Sea turtles

Temperature-dependent Sex Determination: Evolutionary Significance and the Adaptive Potential of Sea Turtles to Climate Change

Research Report

 

Introduction 

Environmental sex determination (ESD) in sea turtles occurs as temperature-dependent sex determination (TSD), whereby the sex of an individual is determined by external thermal cues. All sea turtles exhibit TSD type Ia, which means that higher temperatures produce females and lower temperatures produce males (Heredero Saura et al. 2022). Sex is determined during the thermosensitive period, when temperature cues induce the production of sex hormones that determine the sex of the embryo (Heredero Saura et al. 2022). Pinpointing the evolutionary significance of TSD remains a challenge, despite the prevalence of TSD in sea turtles and other amniote vertebrates (Janzen & Phillips 2006). Sexing hatchlings requires sacrifice and histological examination of gonads (Lockley & Eizaguirre 2020); currently, however, six of the seven species of sea turtles are endangered, and three are critically endangered, which means that until alternative sexing methods are possible, our understanding of TSD in sea turtles—its evolutionary significance and the mechanisms for its maintenance—must remain restricted to laboratory incubation studies (Lockley & Eizaguirre 2006; Sea Turtle Conservation Bonaire, n.d.). In this paper, I will review the hypotheses that have been proposed so far for the evolutionary significance of TSD in sea turtles. I will then consider the plausibility of each of these hypotheses, particularly the Charnov-Bull model of differential fitness. I will also consider the risk that global warming poses for temperature-sensitive sea turtle populations around the globe and explore opportunities for future research. My studies at the University of Chicago trace the effects of climate change around the globe, from the water-stressed regions of Pakistan to the rapidly eroding nesting beaches along the coast of Australia. The topic of this paper is particularly relevant to conversations about how climate change will affect the critical ecosystems on which all life depends. Without sea turtles, 3 marine ecosystems around the globe risk collapsing entirely; sea turtles are considered keystone species, which means that they play a critical role in the productivity and biodiversity of these ecosystems (The Leatherback Trust, n.d.). The opportunities for future research considered in this paper might inform the policy choices and conservation efforts that could save sea turtles—and thus the ecosystems that sustain millions of communities, human and nonhuman, around the globe—from extinction

Date
2023
Data type
Research report
Theme
Research and monitoring
Geographic location
Bonaire
Author

Green and hawksbill turtle detection and abundance at foraging grounds in Bonaire, Caribbean Netherlands

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. 

Date
2023
Data type
Scientific article
Theme
Research and monitoring
Document
Geographic location
Bonaire

How does ecosystem functioning change now sea turtles are returning?

Dutch below

 

Sea turtles have been overharvested in the past, resulting in their endangered population status. But since two decades, their populations have recovered in certain areas around the world, and their ecological role is unfolding. The seagrass meadows they feed on are returning to their naturally grazed state as a result. Some are even becoming overgrazed. How does the return of the sea turtle affect the functioning of coastal ecosystems?

A research team analysed the seagrass ecosystem value (such as coastal protection and carbon storage) along a turtle grazing area from low to high. In experiments, they determined the impact of recovering sea turtle populations on ecosystem functioning. The team, led by Wageningen University, with colleagues from the Netherlands and the Dutch Caribbean, recently published their findings in Global Change Biology.

Seagrass meadows provide us with a range of goods and services. For example, they provide a habitat and food for many commercially important fish species. They also help protect the coast by stabilising sediment, and reducing wave energy. With the return of the large herbivore – the sea turtle – it is important to quantify their impact on the ecosystem services a seagrass meadow can provide. Often these services are measured separately, but in the new study, the most essential services have been assessed together to assess the ecosystem’s multifunctionality.

Combining ecosystem services in one index

“Our approach to measuring different ecosystem functions simultaneously and combining these in one ecosystem-multifunctionality-index is new for marine habitats and led to striking results,” says Marjolijn Christianen, lead investigator of the study. “We found that medium turtle grazing pressure increased carbon storage and nutrient cycling in seagrass meadows. On the other hand, fish biomass and other services were higher in meadows without turtle grazing. More importantly, we found a simultaneous collapse of all these services under severe turtle grazing pressure.”

Cages were used to keep turtles from grazing the seagrass to mimic the – no-grazing scenario

 

Integrative approach for balanced ecosystems

Based on these results, the authors argue that the successful return of the sea turtle should be accompanied by the protection of their habitat, seagrass meadows, as well as their predators, who can influence turtle grazing behaviour through fear effects. “Based on recent insights from studies in the Bahamas, the resulting spatial variation in grazing pressure can probably prevent habitat collapse,” says Fee Smulders, co-author of the study. Christianen: “By taking an integrative ecosystem approach to management, we can maintain high ecosystem multifunctionality, as well as balanced ecosystems that can sustain natural densities of charismatic sea turtles”.

Pristine seagrass meadows

Green sea turtles are considered the megaherbivores of the ocean. Since their populations have been decimated, scientists have become accustomed  to studying and experiencing ungrazed seagrass meadows. These lush seagrass meadows dominated by sturdy species are considered healthy, in ‘pristine’ condition, and very valuable. But with the return of the green sea turtle in many coastal areas in the past decades – due to successful conservation – many seagrass meadows have been transformed into grazed seascapes.

An earlier paper published in Nature, Ecology and Evolution by Wageningen researchers discussed how we must reconsider our view of a pristine seagrass meadow, which likely consists of a mosaic of (heavily) grazed patches and ungrazed patches with high plant species diversity and higher biodiversity in general. This mosaic seascape may be more natural, even though the ecosystem value may be slightly lower than ungrazed meadows. Now that the different grazing scenarios (high turtle abundance, low abundance, sometimes hyper-abundant) coincide in all three ocean basins where turtles are found, the outcomes of these new studies are even more globally relevant and urgent. The findings can also be used to predict ecosystem impacts of future shifts.

Three scenarios of megaherbivore grazing intensity can be observed in tropical seagrass ecosystems with green turtles as megaherbivores worldwide (Fig a). The turtle’s ecological role is rapidly unfolding in numerous foraging areas where populations are recovering through conservation after centuries of decline, with an increase in recorded overgrazing episodes. In field experiments, researchers assessed the effects of simulated grazing intensity scenarios on ecosystem functions and multifunctionality (Fig b) in a tropical Caribbean seagrass ecosystem over an 18-month period.

More info in the Dutch Caribbean Biodiversity Database

 

 

 

 

De zeeschildpad komt terug, wat betekent dat voor het functioneren van ecosystemen?

In het verleden is er veel op zeeschildpadden gejaagd, waardoor ze met uitsterven worden bedreigd. Maar sinds twee decennia hebben ze zich in een aantal gebieden over de hele wereld hersteld. Hun rol in het ecosysteem komt daarmee terug. Zo wordt zeegras – hun voedingsbron – weer op een natuurlijke manier begraasd. Op sommige plekken wordt zeegras zelfs overbegraasd. Wat betekent de terugkeer van de zeeschildpad voor het functioneren van ecosystemen langs de kust?

Een onderzoeksteam heeft in een gebied met grazende zeeschildpadden de waarde van zeegras-ecosystemen geanalyseerd (waarbij bijvoorbeeld naar kustbescherming en koolstofopslag werd gekeken). Via experimenten bepaalden ze het effect van de zich herstellende populaties schildpadden op het functioneren van het ecosysteem. Het team, onder leiding van de Wageningen University, met collega’s uit Nederland en het Caribisch gebied, publiceerde hun bevindingen onlangs in Global Change Biology.

Velden met zeegras voorzien ons van verschillende diensten. Ze vormen bijvoorbeeld een habitat voor veel vissoorten die commercieel belangrijk zijn, en bieden hen voedsel. Ook helpt zeegras de kust te beschermen, doordat het sediment stabiliseert en golfslag vermindert. Nu de grote grazer – de zeeschildpad – is teruggekeerd, is het belangrijk om te bepalen wat voor impact dat heeft op de diensten die een zeegras-ecosysteem kan leveren. Vaak worden deze diensten afzonderlijk gemeten, maar in de nieuwe studie zijn de belangrijkste diensten samen onderzocht om de multifunctionaliteit van het ecosysteem te beoordelen.

Combinatie van ecosysteemdiensten in één index

“Onze aanpak om verschillende functies tegelijkertijd te meten, en die te combineren in één multifunctionaliteitsindex voor ecosystemen, is nieuw voor mariene habitats,” zegt Marjolijn Christianen, hoofdonderzoeker van de studie. “Het leidde tot opvallende resultaten. We zagen dat koolstofopslag en nutriëntencyclus hoger lagen in velden zeegras met een gemiddelde graasdruk. Anderzijds bleken de hoeveelheid vis, en andere diensten, hoger te zijn in onbegraasde velden. Maar nog belangrijker: we zagen dat al deze diensten bezweken bij een zware graasdruk.”

Om het scenario ‘geen-begrazing’ na te bootsen, werden er kooien gebruikt om te voorkomen dat schildpadden bij het zeegras konden.

Integrale aanpak voor een evenwichtig ecosysteem

Op basis van de resultaten stellen de auteurs dat de succesvolle terugkeer van de zeeschildpad moet samengaan met de bescherming van hun habitat (zeegras) en hun natuurlijke vijanden. Deze predatoren kunnen het graasgedrag van schildpadden beïnvloeden door het effect van angst. “Recent zijn er daar op de Bahama’s studies naar gedaan. Op basis van die inzichten, denken we dat ruimtelijke variatie in graasdruk die daaruit ontstaat, waarschijnlijk kan voorkomen dat habitats bezwijken,” aldus Fee Smulders, co-auteur van de studie. Christianen: “Met een integrale benadering van ecoysteembeheer kunnen we de hoge multifunctionaliteit van ecosystemen behouden. En het leidt tot evenwichtige ecosystemen, die natuurlijke aantallen van de zeeschildpad kunnen dragen.”

Ongerepte velden met zeegras

Groene zeeschildpadden worden beschouwd als de grote herbivoren van de oceaan. Sinds hun populaties zijn uitgedund, zijn wetenschappers gewend geraakt aan het bestuderen van onbegraasd zeegras. Deze weelderige velden – gedomineerd door sterke soorten – worden beschouwd als gezond, in ‘ongerepte’ staat, en als zeer waardevol. Maar dankzij succesvolle natuurbescherming is de groene zeeschildpad de afgelopen decennia in veel kustgebieden teruggekeerd. De velden met zeegras zijn daardoor weer veranderd in begraasde gebieden.

Een eerder artikel, gepubliceerd in Nature, Ecology and Evolution door Wageningse onderzoekers, beschreef hoe we ons idee van ‘ongerept’ zeegras moeten bijstellen. Waarschijnlijk bestaat dat uit een mozaïek van (zwaar) begraasde plekken, afgewisseld door onbegraasde plekken met een hogere dichtheid aan plantensoorten, en een hogere biodiversiteit in het algemeen. Dit mozaïek is wellicht natuurlijker, ook al ligt de ecosysteemwaarde iets lager dan bij onbegraasd zeegras. De uitkomsten van deze studies worden wereldwijd nog relevanter en urgenter, nu de verschillende graasniveaus (veel schildpadden, weinig, of soms uitermate veel) samenvallen in de drie oceaanbekkens waar schildpadden voorkomen. De bevindingen kunnen ook worden gebruikt om de gevolgen voor ecosystemen te voorspellen, die toekomtige verschuivingen kunnen veroorzaken.

In tropische ecosystemen met zeegras, waar groene schildpadden als mega-herbivoor voorkomen, worden wereldwijd drie niveau’s aan graasintensiteit waargenomen (a). Na eeuwenlange achteruitgang ontvouwt de ecologische rol van de schildpad zich snel in talrijke foerageergebieden waar populaties zich door natuurbescherming herstellen. Hier wordt een toename van het aantal overbegrazingen geregistreerd. Onderzoekers hebben met veldexperimenten de effecten beoordeeld van gesimuleerde graasintensiteit (op verschillende niveaus) op de functies van ecosysteemfuncties, en op multifunctionaliteit (b). Deze experimenten vonden gedurende 18 maanden plaats in een tropisch zeegras-ecosysteem in het Caribisch gebied.

 

 

More info in the Dutch Caribbean Biodiversity Database

 

 

 

Published in BioNews 59

Date
2022
Data type
Media
Theme
Research and monitoring
Geographic location
Bonaire

STENAPA’s protection of Caribbean Sea turtles

Each year, hawksbill and green turtles, and sometimes even leatherback turtles, come to the beaches of Statia to lay their eggs. New protocols, developed by two research students from Van Hall Larenstein University, will aid in STENAPA’s ability to accurately and safely track sea turtle beach activity and hatchling success in the future.

Three different types of turtles can be found on the beaches and in the surrounding waters of St. Eustatius: the hawksbill turtle (Eretmochelys imbricata), green turtle (Chelonia mydas), and leatherback turtle (Dermochelys coriacea). The hawksbill and green turtle are frequent visitors of the waters of Statia and can be encountered during snorkeling or diving. Every year, nesting hawksbill and green turtles can be found on the beaches. A less frequent visitor is the leatherback turtle. They can only be spotted while nesting since the leatherback is a deep-sea species. Occasionally, a nest of a Leatherback turtle can be found on St. Eustatius.

 

Hawksbill sea turtle. Photo credit: Naturepics: Y.+T. Kühnast

Monitoring

STENAPA monitors the beaches, both morning and night, to identify new nests and track hatching success. The details and information from these beach patrols are collected by filling in data sheets. Those data sheets can be used internationally for the purpose of having comparable data.  Annemieke Borsch and Louise Kramár, two students from Van Hall Larenstein University, recently produced protocols as part of an explanatory report in cooperation and guidance from STENAPA. These protocols cover morning patrol, night patrol and next excavation while the whole report can be used as a guide for how to perform certain tasks in a correct, safe and careful way during patrols.

Importance of data collection

Data collection on turtles is important because it gives information on the status of the species. Baseline information can become important when a new factor comes into play, to see what the effect of the factor is. Besides this, the data collection gives insight in the population trends, if it is declining or growing. It also makes it possible to detect diseases or parasites in a population in an early state. The data collection is also important to get to know the species better, for example habitat, food source and breeding grounds, to protect these necessary factors for the turtles.

Green sea turtle. Photo credit: Naturepics: Y.+T. Kühnast

Why turtles need protection

The green, hawksbill and leatherback turtles are on the IUCN red list of endangered species. The green turtle is listed as ‘’endangered’’ and the hawksbill and leatherback turtles are listed as ‘’critically endangered’’. Sea turtles need protection because they are keystone species. This means, that they are an important part of the marine environment and have an influence on the species living among them. Hawksbill turtles live close to the coral reefs, where they feed on sponges which compete with corals for space. Green turtles are important because they feed on seagrass, which keeps the seagrass ecosystem healthy meaning it can take up more carbon and sustain more species this way. Leatherback turtles are known to control the number of jellyfish in the oceans. Besides the ecological benefits, the turtles are also important for coastal communities, since many people rely on the incomes that are being provided by turtle watching and diving. Some indigenous communities see turtles as a part of their culture and there is said that seeing a turtle in the wild has psychological and emotional benefits.

How to support

There are several ways to contribute to the conservation of sea turtles on Statia. If you are interested in helping, STENAPA offers both part- and full-time volunteering programs (for more information contact volunteer@statiapark.org and/or check out https://www.statiapark.org/vacancies-turtle-program/). Together with the National Parks Staff you can take part in the in-water surveys conducted throughout the Marine Park. If scuba diving is not possible, staff are happy to provide training for beach patrols so that you would be able to assist in the monitoring of nesting turtles on the beaches. Since the turtles are protected, people are not allowed to disturb them. This is the reason why you should contact STENAPA if you want to be involved and not go looking for turtles by yourself.

STENAPA needs volunteers especially for the patrols since it is necessary to be done a few times a week during nesting season. Their wish is for the (local) volunteers to be able to patrol as independently as possible with the help from instructions and the protocols guiding them.

 

More information

To learn more about STENAPA and the turtle species on St. Eustatius you can go to the website: www.statiapark.org. If you are interested in protocols about morning and night patrols and the nest-excavations, send your request to STENAPA.

 

Published in BioNews 54

Date
2022
Data type
Media
Theme
Research and monitoring
Geographic location
St. Eustatius
Author

Sea Turtle Conservation on Bonaire in 2020

Bonaire might have been on lockdown in 2020, but that didn’t keep Sea Turtle Conservation Bonaire from working hard.  Their recently published annual report highlights the latest news concerning sea turtle conservation for the island.  Between monitoring resident populations, nests and deploying a satellite tag, 2020 was a busy year for STCB.

Image from STCB Annual Report

Monitoring

Bonaire’s resident turtle population and nesting turtles are monitored through transect-count surveys and nest patrols. Between the months of May and December, volunteers and staff patrolled the beaches looking for new activities. Once marked, these nests were kept under surveillance and excavated after hatching to calculate hatching success. In addition, 108 in water surveys were conducted, allowing STCB to estimate local populations of 555 green and 70 hawksbill turtles.

Capture-Tag-Recapture

Long-term data on specific turtles is gathered through the capture-tag-recapture project.  Through this effort, recaptured turtles are measured, weighed and checked for signs of fibropapillomatosis to track growth rates and the overall health of the population.  Additional DNA samples provide key insight to the distribution of turtle populations throughout the Caribbean. Using data collected over the past 16 years, STCB has been able to show a slight increase in green turtle populations within Lac Bay.  So far, over 3,500 turtles have been tagged which will continue to provide important information for years to come.

Satellite Tags

Turtles are migratory by nature, spending much of their life in transit. Although there is still much to be learned about their behavior, satellite tracking is giving researchers a never-before-seen glimpse of these routes.  Understanding their migration routes and identifying areas of foraging and nesting will provide important information shaping conservation efforts in the future. STCB has been placing satellite trackers on turtles since 2003.  In 2020, the 26th tracker was placed on “Flappie”, who once tagged, traveled to Aruba before continuing on to the Miskito Cays off the coast of Nicaragua.

Hawksbill turtle. Photo credit: © Marion Haarsma

Sea turtles face a barrage of threats, from being caught as bycatch to degraded habitats from coastal development and climate change.   Conservation groups such as STCB are instrumental for increasing local awareness and driving conservation efforts forward.

Learn more about their important work by visiting STCB’s website (www.bonaireturtles.org) or reading the full annual report, see DCBD link below.

 

https://www.dcbd.nl/sites/default/files/documents/STCB-Year-Report-2020-...

 

Article published in BioNews 45

Date
2021
Data type
Media
Theme
Education and outreach
Research and monitoring
Geographic location
Bonaire
Author

Demographic changes in Pleistocene sea turtles were driven by past sea level fluctuations affecting feeding habitat availability

Abstract

Pleistocene environmental changes are generally assumed to have dramatically af-fected species’ demography via changes in habitat availability, but this is challenging to investigate due to our limited knowledge of how Pleistocene ecosystems changed through time. Here, we tracked changes in shallow marine habitat availability resulting from Pleistocene sea level fluctuations throughout the last glacial cycle (120–14  thou-sand years ago; kya) and assessed correlations with past changes in genetic diver-sity inferred from genome-wide SNPs, obtained via ddRAD sequencing, in Caribbean hawksbill turtles, which feed in coral reefs commonly found in shallow tropical waters. We found sea level regression resulted in an average 75% reduction in shallow ma-rine habitat availability during the last glacial cycle. Changes in shallow marine habitat availability correlated strongly with past changes in hawksbill turtle genetic diver-sity, which gradually declined to ~1/4th of present-day levels during the Last Glacial Maximum (LGM; 26–19   kya). Shallow marine habitat availability and genetic diver-sity rapidly increased after the LGM, signifying a population expansion in response to warming environmental conditions. Our results suggest a positive correlation be-tween Pleistocene environmental changes, habitat availability and species’ demog-raphy, and that demographic changes in hawksbill turtles were potentially driven by feeding habitat availability. However, we also identified challenges associated with disentangling  the  potential  environmental  drivers  of  past  demographic  changes, which highlights the need for integrative approaches. Our conclusions underline the role of habitat availability on species’ demography and biodiversity, and that the con-sequences of ongoing habitat loss should not be underestimated.

 

KEYWORDS: ddRAD sequencing, demographic change, habitat availability, Pleistocene sea turtles, sea level change

Date
2021
Data type
Scientific article
Theme
Education and outreach
Research and monitoring
Geographic location
Aruba
Bonaire
St. Eustatius
St. Maarten

Special Edition: Transboundary Species

There has been a recent increase in public awareness of environmental issues as the effects of climate change have become ever more noticeable in our daily lives. As we enter a new decade, it becomes useful to review what conservation efforts have worked so far, and take inventory of what efforts will be required for the future. Starting with the constitutional referendum creating the Caribbean Netherlands (Bonaire, St. Eustatius and Saba (BES), the response to conservation challenges of all six Dutch Caribbean islands have varied. Since 2010, the BES islands have seen an overall increase in funding support and conservation actions, and therefore presumably also saw greater improvements when compared to Aruba, Curaçao and Sint Maarten, though clearly not enough (Sanders et al, 2019).

The goal of this Transboundary Species special edition of BioNews is to provide an update on the latest published research results and highlight the need for transboundary protection. These species know no boundaries, and thus move between the Dutch Caribbean islands and beyond. Their protection will require broadscale conservation efforts which cover the entire Caribbean, including the six Dutch Caribbean islands. Collaboration between all six islands is of the utmost importance. This is one of the Dutch Caribbean Nature Alliance’s (DCNA) main goals: working together and sharing skills, knowledge and resources to maintain a solid network and support nature conservation in the entire Dutch Caribbean.

 

Date
2019
Data type
Media
Theme
Education and outreach
Research and monitoring
Geographic location
Aruba
Bonaire
Curacao
Saba
Saba bank
St. Eustatius
St. Maarten
Author

Construction setback regulations and sea-level rise: Mitigating sea turtle nesting beach loss

Global sea-level rise of up to 0.6 m is predicted in the next 100 years. In areas where coastal structures prevent landward mi- gration of beaches, a major impact of sea-level rise will be a loss of beach habitat, with repercussions for beach-dependent organ- isms such as sea turtles. Setback regulations, which prohibit construction within a set distance from the sea, have the potential to mitigate loss of beach area by providing a buffer zone which allows for the natural movement of beaches in response to perturbation. The potential impact of a rise in sea level on 11 important sea turtle nesting beaches in Barbados under a range of setback regu- lations was determined. Three sea-level rise scenarios were modelled under five different setback regulations (10, 30, 50, 70 and 90 m). Beach area was lost from all beaches under all sea-level rise scenarios with a 10 and 30 m setback, from some beaches with a 50 m setback and from one beach with a 70 m setback. No beach area was lost with a 90 m setback distance. Sea turtles nest within a range of beach elevations and there was an overall loss of beach habitat within the preferred nesting elevation range with both a 10 and 30 m setback under all sea-level rise scenarios. Considerable variation in the extent of beach and nesting area loss was observed. The implementation and enforcement of adequate setback regulations have the potential to maintain the ecological and economic function of beaches in the face of extensive coastal development and sea-level rise. 

Date
2008
Data type
Scientific article
Geographic location
Bonaire

Trends in Kemp’s Ridley Sea Turtle (Lepidochelys kempii) Relative Abundance, Distribution, and Size Composition in Nearshore Waters of the Northwestern Gulf of Mexico

Long-term monitoring of in-water life history stages of the critically endangered Kemp’s ridley sea turtle (Lepidochelys kempii) is essential for management because it generates information on the species’ at-sea abundance, size composition, distribution, and habitat requirements. We documented trends in Kemp’s ridley size, relative abundance, and distribution using entanglement netting surveys at three study areas adjacent to tidal passes in the northwestern Gulf of Mexico (NWGOM) during intermittent sampling periods from 1991 to 2013. A total of 656 Kemp’s ridley sea turtles were captured ranging in size from 19.5 to 66.3 cm straight carapace length (SCL) (mean 1⁄4 35.0 cm SCL). The dominance of juveniles (25–40 cm SCL) captured during sampling suggests the nearshore waters of the NWGOM are an important developmental foraging ground for Kemp’s ridley. Characterization of Kemp’s ridley long-term relative abundance reveals a generally stable trend in catch- per-unit-effort (CPUE) across all study areas combined. Based on the increasing trend in the number of hatchlings released from the species’ primary nesting beach, Rancho Nuevo, Mexico, since the early 1990s, the lack of a corresponding overall increase in juvenile abundance at nearshore sampling locations is puzzling. This disparity is most likely an artifact of the present study’s sampling design, but could also indicate shifts in Kemp’s ridley recruitment away from the NWGOM. While conservation efforts have contributed to this species’ overall growth since the 1980s, as measured by the increasing number of nests, recent declines in this rate of increase are a concern and call for a more comprehensive approach to managing Kemp’s ridley recovery efforts. 

Date
2016
Data type
Scientific article
Theme
Research and monitoring