Relationship Between Sea Grass And Sea Turtle Biology Essay


Sea turtles are distributed throughout the tropical and subtropical seas. They have a complex history, with juveniles and immature stages shifting foraging habitats and adult females performing long distance breeding migrations (Mazaris et al., 2008). In coastal and marine ecosystem, sea turtles have as important role as a keystone species that transfer nutrient and energy from the ocean to the land at nesting beaches when they deposit their eggs, and they affect the structure and functioning of foraging habitats such as coral reefs, sea grass meadows, algal beds, and soft substrate sea bottom (SWOT, 2006).

Sea turtles select a nest site by deciding where to emerge from the surf and where on the beach to put their eggs (Witherington and Marti, 1996). However, this is still speculative in order to choose to nest on some beaches and not others (Van meter, 1992). For instance, In Japan, an analysis of nesting beaches revealed that factors affecting beach selection by turtles included softness of the sand and beach length (Kikukawa et al., 1999), while in the Mediterranean Loggerhead sea turtle emerge primarily on beaches that are fronted by predominantly sandy areas (Le Vin et al., 1998). Other factors that influence nesting site selection by sea turtle are vegetation cover, distance to vegetation, humidity, temperature of the sand, and distance to high tide line (Kamel and Mrosovsky, 2004; Karavas et al., 2005; Mazaris et al., 2006; Pike, 2008).

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Sea grass is one of the most widespread coastal vegetation types in the world. It protects shorelines against erosion in the middle and lower intertidal and sub tidal zones, because of their gregarious growth and dense root systems (Dahdouh-Guebas et al., 2006). They are also known to accumulate and stabilize sediments from the surrounding environment (Fry et al., 1983). Sea grass beds are also important because they provide breeding and development grounds for many species of fish, shelfish and crustaceans (, 2009), foraging ground for herbivores (Musick and Limpus 1997) and attachment sites to small macroalgae and epiphytic organisms such as sponges, bryozoans, forams, and other taxa that use sea grasses as habitat (, 2009)

So far, there is no research has been done to see what is the relationship occurred between the Loggerhead sea turtle and presence of sea grass. This could be an interesting question to answer the curiosity of researcher about nesting behaviour of this species.

Distribution and Nesting Ecology

Loggerhead sea turtles are circumglobal, occurring throughout the temperate and tropical regions of the Atlantic, Pacific, and Indian Oceans (NOAA-Fisheries., 2009). They are also highly migratory, capable of travelling hundreds to thousands of kilometres between foraging and breeding areas. Female Loggerhead does not appear to migrate to just one foraging area. Rather, they move continuously and thus appear to forage at a series of coastal areas. Moreover, females migrate to nest at their natal beaches about every 3 years (Plotkin, 1997)

In Mediterranean, the important nesting sites of Loggerhead sea turtles are found in Greece, Turkey and Cyprus (Margaritoulis et al., 2003), while others nesting sites with lower density are found in Syria (Kasparek, 1995), Tunisia (Laurent et al.,1990) and Israel (Kuller, 1999). For Greece, nesting Loggerhead are significantly smaller that those other parts of the world (Margaritoulis et al., 2003). Following nesting data from several seasons, Margaritoulis (2000) classified nesting areas in Greece as "major" or "moderate". "Major" nesting areas are those hosting on average more than 100 nests/season and over 6 nests/km/season. Only five areas in Greece fulfil the requirements for "major" areas, there are: Laganas Bay (Zakynthos island), Kyparissia Bay (western Peloponnesus), Rethymno (Crete), Lakonikos Bay (southern Peloponnesus) and the Bay of Chania (Crete island) (Margaritoulis, 2000).

Loggerhead nesting in Greece is highly seasonal. The nesting season usually extends from end of May to late August (Margaritoulis and Rees, 2001). Nesting success varied from area to area, generally caused by diversity of nesting habitat (Table 1-1).

Table 1‑. The main nesting areas monitored during 2002 in Greece

Nesting Area

Beach length (km)

Number of emergences

Number of nest

Overall nesting success (%)

Nesting Density (nest/km)

Laganas Bay (Zakynthos)






Southern Kyparissia Bay





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Lakonikos Bay






Bay of Chania






Bay of Messara


















Sources : Margaritoulis and Rees (2003)

Feeding and Diving Behaviour

Loggerhead sea turtles are known as a carnivorous, foraging primarily on benthic invertebrates throughout their distribution range. Loggerhead populations from different geographic locations forage on different types of prey, and the list of the types of prey eaten by Loggerhead in the wild is extensive. The high diversity in the types of their prey demonstrates versatility in foraging behaviour, suggesting that the Loggerhead is a generalist (Plotkin, et al., 1993).

Dodd(1988) stated that this species eats a variety of foods for each stage. Juvenile Loggerhead particularly feed on coelenterates while sub adult and adult feed on jelly fish but they are primarily feeder on benthic invertebrates. He also mentioned that Loggerhead take alga occasionally, perhaps ingesting it while feeding invertebrates. Table 2 shows diet preference of Loggerhead sea turtles.

Table 1‑. Diet preference Loggerhead sea turtles (Carreta carreta)




4.0 - 5.6 cm (SCL)

Cnidaria, Tar, Synthetics, Sargassum, Crustacean, Hydrozoans, Insects, Gastropods, Plant Material

Atlantic, off at sea

4.5 -47 cm (CL)

Sargassum , Plant material, Insects , Crustacean, Cnidaria, Tar, Fish eggs, Plastics/synthetics

Atlantic Ocean

4.1 - 7.8 cm (SCL)

Sargassum, Plant Material, Cnidaria, Copepods, Insects, Plastics &Tar, Polycheates, Bryozoan

Atlantic Ocean


Sargassum, Gastropods, Crustacean

Florida, stranded

13.5 - 74.0 cm (CCL)

Gastropods, Cephalopods, Crustaceans, Cnidaria, rochordata, Fish, Annelids, Algae

Pacific Ocean

Mean 61.4 cm (SCL)

Pleuroncodes planipes - Pelagic crab

Pacific Ocean

4.6 - 10.6 cm (CCL)

Synthetics, Cnidaria, Crustacea, Gastropods, Plant Material

Pacific Ocean

From various sources. Summarized by Boyle and Limpus, 2008

Diving plays a central role in the lives of all air-breathing marine vertebrates, including sea turtle (Rice and Balazs, 2008) and it is influenced by body size (Schreer and Kovacs, 1997). (2004) reported that the younger of Chelonian mydas (8-10 weeks of development period) dives were usually shallow (≤ 6 m) and consisted of three (V, S, U) profiles. The older can dives only slightly deeper than the younger. In contrast, adult can dive in excess of 100 - 135 m (Rice and Balazs, 2008). Margaritoulis and Teneketzis (2001) reported that most of loggerhead were captured in Lakonikos Bay, Greece are dominated in 18 metres depth.

Oceanic Loggerhead spend 75 % of the time in the top of water column; 80% of dives are 2-5 meter, and reminder of the dives are distributed throughout the top 100 m. Occasionally this species can dive greater than 200 m (Bolton and Rieward, In general, small size should limit diving depth and duration because of volume of tissue to store oxygen is lower and mass specific metabolic rates of smaller animal are higher (Schmidt-Nielsen, 1997).

Research Problem

The most important nesting sites of the Loggerhead in the Mediterranean are located in Greece. The sites are dispersed along Greece's western and southern coast line and on Crete Island (Margaritoulis et al., 2003). However, the population of the Loggerhead in Greece is declined rapidly in the last decade. Human activities such as fishing pressure, coastal development, extensive urban expansion for tourism and recreation, are the most factors that influencing in decreasing of number of this species (Arianoutsou, 1988; Margaritoulis et al., 2003).

Loggerhead sea turtles (Caretta caretta) have defined as endanger species in the world By IUCN. Therefore, many international treaties and agreement have been set up to protect the existence of this species (NOAA-Fisheries., 2009). Many studies have been done in order to understand nesting habitat suitability criteria, but they are rarely reach consolidation (Miller et al., 2003) and mainly focused on the nesting beaches, and take less attention on non nesting beaches that are relative nearby. Knowing that sea turtle spend most of their life in the marine environment, understanding how they interact in their environment is one of important factor for assessing habitat suitability and can lead to enhance successful of management decision and conservation strategies.

Research Objectives

General Objective

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The main objective of this study is to investigate the relationship between sea grass and nesting sites selection by Loggerhead sea turtles (Caretta caretta) in Crete, Greece.

Specific Objective

To characterize percentage cover of sea grass as a parameter to determine nesting site selection by Loggerhead sea turtles (Caretta caretta).

Research Questions

a. What are the differences between coastal physical parameters in nesting beaches and non-nesting beaches?

b. Which are the important coastal physical parameters that are more correlated with the nesting habitat of the Loggerhead sea turtle?

Can the percentage cover of sea grass be confirmed as an indicator to determine sea turtle nesting site selection?

Research Hypothesis

H10: The coastal physical parameters do not have a significant difference in nesting beaches and non-nesting beaches.

H11: The coastal physical parameters do have a significant difference in nesting beaches and non-nesting beaches.

H20: There are no coastal physical parameters that are significantly important to nesting habitat of Loggerhead sea turtle.

H21: There are coastal physical parameters that are significantly important to nesting habitat of Loggerhead sea turtle.

H30: There is no association between percentage cover of sea grass and the number of nests.

H31: There is a negative association between percentage cover of sea grass and the number of nests.

Material and Methods

Study Area

Crete is the largest island in Greece and the second biggest (after Cyprus) of the east Mediterranean (Figure 2-1). It lies at the southern Aegean Sea (23°31' to 26°18' E and from 34°55' to 35°41'N) and at the crossroads of three continents Europe, Asia and Africa. Crete covers an area of 8,336 km2, with a length of 260 km, and a width that from 12 to 60 km. The total length of the Cretan coastline is 1046 km and consists of both sandy beaches and rocky shores (West-Crete, 2008).

Figure 2.1‑ Map of Crete Island, Greece.

Administratively, Crete is one of the 13 regions of Greece and is divided to four prefectures (Hania, Rethymnon, Heraklion and Lassithi) and 70 municipalities. The population of the island is approximately 630.000 (2005), and over a third of it is found in the three major cities, Iraklion (~150.000), Hania(~50.000) and Rethymnon (~30.000) located on the north coast of the island (, 2009).

Crete was chosen as a study area because of this island has been categorized as one of important nesting sites for Loggerhead sea turtle in the Mediterranean. There are 3 areas that were indicated as main nesting sites in Crete i.e Rethimnon, Hania and Messara (Margaritoulis and Rees, 2003).

In the Mediterranean Sea, Posidonia oceanica is the dominant endemic sea grass and its meadows are considered as one of the most important and productive ecosystems in coastal waters. It covering the sea bed from the surface down to about 40 m (Montefalcone et al., 2008).

Research Scheme

Figure 2.2‑. Research Scheme

Data Collection

In situ data collection

Fieldwork was carried out from September 28th to October 18th. The data were collected includes the information about nesting beaches, non nesting beaches, and presence/absence of sea grass. Most of the beaches was been visited in this study are based on the data from the previous study (Asaad, 2009). There are 4 additional sample points that have been added in this study. All of point observations are presented in Table 2-1.

Table 2‑. Point of Observations


Nesting Status

Sea grass Presence/Absence



































Vai (Palm Beach)




























1: Asaad (2009); 2: This study

*: It was indicated as non-nesting sites

Data from Natura 2000 were used to locate sea grass presence. There are 6 areas covered by Natura 2000 project in Crete i.e Setia, Zakros, Rethimnon bay, Kissamos, Paleohora, and Elafonissos (Figure 2-2). The methods that are used to collect the sand samples are based on Asaad, (2009). The sand samples were taken only for new nesting and non-nesting beaches.

Cover fraction of sea grass are identified based on still images taken along the sea grass area using transect sampling method. The photos are captured using an underwater camera, Olympus ST 8000 at 4 minutes interval from rubber boat along the transect line. The camera and GPS were attached to the stainless stick. The leveller was used to keep the position of the camera when submerged in seawater. The time stamp of the GPS position and of the photo allowed the geolocation of each photo.

Figure 2.3‑. Map of Observation points in Crete Island, Greece

Percentage cover of sea grass

Image stitching or photo stitching is the technique of combining numerous images with overlapping fields of view to produce a segmented panorama or high-resolution image (Wikipedia, 2009). It is performed through autostitch Panorama software was used to get the use of computer software, most approaches to image stitching require nearly exact overlaps between images and identical exposures to produce seamless results. It is also known as mosaicing (M. Brown et al., 2003).

Data Analysis

Sand Samples Analysis

Sand samples were analysed at ITC laboratory for 7 parameters of beach characteristics i.e pH, conductivity content, grain shape, grain cleanliness, Sodium Chloride (NaCl) content, Calcium Carbonate (CaCO3) content, and grain size. All methods in this analysis are following methods which is done by (2009).

Statistical Analysis

The statistical analyses were used in this study are focusing on determine the difference in value of each coastal parameters in both nesting and non-nesting, the identification of the relationship between sea grass presence and nesting occurrence, to determine the parameters that are correlated with nesting activity, and the identification of the relationship between percent cover of sea grass and number of nest. All of the analyses were done using SPSS 16.

Each parameter is tested using independent t-test and chi-square significant tests. The relationship between percent cover of sea grass and number of nest is tested using a correlation test. The independent t-test is used to see the significant difference between the means of continuous variable of two groups on some independent variable where those two groups are independent of one another. Nesting and non-nesting beaches are independent variables of two groups while the other independent variables are pH, conductivity, NaCl content, CaCO3 content, and number of nest. The other variables, sand grain shape and sand cleanliness are tested using a chi square test.

The factor that highly correlated to suitable nesting beaches is tested using a logistic regression. To determine whether the coastal parameters are influenced to accessibility of se grass, a multiple linear regression was used, normalized in order to alleviate the influences of the said parameters thus the increasing the variability of the accessibility of sea grass using Multiple Linear regression.


Sand Characteristics

A total of 7 parameters were analysed for sand characteristics in ITC laboratory i.e pH, conductivity content, grain shape, grain cleanliness, Sodium Chloride (NaCl) content, Calsium Carbonate (CaCO3) content, and grain size. All of this data are presented in Appendix .





Figure 3.1‑. Comparison of pH variations of the sand in non-nesting beaches and nesting beaches. A. This study, B. Asaad (2009), and C. All





Figure 3.1‑. Comparison of conductivity variations (µS/cm) of the sand in non-nesting beaches and nesting beaches. A. This study, B. Asaad (2009), and C. All

NaCl content




Figure 3.1‑. Comparison of NaCl content variations (ppm) of the sand in non-nesting beaches and nesting beaches. A. This study, B. Asaad (2009), and C. All

Grain Size

Figure 3.1‑. Comparison of three major grain size of the sand in non-nesting beaches and nesting beaches. A. This study, B. Asaad (2009), and C. All

Grain shape




Figure 3.1‑. Comparison of grain shape of the sand in non-nesting beaches and nesting beaches. A. This study, B. Asaad (2009), and C. All

Grain Cleanliness

Figure 3.1‑. Comparison of grain cleanliness of the sand in non-nesting beaches and nesting beaches. A. This study, B. Asaad (2009), and C. All

Sea grass presence and nesting occurrence

Figure 3.2‑. The relationship between sea grass presence/absence and number of nest.


Precentage cover of sea grass

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Conclusions and Recommendations