How high and low nesting season densities affect the patterns of nest site preferences and nest site fidelity of Caretta caretta, on Maio island, Cape Verde

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2019

UNIVERSIDADE DE LISBOA FACULDADE DE CIÊNCIAS

DEPARTAMENTO DE BIOLOGIA ANIMAL

How high and low nesting season densities affect the patterns

of nest site preferences and nest site fidelity of Caretta caretta,

on Maio island, Cape Verde

Ana Sofia Ribeiro de Oliveira

Mestrado em Biologia da Conservação

Dissertação orientada por:

Doutor Rui Miguel Borges Sampaio e Rebelo (cE3c)

Doutor Juan Patiño Martínez (Maio Biodiversity Foundation)

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Acknowledgments

During the course of this project I collected a significant amount of memories, friends and advisors that contributed to its realization and so I would like to take the opportunity to thank to:

Dr. Juan Patiño-Martinez for giving me the chance to collaborate with FMB in the study of sea turtles and for teaching me all the field work methods involved. Thank you for all the dedication, knowledge and enthusiasm since the beginning of this project.

Dr. Rui Rebelo for accepting this challenge and for all the commitment throughout the last year. When I found myself lost in numbers and results your support, guidance and coffee (I will never forget it, thank you!) were fundamental and really kept me going.

To all FMB members, especially the patrollers and the team from Pedro Vaz (Agostinho, Tucai, Jardel, Juka, Carlins, Tote and Rita). And also, to team leaders, Gelsom and Jairson, for all the support, teachings and histories shared. The final result of this work is yours as well.

All my Pedro Vaz’ family, for receiving me so warmly and teaching me that the perfect mix of happiness, kindness and hard work exists. A giant thank you to Tata and Odete for taking care of me, teaching me crioulo and for really trying to teach me how to “badjar”.

Rita, a big big thank you not just for sharing your dream, the passion for sea turtles and the incredible Maio island, but mostly for making me fell less homesick throughout our time in Cape Verde and less “crioulosick” now that we are back to Portugal.

Carolina for being, as your last name says, the Rock during hard times.

Matilde, Estela and the baby of the family, Kafka, for proving that a house can be a home and more important, that friends can be a family.

Marlene and Verde, because if it was not for you, I would probably be an architect right now.

All my dearest and crazy friends from “Clube das Amogas”, “Cena do Mal”, “Os Vegetais” and, of course, “Ajamigas”. Despite you started to be present in different times during my academic path, all of you made it memorable and remain till today. That we never say “saudade” when we talk about what we are!

Artur, you have an undeniable large dose of “paciência de Alentejano”. For a few years you will probably not stand any sea turtle talk from me anymore but during these last few months you never stopped listening and helping and for that I just want to thank you. And for all the tissues, the hugs and for being the coolest version of Tumbler’s inspirational quotes. You really are a keeper.

My mum and my dad for encouraging me to always chase what seemed impossible and teaching me that challenge and sad times do not last forever. My twin for being my right hand, the bigger version of me and for making me know the definition of “unconditional love”. My grandmother and grandfather, that more than a second mother and father, both are my oldest and most sapient friendships.

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Abstract

When female sea turtles arrive to their natal regions to nest they encounter a heterogeneity of habitats. The choice of the place to lay the nest can affect enormously the survival and the future of the offspring. The assessment of female behaviour concerning nest placement and plasticity regarding this behaviour is important in the context of current climate change and has direct implications for selection of the nesting beaches to protect or manage.

Maio island, in Cape Verde, is one of the main nesting islands of the archipelago for the loggerhead sea turtle, Caretta caretta, and it is known for its heterogeneity, with beaches of different sand colours (black, mixed and white) and lengths around the entire island. The subpopulation of loggerheads that nests there is classified as “Endangered” by the IUCN. Nevertheless, during the last years, the number of females arriving to the island appears to be increasing and, in 2018, the number of nests almost tripled, in comparison to 2017.

This study aimed to compare the nest site distribution, amid these two contrasting years, and assess if the females with different body sizes select different nest sites. Overall, the results revealed that nests were not laid randomly across the island. Beach orientation was evidenced as the main variable influencing the selection of the nesting site in both nesting years, with the majority of individuals selecting beaches facing east and north east. This selection did not differ according to female body size.

Regarding beach fidelity within the same nesting season, we found that individuals selected sites within a mean range of 8003 meters, in 2017, and 7035 meters, in 2018. The mean distance travelled per female between consecutive nesting events seemed to differ according to body size, since larger females tended to travel shorter distances than smaller females.

Moreover, it was also tested if the hatching success varied among beach zones. Hatching success revealed to be similar between the vegetated and the mid-zone of the beach (42.5% and 41.3%, respectively). In the intertidal zone all nests were flooded by spring tide or lost to erosion. We also tested if each female showed fidelity regarding the nest placement along different beach zones and if they selected zones favourable to the development of embryos. An interesting result was that females tended to toggle between areas near the vegetation and in the open sand, avoiding the intertidal zone.

Finally, the intensive monitoring effort allowed the identification of the most important beaches for loggerheads nesting each year on Maio island, as well as the identification of areas that will gain importance in a desirable future when “good” years will be the norm. Beaches facing east and north east of mixed and black sand colour received the highest number of nests in both years, while the areas amid Lagosteira - Pajoana; Casas Velhas -Djampadja; Praia do Morro - Salina Norte and Soca – Santana, , scattered around the island and not only on the east/ north east, were identified as potentially important areas.

Jointly, these findings provide a more profound knowledge about the ecology of Caretta

caretta and provide information that allow the implementation of more effective management

plans.

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Resumo

As tartarugas marinhas são seres carismáticos, com ciclos de vida complexos capazes de abranger uma diversidade de ecossistemas, tanto terrestres como marinhos, ao longo dos diferentes estágios de vida. Reproduzem-se sazonalmente e, quando as condições são favoráveis, as fêmeas migram em direção às regiões onde nasceram para nidificar. A tartaruga-comum,

Caretta caretta, é uma das espécies conhecida por ser altamente fiel às áreas onde nasce, aí

regressando para se reproduzir. Contudo, ao chegar à praia a fêmea encontra um habitat diverso. Sendo, além do mais, uma espécie que não dispõe de cuidados parentais, a escolha do local para nidificar pode afetar bastante o desenvolvimento e a sobrevivência da prole. Vários autores sugerem que a seleção do local para nidificar não é aleatória e, apesar de não se saber ao certo porque preferem determinados locais, fatores como a vegetação, a temperatura, albedo da areia e a distância ao mar são apontados como determinantes para a escolha do mesmo.

Das sete espécies de tartarugas descritas, Caretta caretta é uma das mais difundidas. Estão atualmente descritas dez subpopulações desta espécie, que podem ser encontradas em mares tropicais e subtropicais por todo o mundo. A subpopulação do nordeste do Atlântico está classificada pela IUCN com o estatuto de ameaça “Em perigo”. Desta subpopulação fazem parte as tartarugas que frequentam o arquipélago de Cabo Verde, reconhecido como a principal área de nidificação desta subpopulação e a terceira maior do mundo. A ilha do Maio é uma das 10 ilhas deste arquipélago, conhecida pela elevada heterogeneidade de habitats, abarcando praias de diferentes colorações (branca, mista e preta) e com diferentes características. Estima-se que esta ilha receba cerca de 6% a 8% do total de ninhos registados anualmente em Cabo Verde. Não obstante, nos últimos anos o número de ninhos tem vindo a aumentar, tendo 2018 registado, aproximadamente, o triplo de ninhos comparativamente a 2017 e o maior valor dos últimos anos. Durante as épocas de nidificação de 2017 e 2018, o trabalho de monitorização realizado ao longo de toda a ilha permitiu conduzir um estudo aprofundado relativamente à distribuição dos ninhos de Caretta caretta. Pretendeu-se aumentar o conhecimento relativamente à fidelidade exibida pelas fêmeas desta subpopulação. Tendo em conta o sucesso de eclosão nas diferentes zonas da praia, quisemos também perceber se as fêmeas selecionavam sucessivamente, ao longo das várias posturas que cada fêmea realiza em cada estação, locais que favorecessem a eclosão e o desenvolvimento das ninhadas.

Todas as posturas foram identificadas e registadas tendo em conta as seguintes variáveis: orientação da praia, comprimento da praia, coloração da areia (branca, mista ou preta) e zona ao longo da praia (zona de vegetação, zona intermédia ou zona intertidal). Os resultados revelaram que os ninhos não se encontram distribuídos de uma forma aleatória ao longo da ilha e um padrão semelhante foi evidenciado em ambos os anos: o maior número de ninhos foi encontrado em praias viradas a este e a nordeste e em praias de coloração mista e preta. A caracterização feita inicialmente revelou que grande parte das praias de coloração mista e preta se encontram orientadas a este e nordeste, evidenciando a orientação como a variável mais determinante na escolha do local de desova. De entre as diferentes zonas, a zona intermédia foi a que recebeu um maior número de ninhos e na zona intertidal registou-se o menor número de ninhos.

De forma a perceber se fêmeas com diferentes tamanhos corporais selecionavam diferentes locais para nidificar foi medido o comprimento curvo da carapaça de todos os indivíduos. Nenhuma relação foi encontrada entre o tamanho corporal e a escolha do local de nidificação.

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iv Para as fêmeas que registaram mais do que uma subida com êxito ao longo de uma época foi determinada a frequência do número de ninhos. Os valores médios obtidos, de 2.45 e 2.21 ninhos/fêmea são provavelmente uma subestimativa devido a registos incompletos de recapturas.

Para o estudo de fidelidade, foram selecionadas todas as fêmeas com mais de 3 posturas realizadas ao longo da mesma época de nidificação. Com recurso a um programa de georreferenciação foi calculada a distância entre todas as praias, ao longo da ilha do Maio. De seguida, determinou-se a distância que cada fêmea percorreu, em média, entre posturas. Os resultados demonstraram que em média as fêmeas percorreram 8003 metros e 7035 metros, em 2017 e em 2018, respetivamente, tendo a maioria das fêmeas selecionado praias próximas entre si. Os resultados evidenciaram ainda que, em 2017, fêmeas com um maior tamanho corporal percorreram distâncias mais curtas. Aponta-se como uma possível justificação o facto de indivíduos maiores serem mais experientes e, portanto, investirem em áreas restritas, contrariamente a indivíduos mais pequenos e inexperientes, que provavelmente explorarão várias das praias disponíveis em busca das praias com melhores condições. Apesar da fidelidade demonstrada, provou-se que a distância percorrida pelas fêmeas permite-lhes visitar praias com diferentes orientações e de diferentes colorações.

Em 2018, foi avaliado se o sucesso de eclosão variava ao longo do perfil longitudinal da praia. Para isso, foram selecionados e monitorizados 50 ninhos distribuídos pelas 3 zonas de estudo (zona de vegetação, zona intermédia e zona intertidal). O sucesso de eclosão dos ninhos situados na zona de vegetação e na zona intermédia foi bastante semelhante (42.5% e 41.3%, respetivamente). Todos os ninhos situados na zona intertidal foram inundados ou sofreram erosão, tendo-se, por isso, registado um sucesso de eclosão nulo nessa zona.

Os dados resultantes da monitorização permitiram-nos, não só determinar se as fêmeas utilizavam recorrentemente a mesma zona para nidificar, mas também investigar com mais detalhe se, em posturas consecutivas, selecionavam zonas da praia favoráveis ao desenvolvimento e sobrevivência dos embriões. Considerando os resultados obtidos para cada zona relativamente ao sucesso de eclosão, a zona da vegetação e a zona intermédia foram consideradas favoráveis e a zona intertidal foi considerada prejudicial para o desenvolvimento dos embriões. Contrariamente ao esperado, um grande número de fêmeas tendeu a alternar entre a zona de vegetação e a zona intermédia. A zona intertidal foi evitada pela maioria das fêmeas. Isto sugere, não só que as fêmeas selecionam locais vantajosos para a eclosão e o desenvolvimento dos embriões, mas também que apresentam plasticidade comportamental, uma vez que tendem a alternar entre eles.

Tendo por base a frequência de utilização de cada praia, foram identificadas quais as mais importantes para a subpopulação de Caretta caretta que nidifica na ilha do Maio e quais as áreas que possivelmente irão ganhar relevância quando anos como o de 2018, com elevado número de indivíduos, forem recorrentes. De uma forma geral, as praias viradas a este e nordeste, de areia mista e preta, foram as que receberam um maior número de ninhos e, por esse motivo, as que merecem especial atenção, tendo em vista a conservação da subpopulação. Em 2018, as áreas entre Lagosteira - Pajoana; Casas Velhas -Djampadja; Praia do Morro - Salina Norte and Soca – Santana foram a que se destacaram, indicando que a longo prazo serão áreas que poderão ganhar importância.

A informação recolhida ao longo de dois anos permitiu uma melhor compreensão da dinâmica e distribuição da subpopulação de tartaruga-comum que habita a ilha do Maio. A

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v identificação de praias que deverão ser o foco para o estabelecimento de áreas protegidas permitirá delinear planos de conservação e monitorização mais eficazes a longo prazo.

Palavras-chave: Caretta caretta; áreas de conservação; ilha do Maio; fidelidade;

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List of figures

Figure 1.1. Map of Maio island, showing its location in the Cape Verde archipelago (arrow), and

in the African continent. ... 7

Figure 1.2. Map of Maio Island beaches (north and north east), emphasizing all different types

of sand colour (white, mixed and black). ... 8

Figure 1.3. Map of Maio Island beaches (east), emphasizing all different types of sand colour

(white, mixed and black). ... 9

Figure 1.4 Map of Maio Island beaches (south east), emphasizing all different types of sand

colour (white, mixed and black). ... 9

Figure 1.5. Map of Maio Island beaches (south west), emphasizing all different types of sand

Figure 1.6 Map of Maio Island beaches (west), emphasizing all different types of sand colour

(white, mixed and black). ... 10

Figure 1.7. Map of Maio Island beaches (north west), emphasizing all different types of sand

Figure 1.8. Distribution of nesting beaches, on Maio island, across the island according to

orientation and sand colour. A letter system was used to denominate beach orientation: N= North, NE=North east, E= East, SE= South east, S= South, SW= South west, W=West and NW= North west. Different types of sand colour are represented by different colours: Black= Black beaches, Grey=Mixed beaches, White= White beaches. ... 11

Figure 1.9. Average beach length across the Maio island, according to beach orientation. An

abbreviation system was used to denominate beach orientation N= North, NE=North east, E= East, SE= South east, S= South, SW= South west, W=West and NW= North west. Beach lengths are in meters.. ... 12

Figure 1.10. Beach zonation along the transverse profile, according to probability of nests

inundation. Z1- vegetation zone, with no risk of inundation; Z2- sand bank section that can be subdued to the effects of tides; Z3- intertidal zone. Vegetation and C. caretta images were designed by Mason McNair and James R. Spotila and Ray Chatterji, respectively (www.phylopic.org). ... 12

Figure 1.11. Anterior and posterior anatomical points for measuring curved carapace length

(CCL). Sea turtle image was designed by (after McCulloch 1908) (www.phylopic.org). ... 13

Figure 1.12. Number of C. caretta nests recorded per day on Maio island, during 2017 nesting

season. The first day of the nesting season (24 May) was taken as day “0” and the final day of the nesting season (31 October) as day “125”... 16

Figure 1.13. Number of C. caretta nests recorded per day on Maio island, during 2018 nesting

season. The first day of the nesting season (9 May) was taken as day “0” and the final day of the nesting season (15 November) as day “132”... 16

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Figure 1.14. Frequency distributions of C. caretta female curved carapace length (CCL)

throughout 2017 and 2018 nesting seasons, on Maio island. The black bars represent 2017 female population and grey bars represent 2018 female population. Curved carapace length is in centimetres. ... 17

Figure 1.15. Distribution of C. caretta nests in 2017, across the different types of beaches (mixed,

black and white) existing on Maio island. The size of the red circle is proportional to the number of nests found in each beach. Black circles identify beaches without nesting events. ... 18

black and white) existing on Maio island. The size of the green circle is proportional to the number of nests found in each beach. Black circles identify beaches without nesting events. ... 18

Figure 1.17. Boxplot representing the hatching success (%) of Caretta caretta in the different

beach zones, along the transverse profile of the beach. 1= Zone 1, 2= Zone 2 and 3= Zone 3 Horizontal thick black lines represent the median, the upper and lower boundaries of the box represent the 75th and 25th percentiles, whiskers represent the range of observations within 1.5 times the interquartile range from the edge of the box and outliers (black circles) represents observations farther than 1.5 times the interquartile range. ... 21

Figure 1.18. Boxplot representing C. caretta curved carapace length (CCL) in different beach

orientations, in 2017 (left) and 2018 (right). Curved carapace length is in centimetres. Horizontal thick black lines represent the median, the upper and lower boundaries of the box represent the 75th and 25th percentiles, whiskers represent the range of observations within 1.5 times the interquartile range from the edge of the box and outliers (black circles) represents observations farther than 1.5 times the interquartile range. ... 22

sand colours, in 2017 (left) and 2018 (right). Curved carapace length is in centimetres. Horizontal thick black lines represent the median, the upper and lower boundaries of the box represent the 75th and 25th percentiles, whiskers represent the range of observations within 1.5 times the interquartile range from the edge of the box and outliers (black circles) represents observations farther than 1.5 times the interquartile range. ... 22

zones, in 2017 (left) and 2018 (right). Curved carapace length is in centimetres. Horizontal thick black lines represent the median, the upper and lower boundaries of the box represent the 75th and 25th percentiles, whiskers represent the range of observations within 1.5 times the interquartile range from the edge of the box and outliers (black circles) represents observations farther than 1.5 times the interquartile range. ... 23

Figure 1.21. Relationship between C. caretta curved carapace length (CCL) and beach length, in

2017 (left) and 2018 (right), on Maio island. Curved carapace length is in centimetres. ... 23

Figure 1.22. Frequency distributions of the number of nests for C. caretta females, recorded on

2 or more occasions, in 2017 (left) and 2018 (right) nesting seasons, on Maio island. ... 24

Figure 1.23. Frequency distribution of C. caretta females according to the distance between

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x meters. Vertical line indicates the average distance among beaches for the entire island (23505 meters). Arrow indicates the value of the 50th quartile. ... 25

Figure 1.24. Frequency distribution of C. caretta females according to their fidelity regarding

beach orientation (A), sand colour (B), beach zone (C) and beach length (D), in 2017 nesting season, on Maio island. The x axis denotes the number of different types of beaches visited, being: 1= females visiting 1 beach type, 2= females visiting 2 different beach types, 3= females visiting 3 different beach types, 4= females visiting 4 different beach types and 5= females visiting 5 different beach types. An abbreviation system was used to denote the orientations and zones: N= North, NE=North east, E= East, SE= South east, S= South, SW= South west, W=West, NW= North west, Z1= Zone 1, Z2= Zone 2 and Z3= Zone 3. Beach length is in meters... 26

Figure 1.25. Frequency distribution of C. caretta females according to their fidelity regarding

beach orientation (A), sand colour (B), beach zone (C) and beach length (D), in 2018 nesting season, on Maio island. The x axis denotes the number of different types of beaches visited, being: 1= females visiting 1 beach type, 2= females visiting 2 different beach types, 3= females visiting 3 different beach types and 4= females visiting 4 different beach types. An abbreviation system was used to denote orientations and zones: N= North, NE=North east, E= East, SE= South east, S= South, SW= South west, W=West, NW= North west, Z1= Zone 1, Z2= Zone 2 and Z3= Zone 3. Beach length is in meters. ... 27

Figure 1.26. Relationship between C. caretta curved carapace length (CCL) and the average

distance between consecutive nesting events, in 2017 (left) and 2018 (right), on Maio island. Distances are in meters an curved carapace lengths are in centimetres. ... 29

Figure 1.27. Distribution and variation of beach utilization among years, considering the number

of C. caretta nesting events per beach, on Maio island. Green lozenges designate beaches with highest utilization rates in 2018 in comparison to 2017 (Group A); red lozenges designate beaches with lowest utilization rates in 2018 in comparison to 2017 (Group B); and white lozenges, beaches equally used in 2017 and 2018 (Group C). ... 31

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List of tables

Table 1.1. Observed and expected (according to the available beach extent) number of C. caretta

nests on beaches with different orientations, in 2017 nesting season, on Maio island. N= North, NE=North east, E= East, SE= South east, S= South, SW= South west, W=West and NW= North west. Beach length is given in meters. ... 19

nests on beaches with different orientations, in 2018 nesting season, on Maio island. N= North, NE=North east, E= East, SE= South east, S= South, SW= South west, W=West and NW= North west. Beach length is given in meters. ... 19

nests on beaches with different sand colour types, in 2017 nesting season, on Maio island. Beach length is given in meters... 20

nests on beaches with different sand colour types, in 2018 nesting season, on Maio island. Beach length is given in meters... 20

nests on beaches with different beach zones, in 2017 nesting season, on Maio island. Z1= Zone 1, Z2= Zone 2 and Z3= Zone 3. ... 20

nests on beaches with different beach zones, in 2018 nesting season, on Maio island. Z1= Zone 1, Z2= Zone 2 and Z3= Zone 3 ... 21

Table 1.7. Consecutive beach zones selected by C. caretta, in 2017, on Maio island. Number of

females expected in each pair is shown between brackets. Time t correspond to the first nest location and time t+1 correspond to the second nest location. An abbreviation system was used to denote different zones: Z1= Zone 1, Z2= Zone 2 and Z3= Zone 3 ... 28

Table 1.8. Consecutive beach zones selected by C. caretta, in 2018, on Maio island. Number of

females expected in each pair is shown between brackets. Time t correspond to the first nest location and time t+1 correspond to the second nest location. An abbreviation system was used to denote different zones: Z1= Zone 1, Z2= Zone 2 and Z3= Zone 3 ... 28

Table 1.9. One-way ANOVA of the differences in C. caretta curved carapace length considering

their nest site fidelity, in 2017 nesting season, on Maio island. ... 29

Table 1.10. One-way ANOVA of the differences in C. caretta curved carapace length considering

their nest site fidelity, in 2018 nesting season, on Maio island. ... 30

Table A.1. Summary of Maio island beaches according to their characteristics (sand colour, beach

orientation, length), number of nests and beach utilization in 2017 and 2018. -* corresponds to beaches with no available data………...………...………50

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Table A.2. Number of beaches according to group and sand colour. An abbreviation system was

used to denote the different groups: A= Group A, B= Group B and C= Group C………...………51

Table A.3. Number of beaches according to group and beach orientation An abbreviation system

was used to denote the different groups and orientations: A= Group A, B= Group B; C= Group C, N= North, NE=North east, E= East, SE= South east, S= South, SW= South west, W=West and NW= North west. -* correspond to beaches with no available data………51

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List of equations

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List of abbreviations

ANOVA – One-way analysis of variance CCL – Curved carapace length

E – East

FMB – Maio Biodiversity Foundation GIS – Geographical information system N – North

NE – North east NW – North west

PIT – Passive integrated transponder S – South SE – South east SW – South west W – West Z1 – Zone 1 Z2 – Zone 2 Z3 – Zone 3

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1

Introduction

Sea turtles are long-lived and late-maturing reptiles. This charismatic group can be found across the oceans in tropical, sub-tropical and temperate habitats (Bolten, 2003; Wyneken, Lohmann, & Musick, 2013). All species have a complex life history, encompassing a diversity of ecosystems that range from the terrestrial nesting habitat to neritic and oceanic zones during different life stages.

Currently, there are only seven extant species of sea turtles: green (Chelonia mydas Linnaeus 1758), Kemp’s Ridley (Lepidochelys kempii Garman 1880), olive ridley (Lepidochelys

olivacea Eschscholtz 1829), hawksbill (Eretmochelys imbricata Linnaeus 1766), flatback

(Natator depressus Garman 1880), leatherback (Dermochelys coriacea Vandelli 1761) and loggerhead (Caretta caretta Linnaeus,1758), this last one of the most widespread species. All species are at risk of extinction, and considered either“Critically Endangered”, “Endangered” or “Vulnerable”, except for flatback, which is included in the category “Data Deficient” according to IUCN Red List (2015).

Sea turtles are seasonal breeders and most populations have reproductive cycles constrained by the migration distance between nesting and foraging habitats, environmental conditions at the sea (e.g. sea temperature) and the quality and quantity of prey (Hatase, Omuta, & Tsukamoto, 2013; Saba et al., 2007; Solow, Bjorndal, & Bolten, 2002). The breeding season begins when females have accumulated enough energy to endure its extent. Energy acquisition is largely related to interannual environmental variability at their foraging grounds, that can significantly influence female body size as well as reproductive effort and survival (Ceriani et al., 2015; Miller, du Plessis, Limpus, & Godfrey, 2003; Owens, Hamann, & Limpus, 2002).

Sea turtles life cycle begins in sandy beaches, where oviposition and embryonic development occur (Bolten, 2003). At the end of incubation period, hatchlings emerge from their nests and crawl to the ocean, starting the “swim frenzy stage”, where they can swim uninterruptedly and rapidly for several days (Carr, 1986; Wyneken & Salmon, 1992). The second stage of development differs between species. Natator depressus, for example, has a complete neritic development pattern, passing his juvenile and adult stages in neritic zones, coming inland only to nest. Other species are described as having an oceanic developmental pattern, with both juvenile and adult stages occurring in the oceanic zones. It is also possible to define an intermediate life history pattern in which oceanic-neritic development is observed; in other words, the early juvenile development occurs in the oceanic zone and is followed by later juvenile and adult development stages in the neritic zone. The loggerhead sea turtle, Caretta caretta, is the best-known example of this life history pattern (Bjorndal, Bolten, Dellinger, Delgado, & Martins, 2003).

Loggerhead turtle and its subpopulations

Caretta caretta commonly known as loggerhead turtle can be distinguished from the other

species by their large heads and reddish-brown carapaces and brown to yellowish plastrons, both covered by horny scutes (hard scales that cover the shell). Adults can weigh up to 160 kg and have a carapace length of 82-105 cm.

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2 Currently, ten subpopulations of loggerhead turtles are described, and they can be found living and nesting in regions of the Atlantic, Indian and Pacific Oceans and the Mediterranean Sea (Casale & Tucker, 2017; Hamann et al., 2010; Wallace et al., 2011). The North East Atlantic subpopulation has the second most threatened status and is listed as “Endangered” by the International Union for the Conservation of Nature’s Red List of Threatened Species (IUCN) ( Casale & Marco, 2015). This group includes the Cape Verde archipelago (14 ° 45' – 17 ° 10 'N and 22 ° 40' – 25 ° 20' W), considered the main rookery of this subpopulation and where more than 95% of nesting events occur (Marco et al., 2012; Marco et al., 2011). Mitochondrial and nuclear DNA studies conducted by Monzón-Argüello et al. (2010) revealed genetic differences between Cape Verde and other previously sequenced Atlantic and Mediterranean rookeries considering it as an independent population and therefore a key conservation unit that deserves specific monitoring efforts and conservation measures (Wallace et al., 2010).

Worldwide, Cape Verde hosts the third largest nesting subpopulation for this species, with only South-eastern of United States and the Oman population having higher nesting density (Marco et al., 2012) . In the Atlantic context, this subpopulation is the second most important (Marco et al., 2011). Despite the documented presence of loggerhead turtles in all Cape Verde islands, the density of nests is highly variable among islands. Boa Vista is the island that concentrates the largest number of nests, (aprox. 70-80% of the total) and has been subject of most studies to date. Maio and Sal host 6 to 8% of nests. Turtles can also be found at Santo Antão, São Vicente, Fogo, Santiago and Santa Luzia embodying approximately 1% of the (Monzón-Argüello et al., 2010; Rocha et al., 2015; Martins, Alvarez, & Marco, 2012).

As previously referred, this species follows an oceanic-neritic developmental pattern and, like other sea turtles, adults are migratory and use a wide range of habitats during their lifetime. Upon departing from sandy beaches, and during the “swim frenzy stage”, hatchlings can encounter highly variable habitats, whether in temperature, wind, sea currents, and stochastic weather events that have great influence in their destiny, where they remain until the oceanic stage begins (Bolten, 2003; Monzón-Argüello et al., 2012; Pitcher et al., 2008). Little is known about their feeding habits and distributions during these so-called “lost years” (Carr, 1986). It is known that the Eastern Atlantic juveniles that hatch in the Cape Verde beaches are carried by the North Atlantic Gyre to Azores, Madeira, Canary Islands or to the Mediterranean Sea, in response to secondary currents and stochastic events (Monzón-Argüello et al., 2012).

After 4-19 years in the ocean, they recruitto neritic foraging grounds where juveniles feed on epipelagic invertebrates, being subjected to periods of food abundance and scarcity, up until they reach sexual maturity (Bolten, 2003; Hatase, Omuta, & Tsukamoto, 2010; Vieira, Martins, Hawkes, Marco, & Teodósio, 2014). Sexually mature loggerheads begin their breeding migrations between foraging grounds and the same nesting areas, at intervals of 2.5-3 years, in the case of females, and shorter intervals for males (Hays, Fossette, Katselidis, Schofield, & Gravenor, 2010; Schroeder, Foley, & Bagley, 2003). In nesting areas, females deposit from 1 to7 clutches per nesting season followed by the non-breeding interval before returning again to their natal beaches (philopatry) (Hatase et al., 2002; Lee, Luschi, & Hays, 2007; Schroeder et al., 2003). The loggerhead sea turtle is one of the species known to exhibit high nest site philopatry. This has been demonstrated in different nesting areas worldwide, such as Cape Verde and Tongland (South Africa) (Abella. et al., 2010; Hawkes, Broderick, Godfrey, & Godley, 2005; Miller et al., 2003).

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3 The North East Atlantic loggerhead subpopulation displays two distinct adult foraging strategies that correspond to two distinct breeding subpopulations: small body sized adults that continue to forage in the oceanic zone of Gambia and Guinea Bissau, and larger and more fecund females that forage in the neritic zones from Mauritania and Sierra Leone (Eder et al., 2012; Hawkes et al., 2006). This size dichotomy can probably be attributed to differences in prey quality and abundance at non-breeding habitats (Ceriani et al., 2015; Rees et al., 2010). In oceanic areas individuals forage on nutrient-poor planktonic preys (e.g. gelatinous zooplankton), while in neritic waters they feed on nutrient-rich benthic preys (e.g. mollusks, crustaceans) (Hatase et al., 2013).

Nest site selection

As mentioned earlier, some sea turtle species exhibit high degree of nest site fidelity, making more likely to females to return to the same areas to reproduce within and between reproductive seasons (Miller et al., 2003; Pfaller, Limpus, & Bjorndal, 2009; Wyneken et al., 2013). However, after selecting the beach in which to lay the eggs, females encounter a diverse habitat. In species with no parental care, like sea turtles, the conditions of the habitat in which the nests are laid can affect the proper development of the embryos (Hays et al., 1995; Silva et al., 2017; Spencer, 2002). Thus, the hatching success, as well as the future of the clutches is extremely dependent of the choice of the nest site by the female (Bjorndal & Wood, 2000). It is suggested that nest placement is non-random and, although remains unclear why some beaches are preferred and why they choose to nest in a particular site, it is known that females use multiple environmental cues when arriving to the beach (Mazaris, Matsinos, & Margaritoulis, 2006; Pike, 2008). Although nest site preferences can differ amid species and populations, numerous studies have identified vegetation, beach length and sand characteristics (i.e. sand albedo, temperature, grain size, salinity, pH, water content and conductivity) as potential factors for nest site selection (Hays et al., 2001; Hays et al., 1995; Kikukawa, Kamezaki, & Ota, 1999; Lamont & Houser, 2014; López-Castro, Carmona, & Nichols, 2004; Turkozan, Yamamoto, & Yilmaz, 2011).

The choice of site for nesting has consequences on hatching success. For example, to ensure a healthy embryonic development, females must nest in places with suited temperatures and water content (Ackerman, 1980). During the middle third of the incubation, temperature is responsible for sex determination with cooler incubation temperatures producingmales, warmer incubation temperatures producing females and a mixture of sexes being produced within a narrow threshold range of temperatures (Abella, Marco, & López-Jurado, 2007; Ackerman, 1997). Additionally, temperature is negatively correlated with incubation period – higher temperatures result in a shorter incubation period and lower temperatures in a longer incubation period (Abella et al., 2007; Matsuzawa, Sato, Sakamoto, & Bjorndal, 2002). This may have high influence in the success of the offsprings. Warmer nests are known to produce hatchlings with smaller body size and inferior locomotor abilities, while cooler nests produce hatchlings with bigger body size, capable of swim faster and with a higher chance of survival (Hart, Tello-Sahagun, Abreu-Grobois, & Zavala, 2019; Ischer, Ireland, & Booth, 2009; Wood, Booth, & Limpus, 2014).

Incubation temperatures can vary within clutches and are profoundly influenced by environmental factors, as air temperature, vegetation or sand albedo (Esteban, Laloë, Mortimer, Guzman, & Hays, 2016; Hays et al., 2001). Sea turtles, when arrive to a beach, can encounter

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4 changes in sand albedo due to differences in sand colour. Beaches of whiter sand colour are cooler and have lower incubation temperatures, than beaches of darker sand colour, due to the differences in light absorption (Hays et al., 2001; Laloë, Cozens, Renom, Taxonera, & Hays, 2014; Laloë et al., 2017). The higher incubation temperature experienced in beaches with darker sand colour can, that way, produce clutches biased towards feminization or ultimately become lethal for embryos (Abella et al., 2007; Delgado, Dellinger, Varo, Cejudo, & Lopez-Jurado, 2007). Nonetheless Marco et al. (2013) revealed that despite the presence of beaches with different sand colours on Maio island (Cape Verde), several loggerhead females nest on black beaches instead of selecting the white ones.

The preference for some beach zones according to the distance to vegetation and high tide are also pointed as some of the characteristics used by the females to choose the location of the nest (Garmestani, Percival, Portier, & Rice, 2000; Miller et al., 2003). Previous research has shown that Caretta caretta, nests preferably on open sand, close to the vegetation line (Garmestani et al., 2000; Hatase & Omuta, 2018; Hays et al., 1995; Hays & Speakman, 1993). The same was reported for Chelonia mydas and Eretmochelys imbricata (Kelly, Leon, Gilby, Olds, & Schlacher, 2017; Santos, Livesey, Fish, & Lorences, 2015; Zare, Vaghefi, & Kamel, 2012). This tendency for clumping nests close to the vegetation minimizes the risk of clutches being flooded or washed away by beach erosion. However, if females nest within or in the vegetation line, clutches are more likely to be destroyed or penetrated by roots (Bjorndal & Wood, 2000; Hays et al., 1995; Kamel & Mrosovsky, 2004; Wood et al., 2014).

Threats and conservation actions

Sea turtles face numerous threats that may result in death or lead to diseases and disabilities that can affect their survival (Hamann et al., 2010; Hancock, Furtado, Merino, Godley, & Nuno, 2017). In species with later maturation, like loggerheads, the recovery from environmental and human threats takes much longer compared to other species with early maturation and are a crucial topic of concern to scientists (Lutz, Musick, & Wyneken, 2003).

The most severe threats are caused by human activities. Incidental capture in fishing gear (fisheries by-catch), egg poaching for consumption and illegal harvesting and trade are some of the major concerns for sea turtle species like the loggerhead (Marco et al., 2012; Wallace et al., 2011). During the embryonic stages sea turtles are also exposed to several risks that result in embryo mortality. Tidal flooding, beach erosion, plant root penetration, predation, pathogenic agents and infections represent some non-antropogenic threats (Marco et al., 2011; Sarmiento-Ramírez et al., 2010; Witt, Hawkes, Godfrey, Godley, & Broderick, 2010). Predation is one of the major causes of egg mortality (aprox. 90%), with crabs, carnivorous mammals (for example, dogs, jaguars and raccoons), birds (for example, seagulls and crows) and reptiles (for example monitor lizards) as the most relevant predators (Silva et al., 2017; Tucker, Paukstis, & Janzen, 2008; Veríssimo, Jones, Chaverri, & Meyer, 2012).

With current climate change occurring so rapidly, the increasing temperatures, as well as increasing storm frequency and intensity and the rising of sea levels represent potential threats to sea turtles’ success (Bjorndal & Wood, 2000; Butt, Whiting, & Dethmers, 2016; Hawkes, Broderick, Godfrey, & Godley, 2009; Kamel, Mrosovsky, & Mrosovsky, 2004). Shifts in sex ratios have been detected, with a tendency towards feminization of certain populations, an

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5 increase in hatchlings’ mortality rate and loss of nesting sites, which will have a major impact on population dynamics (Fish et al., 2008; Fish et al., 2005; Fuentes, Limpus, Hamann, & Dawson, 2009; Hawkes et al., 2009; Jensen et al., 2018; Patiño-Martinez, Marco, Quiñones, & Hawkes, 2012).

Nowadays, in Cape Verde, excessive harvesting of adult females for their meat and of males for their penis, which is sold as an aphrodisiac (5-36% of the captured turtles), egg poaching and fishery by-catch are some of the major threats. The continuing decline of habitat area and quality, as a result of tourism development and sand extraction activity, has also led to a reduction of the nesting levels and presently only loggerheads use these beaches to nest (Casale & Marco, 2015; Cozens, Taylor, & Gouveia, 2011; Loureiro & Torrão, 2008; Marco et al., 2008).

Due to the conservation status of sea turtles and the extreme urge for their protection over the past years, this species is receiving more attention worldwide (Hamann et al., 2010). These reptiles are now considered as “flagship species”, a term applied to charismatic species representative of the biodiversity of the ecosystem in which they inhabit (Caro, Engilis, Fitzherbert, & Gardner, 2004). Thus, they are considered symbols for several conservation projects.

Conservation actions usually involve the protection of nesting and non-nesting sea turtles, with the creation of marine or coastal protected areas, nest protection programmes and, finally, the implementation of monitoring programmes in order to evaluate the population long-term response (Metcalfe et al., 2015; Piacenza, Richards, & Heppell, 2019). Some nest protection programmes involve the translocation of nests to safer areas, when they are at risk due to erosion, sea currents or predators (Abella, Marco, & Lopez-Jurado, 2007; Kasparek, Godley, & Broderick, 2001).

All these measures have already been implemented in several places where sea turtle protection is necessary, such as in South Africa and the Brazilian coast, where the TAMAR-IBAMA Project created several research and conservation stations (Marcovaldi & Marcovaldi, 1999; Nel, Punt, & Hughes, 2013).

In Cape Verde the protection of the principal nesting beaches began in 1998 and, in 2008, the Cape Verde government developed a national plan for the conservation of sea turtles (PNCTM-CV) (Neves, 2010). The Maio Biodiversity Foundation (FMB) in a non-governmental organization that operates on this island since 2013 and is mainly responsible for most of the studies regarding Caretta caretta. In 2013, a sea turtle conservation programme was implemented with the main goals of decreasing poaching and harvesting by conducting patrols, every night, during the nesting season. This community-based programme represents another approach that aims the protection of this species by helping to improve the perception that locals have on sea turtles and the threats that they face (Dutra & Koenen, 2014).

In 2018, and for the first time since the program started, there was a marked increase (of almost 300%, when comparing with 2017) in the number of nesting events recorded at Maio beaches during the monitoring program. This increase in the number of nests was tentatively interpreted as a result of the conservation efforts (Marco, Martins, Abella, & Patiño-Martinez, 2018).

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6 Nonetheless, it should be borne in mind that high-density nesting seasons enhance competition amid females. Cases were reported where females adopt aggressive behaviours or disturb the nesting activity of other females (which may delay successful oviposition), and it is still not excluded the possibility of other behavioural effects (Jessop, Limpus, & Whittier, 1999).

Study purposes

Our primary goal was to analyse the nest site distribution and fidelity of the loggerheads’ North East Atlantic population, particularly the Cape Verde subpopulation reproducing on Maio island.

Collecting information about nesting beaches and having an appreciationof key habitat use is essential to assess population trends and patterns, since favourable nest placement is crucial to the survival and recovery of this threatened species (Marco et al., 2012; Schroeder et al., 2003).

Nesting site selection (behaviour that may result in a disproportional use of nesting sites) and nest site fidelity (tendency of a turtle to return to the nest near the site where it previously nested, within a season or in a previously nesting season (Schroeder et al., 2003)) were some of the parameters compared.

This study is based on the results of two contrasting years concerning the number of nests recorded (2017 and 2018), and therefore also aims to compare nesting patterns in these two years. We investigated if: (i) nest distribution differs regarding beach orientation, length, zone and sand colour; (ii) hatching success varied among beach zones, (iii) females exhibit high nest site fidelity (iv) females select beach zones that increase hatching success; (v) there was a relationship between body size and female nesting preferences. Finally, this survey allowed the identification of main nesting beaches for loggerhead sea turtles arriving on Maio island as well as the identification of areas that will gain importance in the future, with the increase of the population.

The final purposeof the present study is to contribute to a finer knowledge of the Maio island nesting population, to help detect population trends and nesting patterns. Moreover, we hope that this work provides key information that allows the implementation of new or more effective conservation actions.

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7

Materials and methods

Study area

This study was carried out on Maio island (15o_{07’ N, 23}o_{15’ W) (Figure 1.1.), one of the}

ten smaller volcanic islands of the Cape Verde Archipelago, with an extension of 269 Km2 _(Duarte

& Romeiras, 2005; Stillman, Furnes, LeBas, Robertson, & Zielonka, 1982). This island is included in the African Sahelian region. The climate is influenced by dry and hot East winds being characterized by a tropical dry to semi-desertic climate with a long dry season (October – July) and a short rainy season (August - September) (Santos, 2005). The island maintains pristine sandy beaches with different sand colours (white, black and mixed) and low anthropogenic impact, with fishing as the most common human activity in most beaches (Marco et al., 2012). Supralittoral dune flora is constituted by xerophytes and grass species well adapted to extreme arid conditions (Suaeda vermiculata, Zygophyllum waterlotii and Cyperus conglomeratus)

(Santos, 2005).

The loggerhead nesting season extends from mid-June to mid-October, with some seasonal variations, and the nesting activity occurs every night. Thus, for this study, all beaches were patrolled ca 8 hours each night during the nesting season.

Map production

During the 2017 and 2018 nesting seasons a Garmin etrex10 GPS (Garmin etrex10 with an associated error of, approximately, 3 meters) was used to record track-logs of the vegetation line and high tide line given by FMB. The geographical information collected was gathered and analysed in a Geographic Information System (GIS) - QGIS version 2.16.2 software (QGIS Development Team, 2009). Maps of Africa and Cape Verde Archipelago (land cover shapefile)

Figure 1.1. Map of Maio island, showing its location in the Cape Verde archipelago (arrow), and in the African

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8 were imported along with the data collected on the field (GPS track-logs). The projection used for all data collection was WGS84/ Cape Verde National (EPSG 4826). The integration of all this information on GIS allowed a spatial analysis of sand colour and beach orientation distribution along the profile of the island as well as the nests distribution and a comparison between both nesting seasons. This system also allowed to estimate the distance between all beaches, used in fidelity analysis.

Nest distribution

Loggerhead turtle nesting activity was monitored daily from 24 May to 31 October 2017 (160 days) and from 9 May to 15 November 2018 (190 days). All beaches of Maio island were patrolled. The distribution of all loggerhead turtles nesting activity was assessed in accordance to name of the beach, sand colour, beach orientation and beach zone.

During 2017, 47 beaches were assigned according to their sand colour as white (n=12), mixed (n=29) or black (n=6) (Figures 1.2., 1.3., 1.4., 1.5., 1.6., 1.7. and 1.8. and Appendix – Table A.1.). The length of all beaches was measured, midway between the high tide mark and the dune vegetation, using a 50-meters measuring tape. The geographic orientation (north, north east, east, south east, south, south west, west, northwest) of each beach was determined using a Geographic Information System (GIS) - QGIS version 2.16.2 software (QGIS Development Team, 2009).

Figure 1.2. Map of Maio Island beaches (north and north east), emphasizing all different types of sand colour (white,

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9

Figure 1.4 Map of Maio Island beaches (south east), emphasizing all different types of sand colour (white,

mixed and black).

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10

Figure 1.6 Map of Maio Island beaches (west), emphasizing all different types of sand colour (white, mixed and black). Figure 1.5. Map of Maio Island beaches (south west), emphasizing all different types of sand colour (white, mixed and

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11 The 47 nesting beaches surveyed are not distributed homogenously throughout Maio island. Most beaches face north east (10 beaches) and east (13 beaches), with black sand beaches being recorded only in these orientations. White beaches mostly face south west (3 beaches) and west (4 beaches). Beaches facing south east are characterized by mixed sand colour (3 beaches) (Figure 1.8.). The same heterogeneity is observed regarding the mean length of the beaches. In the northern part of the island the few beaches that can be found there are, in average, wider than the rest. In contrast, the beaches facing south west, north east and east, are, in average, smaller and have less space for loggerheads to nest (Figure1.9.).

Figure 1.8. Distribution of nesting beaches, on Maio island, across the island according to orientation and sand colour.

A letter system was used to denominate beach orientation: N= North, NE=North east, E= East, SE= South east, S= South, SW= South west, W=West and NW= North west. Different types of sand colour are represented by different colours: Black= Black beaches, Grey=Mixed beaches, White= White beaches.

0 1 2 3 4 5 6 7 8 9 N NE E SE S SW W NW Mixed Black White

Figure 1.7. Map of Maio Island beaches (north west), emphasizing all different types of sand colour (white, mixed and

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12

Figure 1.9. Average beach length across the Maio island, according to beach orientation. An abbreviation system was

used to denominate beach orientation N= North, NE=North east, E= East, SE= South east, S= South, SW= South west, W=West and NW= North west. Beach lengths are in meters..

Each beach was divided, along the transverse profile, in three sections (zones), according to the probability of inundation of nests: Z1, with very low risk of inundation, including the vegetation zone; Z2, comprehending the sand bank that can be subdued to the effects of tides during storm tides; Z3, intertidal zone ranging between the lines of maximum high tide and the minimum low tide (Figure 1.10.) (Quiñones, Patiño-Martinez, & Marco, 2007).

Figure 1.10. Beach zonation along the transverse profile, according to probability of nests inundation. Z1- vegetation

zone, with no risk of inundation; Z2- sand bank section that can be subdued to the effects of tides; Z3- intertidal zone. Vegetation and C. caretta images were designed by Mason McNair and James R. Spotila and Ray Chatterji, respectively (www.phylopic.org).

Due to the high number of female emergences during the nesting season, these tend to mask one another’s activities. In order to correctly identify nests distribution, and avoid over-sampling, turtles nesting activity was monitored every night from 8 p.m. to 4 a.m. and all tracks were then erased. Successful nests were only counted when females were found covering, camouflaging the nests or there were clear signs of this behavior (nesting crawl).

All sea turtles found were checked for Monel flipper tags and Passive Integrated Transponders (PIT) and the corresponding information registered. If no tag was found, females were marked with one Monel flipper tags on each front flipper, and a PIT at the right front flipper. Females were tagged only after laying in order to minimize disturbance. Female turtle curved

0 500 1000 1500 2000 2500 3000 3500 N NE E SE S SW W NW

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13 carapace length (CCL) was measured from the nuchal scute to the posterior notch at midline between the supracaudals, using a flexible tape to the nearest 0,1 cm (Figure 1.11.) (Bolten, 1999).

All data was collected by FMB members (2/3 patrollers per beach). Between July and September 2018, I assisted in the data collection.

Figure 1.11. Anterior and posterior anatomical points for measuring curved carapace length (CCL). Sea turtle image

was designed by (after McCulloch 1908) (www.phylopic.org).

Determination of hatching success

In 2018, 50 nests (NPraiona=25, NBoca Ribeira=25) were randomly chosen to estimate hatching

success, in two mixed sand colour beaches. The number of eggs was counted after laying had begun, so that disturbance and aborted nesting events were avoided. Nests were marked by triangulation, using three wooden sticks, each one distancing 1 meter from the nest center, and a rope placed in the nest chamber, at the moment of oviposition, to easily locate the nest. The hour and date of oviposition and coordinates were registered.

All nests were checked daily during the incubation period (period between oviposition and emergence), looking for evidences that the nest had suffered erosion, inundation, predation, human impact and/or emergence events. Nests were marked as inundated or eroded if these events happened at least once during the incubation period. All nests with any evidence of disturbance by crabs were considered to have been predated. Human impact was considered when nests showed signs of excavation attempts or when eggs had been removed.

After the incubation period, nest exhumation was carried out. This process was performed 5 days after the last emergence or at the end of the incubation period known for this turtle species (60-65 days), if no sign of emergence was observed. Each marked nest was excavated, and all successful eggs (hatched eggs fragments constituting more than 50% of the eggshell) were counted (Patiño-Martinez, Marco, Quiñones, & Godley, 2008). Hatchling success was calculated dividing the total number of hatched eggs by clutch size (Eckert, Bjorndal, Abreu-Grobois, & Donnelly, 1999).

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14

Nest site fidelity

For this survey we recorded females that were seen nesting two or more times during the same nesting season (Neeman, Harrison, Wehrtmann, & Bolaños, 2015; Santos et al., 2016).

Females were checked for flipper tags and PITs and the corresponding information recorded. If no marks were found, turtles were tagged, following the method previously described. Female turtle CCL was measured, following the method previously described. We recorded information on date, name of the beach, beach length, sand colour, zone and beach orientation. Female tracks were erased at the end of the procedures to avoid nest excavation attempts or stolen eggs.

Statistical methods

An independent two sample t-test was performed to compare if the number of nests recorded, per day, was different among nesting seasons. Normality and homoscedasticity were assumed due to the large sample size (Hoshmand, 2017).

To verify if there were differences in CCL between 2017 and 2018, since normality and homoscedasticity assumptions were admitted, an independent two sample t-test was performed. To analyse the CCL distribution in each year, skewness (measure of the asymmetry of a distribution) and kurtosis (measure of “peakedness” of a distribution) were calculated, running the e1071 package from R studio (Meyer et al, 2019). In cases where there were two or more CCL measurements of a single female only the first was considered.

Chi-squared goodness of fit tests were carried out to determine if the number of nests differed according to: (i) orientation, (ii) sand colour and (iii) zone. The Pearson correlation coefficient was calculated to test if beach length was associated with the number of nests in each of the surveyed seasons. The number of expected nests in each orientation and sand colour classes were calculated based on the total length of each beach class.

A Mann-Whitney U was used to evaluate if there was variation in hatchling success according to the beach zone.

To verify if female CCL was correlated with beach length a Pearson correlation was executed. To determine if there were differences in female length among beach orientation, sand colour and zone a one-way ANOVA and the post-hoc Tukey test were used.

The observed clutch frequency is designated as the number of nests per female, in the same nesting season and was calculated as the number of nesting events that were detected per female (Saba et al., 2007). Assuming normality and homoscedasticity, a t-test was performed to assess if clutch frequency varied among nesting seasons. A Spearman correlation test was made to evaluate if CCL and clutch frequency were associated.

The average distance travelled between beaches, in consecutive nesting events was calculated for each female, as well as the average overall distance between all beaches of the island. An one sample t-test was used to compare the average distances calculated, in each year, with the average distance between beaches calculated for the Maio island. An independent two sampled t-test was done to compare if the mean distances calculated per female differed between

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15 2017 and 2018. For both years, a Pearson correlation was made to evaluate if carapace length and average distance between nesting beaches were correlated.

To assess nest site fidelity a graphical analysis was made. In order to do that, the number of sites with different characteristics visited by each individual was counted according to: (i) orientation; (ii) sand colour, (iii) zone and (iv) beach length. The results were displayed in a grouped bar plot (for graphics (i), (ii) and (iii)) and a scatterplot (for graphic (iv)). All graphs were made using the ggplot2 and ggpubr packages from R studio (Wickham, 2016; Kassambra, 2019). To ascertain if the fidelity exhibited regarding (i) orientation, (ii) sand colour, (iii) zone differed with female CCL, one-way ANOVAs followed by post-hoc Tukey tests were performed.

For the statistical tests referred above, to assess with more accuracy the average distance between beaches, we considered only females that were recorded nesting more than three times.

Then the hypothesis of nest site fidelity according to beach zone was investigated in more detail. In order to test this, we selected pairs of consecutive nesting events of females that were recorded nesting 2 or more times within a season. This analysis included only consecutive nests within an interval of 11 to 17 days as the mean internesting period is of 14 days for this species and intervals greater than 17 days may result of incomplete capture and recapture records (Tucker, 2010). The dependence amid consecutive nest sites was tested using a chi-squared goodness of fit test (Nordmoe et al., 2004).

To compare the importance of all the island beaches in 2017 and 2018, the beach utilization rate was calculated based on the following formula:

Equation 1.1

Beach utilization rate =Number of nests per beach Number of nests per year

To analyse if there were differences in the same beach utilization rate between 2017 and 2018, a two proportions Z-test was performed and three groups created, based on the relative use in 2018: A, more important nesting beaches in 2018; B, less important nesting beaches in 2018 and C, nesting beaches equally used among years. A Fisher’s exact test was used to analyse differences between the three groups considering sand colour and beach orientation. When differences were found a pairwise comparison using Fisher exact test was performed using the RVAideMemoire package from R studio (Hervé, 2019).

A significant level of 0.05 was considered for all statistical tests.

All statistical analyses were performed using R studio version 3.1.1 software (R Development Core Team, 2014).

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16

Results

Annual number of nests

A total of 5219 and 13925 nesting events were recorded during 2017 and 2018 nesting seasons, respectively. There was a highly significant difference in the mean number of nests dug per day between both years (t252=-8.50, p<0.001), with an average of 42.78 (±28.55 SD) nests per

day recorded in 2017 and an average of 105.50 (±76.55 SD) nests per day in 2018 (Figures 1.12., 1.13. and Appendix – Table A.1.)

Figure 1.12. Number of C. caretta nests recorded per day on Maio island, during 2017 nesting season. The first day of

the nesting season (24 May) was taken as day “0” and the final day of the nesting season (31 October) as day “125”.

Figure 1.13. Number of C. caretta nests recorded per day on Maio island, during 2018 nesting season. The first day of

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17 The overall average CCL for the females that nested in 2017 was 78.69 cm (±3.74 SD, range 67-108 cm) and the average CCL for females that nested in 2018 was 79.78 cm (±4.41 SD, range 64–98.5 cm). Overall, the distribution of females CCL was leptokurtic (g2=5.02), in 2017.

In contrast, in 2018, the distribution of female CCL was normal. There were differences in female CCL between both nesting seasons (t-test: t3000.5 = -9.5534, p<0.001), with females from the 2018

nesting season having a larger mean CCL than those from 2017 (Figure 1.14.).

Figure 1.14. Frequency distributions of C. caretta female curved carapace length (CCL) throughout 2017 and 2018

nesting seasons, on Maio island. The black bars represent 2017 female population and grey bars represent 2018 female population. Curved carapace length is in centimetres.

Nest site distribution

In 2017, there was no significant relation between the number of nesting events and beach length (Pearson correlation: r=0.07, n=44, p= 0.63). In the second surveyed year (2018), the pattern evidenced the same absence of relation (Pearson correlation: r= 0.24, n=40, p= 0.14).

Nesting activity was not equal across the island regardless of the year - with some beaches receiving a larger number of nests than others (Figures 1.15. and 1.16).

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18

Figure 1.15. Distribution of C. caretta nests in 2017, across the different types of beaches (mixed, black

and white) existing on Maio island. The size of the red circle is proportional to the number of nests found in each beach. Black circles identify beaches without nesting events.

black and white) existing on Maio island. The size of the green circle is proportional to the number of nests found in each beach. Black circles identify beaches without nesting events.

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19 Both in 2017 and 2018, the number of nests was different among beach orientations (chi-square test: χ2

7=12819.18, p<0.001, in 2017, and χ27=25129.99, p<0.001, in 2018). Higher nest

frequencies were found on beaches facing east (44.15%, in 2017, and 44.06%, in 2018) and north east (23.63%, in 2017, and 19.88%, in 2018), where nesting activities, per meter, were more frequent than expected by chance (Tables 1.1. and 1.2.).

Table 1.1. Observed and expected (according to the available beach extent) number of C. caretta nests on beaches with

different orientations, in 2017 nesting season, on Maio island. N= North, NE=North east, E= East, SE= South east, S= South, SW= South west, W=West and NW= North west. Beach length is given in meters.

Orientation Beach Length (m) Number of nests Expected number of nests N 5391 192 553.49 NE 2023 1233 207.70 E 5509 2304 565.61 SE 1911 334 196.20 S 22176 331 2276.80 NW 2563 62 263.14 W 6577 638 675.26 SW 4683 125 480.80

Table 1.2. Observed and expected (according to the available beach extent) number of C. caretta nests on beaches with

different orientations, in 2018 nesting season, on Maio island. N= North, NE=North east, E= East, SE= South east, S= South, SW= South west, W=West and NW= North west. Beach length is given in meters.

Orientation Beach Length (m) Number of nests Expected number of nests

N 4291 650 1229.27 NE 1922 2768 550.61 E 6104 6135 1748.65 SE 1911 774 547.45 S 22038 1241 6313.35 NW 679 13 194.52 W 7139 1913 2045.15 SW 4524 431 1296.02