to determine the population trend between 2019 and 2022.

Hawksbill turtle. Photo credit: Brenda Kirkby

Sea turtles are an iconic species that face a wide variety of threats.  Since these species can travel vast distances, they are susceptible to a number of transboundary threats, including climate change and poaching as well as local concerns such as pollution and habitat loss.  Monitoring nesting and foraging grounds is a critical part in understanding long-term sea turtle trends throughout the Caribbean.

Unfortunately, sea turtles can be difficult to accurately monitor due to issues with detection and hence incomplete counts. To combat this, research statisticians have developed methods to estimate detection and population size, allowing for more realistic counts. STCB has been working many years to monitor local turtle populations, and in collaboration with Frank Rivera-Milan, to determine trends and test the accuracy of prediction models. For this particular research, the goal was to determine whether the population increased, decreased on remained the same between 2019 and 2022, compared to 2003-2018 and predictions for 2019-2030.

Results

Between 2019 and 2022, surveys were conducted along western Bonaire and Klein Bonaire. In total 703 green turtles and 56 hawksbill turtles were recorded.  Researchers also looked at how easy it was to spot the turtles, and they found that some factors, like the substrate (e.g., sand, rubble) or the level of disturbance (e.g., boats, divers), influenced the number of turtles that were detected.

Green turtle. Photo credit: Brenda Kirkby

When comparing historical data to the prediction data, this study yielded interesting results.  For green turtles, the number of turtles counted between 2019 and 2022 did not change much but was comparatively lower than data collected from 2003-2018 and lower than estimates predicted in the 2019-2030 model.  The number of hawksbill turtles, on the other hand, fluctuated between 2019 and 2022 but was similar to previous surveys (conducted between 2003-2018) and was closer to the predictive models.

Implications

The study’s density estimates aligned with previous research on sea turtles in other locations, and the researchers emphasized the importance of accounting for detection when estimating population numbers. This research concluded that this methodology provided reliable detection and population estimates for monitoring sea turtles within foraging grounds in the Caribbean, therefore this approach could be valuable for similar studies in coastal areas.

DCNA

The Dutch Caribbean Nature Alliance (DCNA) supports science communication and outreach in the Dutch Caribbean region by making nature related scientific information more widely available through amongst others the Dutch Caribbean Biodiversity Database, DCNA’s news platform BioNews and through the press. This article contains the results of one of those scientific studies, but this study is not a DCNA study. No rights can be derived from the content. DCNA is not liable for the content and the in(direct) impacts resulting from publishing this article.

In hun wetenschappelijk artikel uit 2019 gebruikte Sea Turtle Conservation Bonaire (STCB) gegevens over zeeschildpadden die tussen 2003 en 2018 waren verzameld om de populatie te schatten en toekomstige populatietrends voor groene en karetschildpadden aan de westkust van Bonaire te voorspellen. STCB heeft dat onderzoek nu uitgebreid in hun nieuwste wetenschappelijk artikel om de populatietrend tussen 2019 en 2022 te bepalen.

Karetschildpadden. Photo credit: Brenda Kirkby

Zeeschildpadden zijn een iconische soort die met een breed scala aan bedreigingen wordt geconfronteerd. Omdat deze soorten grote afstanden kunnen afleggen, zijn ze vatbaar voor een aantal grensoverschrijdende bedreigingen, waaronder klimaatverandering en stroperij, maar ook voor lokale problemen zoals vervuiling en verlies van leefgebied. Het monitoren van broed- en foerageergebieden is een cruciaal onderdeel om inzicht te krijgen in de langetermijntrends van zeeschildpadden in het Caribisch gebied.

Helaas kunnen zeeschildpadden moeilijk nauwkeurig te volgen zijn vanwege problemen met detectie en dus onvolledige tellingen. Om dit tegen te gaan, hebben onderzoeksstatistici methoden ontwikkeld om de detectie en de populatiegrootte te schatten, waardoor meer realistische tellingen mogelijk zijn. STCB werkt al vele jaren aan het monitoren van lokale schildpaddenpopulaties, en in samenwerking met Frank Rivera-Milan, aan het bepalen van trends en het testen van de nauwkeurigheid van voorspellingsmodellen. Voor dit specifieke onderzoek was het doel om te bepalen of de bevolking toenam, daalde of gelijk bleef tussen 2019 en 2022, vergeleken met 2003-2018 en voorspellingen voor 2019-2030.

Resultaten

Tussen 2019 en 2022 zijn tellingen uitgevoerd langs westelijk Bonaire en Klein Bonaire. In totaal werden 703 groene schildpadden en 56 karetschildpadden geregistreerd. Onderzoekers keken ook naar hoe gemakkelijk het was om de schildpadden te spotten, en ze ontdekten dat sommige factoren, zoals de ondergrond (bijvoorbeeld zand, puin) of de mate van verstoring (bijvoorbeeld boten, duikers), van invloed waren op het aantal schildpadden dat werd gedetecteerd.

Groene schildpad. Photo credit: Brenda Kirkby

Bij het vergelijken van historische gegevens met de voorspellingsgegevens leverde dit onderzoek interessante resultaten op. Voor groene schildpadden veranderde het aantal getelde schildpadden tussen 2019 en 2022 niet veel, maar was relatief lager dan de gegevens verzameld van 2003-2018 en lager dan de schattingen die in het 2019-2030-model waren voorspeld. Het aantal karetschildpadden fluctueerde daarentegen tussen 2019 en 2022, maar was vergelijkbaar met eerdere onderzoeken (uitgevoerd tussen 2003-2018) en lag dichter bij de voorspellende modellen.

Implicaties

De schattingen van de dichtheid tijdens de studie kwamen overeen met eerder onderzoek naar zeeschildpadden op andere locaties, en de onderzoekers benadrukten het belang van het verantwoorden van detectie bij het schatten van populatieaantallen. Dit onderzoek concludeerde dat deze methodologie betrouwbare detectie- en populatieschattingen opleverde voor het monitoren van zeeschildpadden in foerageergebieden in het Caribisch gebied, daarom zou deze aanpak waardevol kunnen zijn voor soortgelijke studies in kustgebieden.

DCNA

De Dutch Caribbean Nature Alliance (DCNA) ondersteunt wetenschapscommunicatie en outreach in de Nederlandse Caribische regio door natuurgerelateerde wetenschappelijke informatie breder beschikbaar te maken via onder meer de Dutch Caribbean Biodiversity Database, DCNA’s nieuwsplatform BioNews en via de pers. Dit artikel bevat de resultaten van een van die wetenschappelijke onderzoeken, maar dit onderzoek is geen DCNA-onderzoek. Aan de inhoud kunnen geen rechten worden ontleend. DCNA is niet aansprakelijk voor de inhoud en de indirecte gevolgen die voortvloeien uit het publiceren van dit artikel.

Published in BioNews 67

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. 

This dataset contains ecosystem metabolism and seagrass meadow data from five locations in the Greater Caribbean and Gulf of Mexico regions at which green turtle populations had established foraging areas. Ecosystem metabolic rates were compared between grazed and adjacent ungrazed areas of seagrass (Thalassia testudinum) to investigate the effects of green turtle grazing on metabolic carbon capture rates in seagrass meadows across a wide geographic area. Seagrass data are provided for site descriptions and drivers of variation in metabolic rates. Ecosystem metabolic rates are also included for meadows of the invasive seagrass Halophila stipulacea from two locations for comparison to rates in the native seagrass meadows where this invasive seagrass is encroaching upon green turtle foraging areas. Data were collected from one location (Little Cayman) in 2016, and from the remaining four locations (Bonaire; St. Croix; Eleuthera, Bahamas; west coast of Florida) in 2018.

From Bonaire, we here provide the first documented case of the green turtle feeding on the invasive seagrass, Halophila stipulacea, in the Caribbean. The seagrass is rapidly invading existing seagrass meadows and altering key foraging habitat of this endangered marine reptile throughout the eastern Caribbean. We expect that more records of green turtles feeding on this invasive species will gradually follow from throughout the region and that the green turtle might alter its foraging behavior in response to the changing species composition of its foraging habitat. 

Many pelagic organisms, including sea turtles, host unique communities of epibionts on the surfaces of their bodies. Although sea turtle epibiota have been studied in other areas of the world, very little research has been conducted on the epibionts found on sea turtles inhabiting the water around Bonaire, Netherland Antilles. In this study, epibiont samples were obtained from 33 sea turtles found in Bonaire. Epibionts included green and red algae, polychaete worms, skin barnacles, and turtle barnacles. Barnacle abundance and epibiont biodiversity was determined for each size class (Small, Medium, Large juveniles) of the two most common species of sea turtles found on Bonaire (Eretmochelys imbricata and Chelonia mydas). There was no significant difference in number of barnacles between E. imbricata and C. mydas. However, there was a significant increase in the number of barnacles with increasing size class in both E. imbricata and C. mydas. Epibiont biodiversity was significantly higher on E. imbricata but did not increase with size class for either species. Such findings indicate that the distinct life histories of C. mydas and E. imbricata may lead to varying degrees of epibiont accumulation.

This student research was retrieved from Physis: Journal of Marine Science V (Spring 2009)19: 32-36 from CIEE Bonaire.

Invasive seagrass mega herbivore interactions
a study on the invasion of seagrass Halophila stipulacea in a Southern Caribbean lagoon affected by C. mydas grazing
In the Caribbean, the recent invasion of the seagrass species Halophila stipulacea has raised concerns regarding its impact on the invaded seagrass ecosystem and its associated flora and fauna. The main purpose of the experimental set-up was to understand the mechanisms and impacts of invasive species on a native seagrass in interaction with grazing impacts by the green sea turtle (C. mydas). The aims of the study were i.) to determine the colonization capacity of native seagrass species T. testudinum as affected by the presence of the invasive species (Halophila stipulacea) and vice-versa in a Caribbean lagoon (Lac Bay, Bonaire); ii.) To determine whether sexually and/or vegetatively colonization is affected by turtle grazing. and iii) to determine whether architectural properties of T.testudinum and H. stipulacea are affected by presence of other species and whether these are related to C. mydas grazing. For this study, four seagrass bed types were selected that naturally occur in the bay: (1) monoculture of T. testudinum, (2) monoculture of H. stipulacea , (3) mixed bed of H. stipulacea and T. testudinum and (4) mixed bed containing H. stipulacea, T. testudinum and S. filiforme. In each seagrass bed type, 12 experimental units were created divided over three experimental periods of six weeks. Within each unit, two patches of 150 x 150 mm were cleared of above and below ground biomass. Cages were placed over half of the cleared patches to prevent turtle grazing. After six weeks, recolonization of the patches by native species and invasive species were measured by resampling biomass. To assess whether turtle grazing changed architectural properties, measurements on length and width with and without grazing were taken. Lastly, lines around two T. testudinum turtle grazing plots were placed to measure the lateral expansion rate of the surrounding H. stipulacea patches.
Our results indicate that H. stipulacea is a ~11 times faster colonizer than T. testudinum. Effects of grazing on their colonization rate were different with T. testudinum colonization rate under C. mydas grazing being lower and H. stipulacea’s colonization rate being higher. These effects were not statistically proven, but strong trends were observed. The presence of other seagrass species did not seem to influence competitive abilities (colonization capacity and architectural properties). C. mydas grazing, on the other hand, clearly influenced T. testudinum’s architectural properties. Regarding T. testudinum’s grazing plots, an average lateral expansion of 0.35 cm day-1 by H. stipulacea was detected.
This study demonstrates that there is no direct competition between T. testudinum and H. stipulacea. It seems that H. stipulacea is colonizing areas unsuitable to T. testudinum. Sea turtle grazing creates less dense seagrass beds and therefore might further stimulate the expansion of H. stipulacea. The impact of the establishment of H. stipulacea on C. mydas is not yet clear: Even though it seems not to be the preferred seagrass species, C. mydas does graze on the invasive species in Lac Bay. It is, however, unknown how this new food resource will affect their fitness. Though the invasive may alter abiotic conditions in their habitat, the sea turtles may benefit from an extended cover of seagrass beds as the invasive seagrass is able to grow in places where native seagrass species currently cannot survive. It is recommended to keep monitoring changes and investigating the impact of H. stipulacea on the whole ecosystem.
 
Retreived from http://www.bonaireturtles.org on Aprile 13, 2015
 

Fibropapillomatosis is a disease that is affecting sea turtles all around the world. Turtles that are most at risk are those that live in near-shore waters and lagoons, especially areas next to large human populations with poor sewage treatment facilities. In this research project the main focus is fibropapillomatosis (FP) and green sea turtles captured by netting in Lac Bay, Bonaire. The research goal was to see how turtles living in Bonaire are affected by the disease. The two main research questions were: “What is the true rate of fibropapillomatosis affecting green sea turtles in Lac Bay?” and “What is the difference between healthy turtles and infected turtles that are caught by netting?”
To determine the true rate of FP, the percentage of diseased turtles is calculated as the percentage of captured turtles with FP compared to the whole amount captured during netting conducted from 2006 until 2014. In 2006 rates of FP were 20 percent, the infection rates then decreased dramatically, even reaching zero percent in 2010. FP rates started increasing again after 2012, and in 2014 the rates of FP now stand at 34 percent (n=89). It is still uncertain what causes FP to increase. To determine the difference between healthy turtles and diseased turtles, the length, weight, and overall growth rates have been assessed. Recapture rates were also assessed, to determine if diseased turtles were captured more, because of their limitations. There was a significant difference (p < 0.001) found between recapture rates of healthy and diseased turtles indicating that healthy turtles are recaptured more often than diseased turtles. Assessed length and weight of diseased turtles are not significantly different than from healthy turtles (p < 0.001). The growth rate in this research was not significantly different between healthy and diseased turtles. Overall there was no significant difference found between healthy turtles and diseased turtles living in Lac Bay, not in length, weight or in growth rates. The implications of this research suggest that the overall survival rate of turtles with FP on Bonaire is relatively high in comparison to other areas of the Caribbean. This could be due to the tumors not restricting the turtles to such a degree that they are unable to forage or flee.
 
Retrieved from http://www.bonaireturtles.org on April 13, 2015