Nest site selection and hatching success of hawksbill and ... · Loggerhead turtles nested...

Phyllomedusa - 8(1), July 2009

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Nest site selection and hatching success of hawksbilland loggerhead sea turtles (Testudines, Cheloniidae)at Arembepe Beach, northeastern BrazilThiago Zagonel Serafini1, Gustave Gilles Lopez2 and Pedro Luís Bernardo da Rocha1

1 Departamento de Zoologia, Instituto de Biologia, Universidade Federal da Bahia. Rua Barão de Geremoabo, s/n, Campusde Ondina, 40170-290, Salvador, BA, Brazil. E-mails: [email protected], [email protected].

2 Fundação Pro-TAMAR. Caixa Postal 2219, 41950-970, Salvador, BA, Brazil. E-mail: [email protected].

Received 7 November 2008.Accepted 13 May 2009.Distributed July 2009.

Phyllomedusa 8(1):3-17, 2009© 2009 Departamento de Ciências Biológicas - ESALQ - USP

ISSN 1519-1397

AbstractNest site selection and hatching success of hawksbill and loggerhead sea turtles(Testudines, Cheloniidae) at Arembepe Beach, northeastern Brazil. Nest siteselection influences the hatching success of sea turtles and represents a crucial aspectof their reproductive process. Arembepe Beach, in the State of Bahia, northeasternBrazil, is a known nest site for Caretta caretta and Eretmochelys imbricata. For thenesting seasons in 2004/2005 and 2005/2006, we analyzed the influence of beachprofile and amount of beach vegetation cover on nest site selection and the hatchingsuccess for both species. Loggerhead turtles nested preferentially in the sand zone,while hawksbill turtles demonstrated no preferences for either sand or vegetation zone.Beach vegetation was important in the modulation of nest site selection behavior forboth species, but the amount of beach vegetation cover influenced (negatively)hatching success only for the hawksbill, mainly via the increment of non-hatched eggs.Hatching success, outside the tide risk zone, was not influenced by the position ofthe nests along the beach profile. The pattern of nest distribution by species indicatedthat management of nests at risk of inundation and erosion by the tide is moreimportant for loggerhead turtles than for hawksbill turtles. Beach vegetation is animportant factor in the conservation of these sea turtle species. Nests that are at riskdue to tidal inundation and erosion can be translocated to any position along the beachprofile without producing any significant effect on hatching success, as long as highdensities of beach vegetation cover are avoided for hawksbill nests. It is importantto point out that the pattern we report here for distribution of hawksbill nests alongthe beach profile could be due in part to the influence of pure and hybrid individuals,since there are reports of hybridization among hawksbills and loggerheads to the studysite.

Keywords: Testudines, Cheloniidae, Caretta caretta, Eretmochelys imbricata, nestingpreference, conservation, Brazil.


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ResumoSeleção de locais de nidificação e sucesso de eclosão da tartaruga-de-pente e da tartaruga-cabeçu-da (Testudines, Cheloniidae) na Praia de Arembepe, nordeste do Brasil. Em tartarugas marinhas, aseleção de locais de desova influencia o sucesso de eclosão e representa um aspecto crucial de seu pro-cesso reprodutivo. A Praia de Arembepe (BA, Brasil) é um local de desova de Caretta caretta eEretmochelys imbricata. Durante as temporadas de nidificação de 2004/2005 e 2005/2006, analisamos ainfluência da posição do ninho ao longo do perfil da praia e da cobertura vegetal sobre a seleção delocais de desova e o sucesso de eclosão das duas espécies. Caretta caretta desovou preferencialmenteem zonas da praia sem vegetação (zona de areia), enquanto E. imbricata não mostrou preferência pelazona de areia nem por locais dotados de cobertura vegetal (zona de vegetação). A vegetação foi impor-tante para o comportamento de seleção de locais de desova para ambas as espécies, mas a quantidade decobertura vegetal da praia influenciou (negativamente) apenas o sucesso de eclosão de E. imbricata, prin-cipalmente por meio do aumento de ovos não-eclodidos. Em locais sem risco de erosão e inundação pelaação da maré, o sucesso de eclosão não sofreu influência da posição dos ninhos ao longo do perfil dapraia. O padrão de distribuição dos ninhos por espécie indicou que o manejo dos ninhos sujeitos a inun-dação e erosão pela ação da maré é mais importante para C. caretta do que para E. imbricata. A vegeta-ção da praia é um fator importante para a preservação dessas espécies de tartarugas marinhas. Os ninhosameaçados de inundação e erosão poderiam ser transferidos para qualquer outro local ao longo do perfilda praia sem qualquer efeito significativo sobre o sucesso de eclosão, contanto que locais com altas den-sidades de cobertura vegetal sejam evitados para os ninhos de E. imbricata. É importante salientar que opadrão encontrado para a distribuição dos ninhos de E. imbricata ao longo do perfil da praia poderiadever-se, em parte, à influência de indivíduos puros e híbridos, já que existem relatos de hibridizaçãoentre essa espécie e C. caretta para o local de estudo.

Palavras-chave: Testudines, Cheloniidae, Caretta caretta, Eretmochelys imbricata, preferência por lo-cais de nidificação, conservação, Brasil.

Introduction

Nest site selection by turtles represents acrucial aspect of their reproductive biology asthe environment where eggs are incubateddirectly influences hatchling survival (Ackerman1997). Some environmental variables, like sandtemperature (Yntema and Mrosovsky 1982),sand humidity (McGehee 1990) and rate of gasexchange between the nest and the environment(Ackerman 1980) are directly related toembryonic development. These variables differacross available nesting habitats due to suchfactors as grain size (Mortimer 1990, Foley etal. 2006), beach profile (Hays and Speakman1993, Kamel and Mrosovsky 2006), andpresence (or absence) of vegetation (Janzen1994). Therefore, selection of a nest site byfemale turtles directly affects hatching successand thereby reproductive success.

Nest site locations vary among sea turtlespecies. Studies on leatherbacks (Dermochelys

coriacea) have shown a great deal of intra-individual variation in nest site selection. Forexample, a single female will nest at differentdistances from the high tide line at eachoviposition, resulting in a wide dispersion of thenests she produces. Such behavior results inmany nests being laid near the high tide line,with a considerable loss of eggs due to beacherosion (Eckert 1987, Kamel and Mrosovsky2004). On the other hand, studies on hawksbills(Eretmochelys imbricata) have detected a moreselective nesting behavior, with individualfemales systematically nesting far away fromthe high tide line and closer to vegetation(Kamel and Mrosovsky 2005). The differencesbetween these species results in differentfrequencies of egg loss due to naturalphenomena, like beach erosion and incubationenvironment. The presence of beach vegetationmay also affect nest site selection, as some seaturtle populations choose nest sites associated(or not associated) with beach vegetation


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Nest site selection and hatching success of hawksbill and loggerhead sea turtles

(Horrocks and Scott 1991, Hays and Speakman1993, Hays et al. 1995, Kamel and Mrosovsky2004, 2005, 2006). Usually nesting beachespresent a vegetation zone beyond the beachslope, namely on the beach berm. The influenceof this vegetation zone on hatching success ispoorly evaluated in the scientific literature.

Although populations differ in their choiceof nest sites relative to the proximity of vegeta-tion, studies focusing on this phenomenon haveusually been restricted to implications for sexdetermination (Morreale et al. 1982, Spotila etal. 1987). Studies focusing on hatching successusually compare the open beach zones withvegetated zones (Horrocks and Scott 1991,Kamel and Mrosovky 2005, Karavas et al.2005), without taking into account the effectsof the amount of vegetation cover on top of thenests located in vegetated zones.

Understanding the influences ofenvironmental variables, such as vegetativecover, on sea turtles nests is important for nestmanagement. Nest management is a valuablepractice adopted in many nesting sites, as it mayreduce risks of predation, erosion and/or humanthreat (Wyneken et al. 1988, Eckert and Eckert1990, Marcovaldi and Laurent 1996, Hitchinset al. 2004). Nests at risk due to erosion andflooding by tides are often transported tohatcheries or to safer places on the beachmainly on the beach berm, where the sand iscovered by beach vegetation of varying density.This vegetation may influence the variablesaffecting egg incubation and hatchling emergence(Ackerman 1997, Carthy et al. 2003). Therefore,knowledge of the variability of hatching successalong a variable vegetation profile is important forevolving management procedures aimed atincreasing hatchling production.

The northern coast of state of Bahia, Brazil,including Arembepe Beach, is an importantnesting area for sea turtles. Four sea turtlespecies nest on this beach, the loggerhead(Caretta caretta) and the hawksbill beingpredominant. The International Union forConservation of Nature have categorized these

species are as endangered and criticallyendangered, respectively (IUCN 2006). Theother two species that nest on Arembepe Beachare the olive ridley (Lepidochelys olivacea) andthe green turtle (Chelonia mydas) (Marcovaldiand Marcovaldi 1999). It has been known formany years that at this site nests very close tothe high tide line are at risk of erosion andinundation (Marcovaldi and Marcovaldi 1999).Therefore, such nests are transported to higherareas in the same beach on the morningfollowing deposition. To increase the success ofnest translocation on Arembepe Beach, a betterunderstanding of species-specific nest siteselection and the hatching success resultingfrom this selection is required. Additionally,although removal of beach vegetation forrecreational purposes is common, the impact ofvegetation removal on nesting sea turtles iscurrently unknown.

In this study we compared nest site selectionbetween the loggerheads, Caretta caretta(Linnaeus, 1758), and the hawksbills,Eretmochelys imbricata (Linnaeus, 1766), onArembepe Beach as well as the manner in whichnest-site selection affects hatching success in thesetwo species. In order to determine suitable areasfor translocating nests at risk for both sea turtlespecies, and to maximize the hatching successof all nests at Arembepe, we evaluated theeffects of nest location and vegetation cover onhatching success.

In Bahia’s northern coast there are reportsof hybridization among hawksbills andloggerheads. Lara-Ruiz et al. (2006) detected afrequency of 42% of the hawksbill femalessampled in the region as hybrids of this twospecies. Behavioral comparisons between pureand hybrid hawksbills have not yet beenundertaken but if there is a genetic aspect todetermination of nesting behavior, it is possiblethat there are differences between pure andhybrid individuals. Therefore, the pattern wereport here for distribution of hawksbill nestsalong the beach profile could be due in part tothe influence of pure and hybrid individuals.


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Specifically, we examined the effect of nest-to-vegetation distance and amount of vegetationcover on the success of loggerhead andhawksbill sea turtle nests. We tested thefollowing hypotheses for both species: (i) thereis preference for nesting either in exposed sandor in vegetated zones in the beach; (ii) nests arenot uniformly distributed along the beachprofile; (iii) for nests laid outside the risk zone(where erosion and flooding by tides results inlittle or no hatching success), hatching successvaries with the position of the nest on the beach;and (iv) hatching success is affected by thedensity of beach vegetation cover around thenests.

Materials and Methods

Study Area

Arembepe Beach is located on the northerncoast of Bahia, Brazil (12º45’45.7’’S,38º10’05.5’’W) (Figure 1). Along its 3 kmlength, there is a small amount of urbandevelopment. During the summer season(December to March), the beach is used forrecreation by large numbers of people, and

Figure 1 - Location of Arembepe Beach in Brazil.

Figure 2 - Partial view of Arembepe Beach. (a) sandzone, (b) vegetated zone, (c) dune, (d) stakeindicating a nest.

frequented by a low volume of vehicular traffic.The Brazilian Program for Sea TurtlesConservation and Protection (Projeto TAMAR-ICMBio) has monitored sea turtle nestingactivity on this beach since 1983 (Marcovaldiand Marcovaldi 1999).

Sand grains in the study area are medium-sized (0.25 mm). The beach is moderatelyinclined and reaches a frontal dune (of 4 maverage height) fixed by a vegetation cover. Thedominant beach vegetation species found on theberm are Blutaparon portulacoides, Ipomoeaasarifolia, I. pes-caprae, Spartina alterniflora,Canavalia rosea, Suriana maritima andcoconuts (Cocus nucifera), mostly on the dunes(Figure 2). Arembepe has a tropical climatewith a rainy season lasting from March toAugust, and a dry season from September toFebruary (Mafalda et al. 2004). The tide has amedium amplitude of 1.8/1.9 m. Nestingseasons for both species are predominantly fromSeptember to March.

Field seasons and nest monitoring

We collected data during the nesting seasonbetween August 1st and April 1st in 2004/2005

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and 2005/2006. Nest monitoring, excavation,translocation, and analysis of nestingdistribution along the beach took place in both2004/2005 and 2005/2006. Analyses of nest siteselection and hatching success were onlyconducted in 2005/2006. Nest monitoringfollowed Projeto TAMAR-CMBio’smethodology (Marcovaldi and Marcovaldi,1999) and consisted of daily morning beachsurveys, performed by a local agent hired byTAMAR. Eggs from nests at risk of tidalinundation and/or erosion (i.e., those very closeto the high tide line) were removed from theiroriginal location on the morning followingoviposition. They were carefully placed in aStyrofoam box in the same orientation in whichthey were removed from the nest, and thentransported to a man-made nest higher than thehigh tide in the same part of the beach, usuallyon the berm of the beach. These sites varied invegetation cover. Eggs were buried again inhand-made nests similar to those built by theturtles. All nests (in situ and translocated) weremarked and monitored until hatchlingsemerged. On the morning following emergence,we excavated nests to identify species anddetermine hatching success.

Beach zone preference

In 2005/2006 in order to analyze speciespreference for nesting sites, we quantified theavailability of sand and vegetated zones (seebelow) and their use, in terms of proportion ofnests per zone. Using a chi-square test, wetested the hypothesis that the proportion of nestslaid by each species in the two zones differsfrom the proportion of available habitat madeup by each zone.

To describe the pattern of nesting along thebeach profile, we measured the followingvariables of all nests on the day after nestconstruction: (i) distance from the center of thenest cavity (=distance from the nest) to thehighest tide line of the previous night; (ii)distance from the nest to the edge of thevegetated zone (nest-to-vegetation distance);and (iii) distance from the nest to the dune base(Figure 3). We considered the dune base to bethe upper limit of availability of the nestinghabitat, as the dune’s incline poses difficultiesfor turtle nesting. The width of available sandzone and available vegetated zone wasconsidered to be the perpendicular distancefrom the high tide line to the edge of the

Figure 3 - Schematic representation of Arembepe Beach profile showing nest site variables measured in the present study.Legend: i = distance between the nest and the high tide line of the prior night; ii = nest-to-vegetation distance;iii = distance between the nest and the dune base; SZ = sand zone; VZ = vegetation zone; BW = beach width.



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vegetated zone and the edge of the vegetatedzone to the dune base, respectively. Theproportion of the total beach width made up bythe sand zone and vegetated zone were thencalculated for each nest to determine theavailability of each zone specific to eachindividual nest. The sand zone wascharacterized by the presence of only sand andthe vegetation zone had sand and somevegetation covering it. The beach width wasconsidered to extend from the high tide line tothe dune base (Figure 3).

We compared the distribution of nests alongthe beach profile between the two sea turtlespecies using the nest-to-vegetation distance ofeach nest (data from 2004/2005 and 2005/2006). We then used a multiple regression testto analyze the influence of the beach width andthe size of vegetated zone on the straightdistance crawled by the turtle for nesting.

Hatching success

Hatching success was calculated as thenumber of offspring that left their egg shells asa proportion of the total number of the eggspresent in the nest (Almeida and Mendes 2007).We excavated the nests on the morningfollowing the emergence of the majority ofhatchlings and counted the number of emptyegg shells (hatchlings that emerged from thenest), of dead hatchlings (individuals thathatched, but died inside the nest) and non-hatched eggs (intact eggs with or without deadembryos). Hatching success was determined asthe ratio of the number of live hatchlings (thathad emerged or were still in the nest whenexcavated) to the total number of eggs in theclutch and calculated using the followingequation: empty shells · 100 / (empty shells +dead hatchlings [and their empty shells] + non-hatched).

On the morning following emergence, wetook digital pictures of a one square meter areawith the nest at its center. Using the Jasc PaintShop 7 software, we converted each picture to

a black and white image in which black pixelsrepresented areas covered by beach vegetationand the white pixels represent sand (Figure 4).We counted the number of white and blackpixels per picture using the software Áreafeature, developed by the Physics Statistics andComplex Systems Group from UniversidadeFederal da Bahia (available at http://www.vivas.ufba.br/Area/Area.rar), and then wecalculated the percentage of beach vegetationcover for each nest (Camacho et al. 2007).

Based on data collected during the 2005/2006 nesting season and using a multipleregression test, we evaluated the hypotheses thatthe nest-to-vegetation distance and thepercentage of beach vegetation cover around thenest influence the hatching success for each ofthe two most common species.

In order to better understand the processunderlying the effects of the nest-to-vegetationdistance and the percentage of beach vegetationcover around the nest, we also evaluated scatterplots of vegetation cover versus the componentsof the hatching success (percentage of deadhatchlings and of the non-hatched eggs).

For the multiple regression analysis, weused only the nests that were not translocated(that is, only those that were left in situ). Inorder to analyze the effects of management onhatching success, we used an ANOVA tocompare the hatching success between nestsremaining in situ and those translocated (movedto other beach sites).

Statistical analysis

We set the significance level at 0.05 forstatistical analysis. However, as the same testswere repeated for two species, we adjustedalpha values applying Bonferroni‘s correction(alpha/2) for each set of tests (Quinn andKeough 2004). We treated the following asdifferent sets of tests: nesting zone preferenceanalysis (chi-square test); nesting distributionanalysis (multiple regression tests); andhatching success analysis (multiple regression

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Figure 4 - Example of conversion of the digital picture of one square meter area around the nest (a) into black and whitepixeld (b) used to quantify percentage of beach vegetation cover.

Table 1 - Number (and percentage) of nests of loggerhead and hawksbill by management type (top) and beach zone(bottom) in Arembepe Beach during 2004/2005 and 2005/2006 nesting seasons.

Loggerhead Hawksbill

2004/2005 2005/2006 Total 2004/2005 2005/2006 Total

In situ 29 (78%) 55 (71%) 84 (73%) 30 (88%) 33 (97%) 63 (93%)

Relocated 8 (22%) 23 (29%) 31 (27%) 4 (12%) 1 (3%) 5 (7%)

Sand zone 28 (76%) 67 (86%) 95 (83%) 14 (41%) 18 (53%) 32 (47%)

Vegetation zone 9 (24%) 11 (14%) 20 (17%) 20 (59%) 16 (47%) 36 (53%)

Total 37 78 115 34 34 68

tests). All analyses were performed using SPSS13.0 Software.

Results

Nesting and nest management

Loggerhead nests were more frequent (37and 78 nests during the 2004/2005 and 2005/2006 nesting seasons, respectively) thanhawksbills (34 nests per season) (Table 1). To

avoid inundation and erosion risks due to tidemovements, 31 loggerhead and five hawksbillnests were translocated from their original sites(Table 1).

Beach zones preferences

The availability of sand and vegetated zoneson the beach based on measurements taken forboth species at individual nests during the 2005/



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2006 nesting season was 62% and 38%,respectively. Loggerheads showed a significantpreference for the sand zone (χ²=18.5; gl=1;p<<0.001), while hawksbills showed nopreference for either zone (χ²=0.8; gl=1;0.5>p>0.25) (Table 1).

Most of the nests for both species that werelaid in the vegetated zone, were in areas withlow beach vegetation cover (≤ 10%) (Figure 5).

The distribution of nest-to-vegetationdistances differed between the species (Figure6). Loggerhead nests were more widelydistributed across the beach profile, and mostof them (83%) were in the sand zone, whilehawksbill nests were more frequent (53%) inthe vegetated zone (Table 1). For both species,the majority of nesting occurred within 2 m ofthe edge of the vegetated zone. No nests werefound on the dune.

The multiple regression models used toevaluate the effect of beach width and ofvegetated zone on the straight-line distancecrawled by the sea turtles for nesting weresignificant both for loggerheads (F=21.41;r²=0.37; p<0.001) and for hawksbills (F=24.97;r²=0.62; p<0.001). The tolerance was high forboth tests (T=0.986 and T=0.793, respectively)showing that these factors are quite independentfrom each other. The partial analysis showedthat beach width positively influenced thedistance crawled by loggerhead (p<0.001) andby hawksbills (p<0.001), while the width ofvegetated zone negatively influenced thedistance crawled by loggerheads (p=0.004), butdid not affect hawksbills (p=0.080) (Figure 7).

Hatching success

The multiple regression model used toevaluate the effect of the nest-to-vegetationdistance (nest position on beach profile) and thepercentage of beach vegetation cover, on thehatching success was significant for hawksbills(F=6.46; r²=0.31; p=0.005) but not forloggerheads (F=2.42; r²=0.09; p=0.100). Thetolerances were high for both tests (T=0.694

and T=0.772, respectively). The partial analysisfor hawksbills detected significant (negative)influence of percentage of beach vegetationcover (p=0.006) on hatching success, butshowed no effect of distance to vegetative cover(p=0.673) (Figure 8).

Figure 5 - Number of nests constructed by percentvegetation covers in Arembepe Beach (2005/2006 nesting season) for loggerhead (top) andhawksbill (bottom). The classes of percentageof vegetation cover were: 0 = 0%, 10 = 1 to10%, 20 = 11 to 20% etc.

Figure 6 - Frequency distribution of nest-to-vegetationdistance in Arembepe Beach (2004/2005 and2005/2006 nesting season) for loggerhead(gray bars) and hawksbill (black bars).

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Figure 7 - Partial regression scatter plots showing the relationship between beach width (m) and straight distance crawled(m) by turtles to nest (top) and between percentage of the vegetation zone and straight distance crawled (m)(bottom) for loggerhead (left) and hawksbill (right) in Arembepe Beach (2005/2006 nesting season). The valuesare: b=slope, r2=coefficient of determination, p=probability. Values on both axes represent residuals.

To better understand the negative effect ofbeach vegetation cover on hawksbill hatchingsuccess, we evaluated scatter plots of thisfactor against both the percentage of deadhatchlings and the percentage of non-hatchedeggs (Figure 9). The percentage of deadhatchlings tended to decline with increasingpercentage of beach vegetation cover. Thepercentage of non-hatched eggs tended toincrease with increasing vegetation cover.

Therefore, problems related to survival beforehatching seemed to be the main cause for thesignificant relationship detected.

The hatching success of loggerhead nestsleft in situ (75.9±19.6%; n=84) wassignificantly higher (F=7.189; p=0.008) than oftranslocated nests (65.3±16.6%; n=31). Forhawksbills, such a comparison was not possibledue to the small number of translocated nests(n=5).



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Figure 8 - Partial regression scatter plots showing the relationship between nest-to-vegetation distance (m) and hatchingsuccess (%) (top) and between percentage of beach vegetations cover and hatching success (%) (bottom) forloggerhead (left) and hawksbill (right) in Arembepe (2005/2006 nesting season). The values are: b=slope;r2=coefficient of determination; and p=probability. Values on both axes represent residuals.

Discussion

Most nests recorded at Arembepe Beach inboth seasons were of loggerheads. Bahia’snorthern coast is the main nesting area for thespecies in Brazil and one of the most importantin the world (Marcovaldi and Chaloupka 2007).The hawksbill is the species responsible for thesecond highest number of nests in the Bahianorthern coast, which represents its main and

almost exclusive nesting site on the Braziliancoast (Marcovaldi et al. 2007).

In Arembepe, loggerheads nested preferen-tially in the sand zone, which is devoid of beachvegetation, while hawksbills presented nopreferences either for the sand or the vegetationzone). Our methodological approach ofevaluating preferential nesting zones based onthe quantification of the availability of habitat(Lawlor 1980) is not normally found in the

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Figure 9 - Scatter plots showing the relationship between percentage of dead hatchlings and percentage of beachvegetation cover (left) and between percentage of non-hatched eggs and percentage of beach vegetation cover(right) for hawksbill in Arembepe (2005/2006 nesting season).

literature on sea turtles (Whitmore and Dutton1985, Bjorndal and Bolten 1992, Blamires et al.2003).

The frequency distribution of nests along thebeach profile in Arembepe was differentbetween species. Similar to the results of Haysand Speakman (1993) for the Mediterranean Seaand Hays et al. (1995) in Southeast Florida, theloggerheads nested predominantly in the sandzone, and most nests were found close to, butnot within, the vegetation zone. Loggerheadfemales tended to crawl longer distances to nestin wider places on the beach, but tended tocrawl shorter distances when the vegetationzone was wider. This suggests that vegetationmay represent a landmark for unfavorablenesting habitat for this species. At variance withour results, Garmestani et al. (2000) reportedthat 111 of 236 loggerhead nests found in theTen Thousand Islands, Florida were laid in thevegetation zone.

Hawksbills nested with equal frequency inthe sand and vegetated zones. Occurrence ofhawksbill nests in the vegetation zone isreported in other areas as well (Witzell 1983,

Horrocks and Scott 1991, Kamel andMrosovsky 2005, 2006). Most of the nestsfound outside the vegetation zone (as far awayas 2 m) were very close to the periphery of thiszone, showing that the species nested at greaterdistances from the sea than loggerheads. Indeedas the beach width increased, hawksbills nestedfarther from the sea. The relative widths of thevegetation and sand zones did not significantlyaffect the nesting by female hawksbills. In thepresent work, we evaluated populationpreferences not individual preferences. It ispossible that there are individual differences innesting site selection, as recorded for anotherhawksbill population in the Caribbean (Kameland Mrovsky 2005).

Although most records of hawksbill nestsindicate an association with the vegetation zone(Witzell 1983, Horrocks and Scott 1991, Kameland Mrosovsky 2005), in Arembepe such nestsoccurred in areas with low percentages of beachvegetation cover. Usually, published worksevaluate vegetation cover only as a subjectivecategorical variable (with and withoutvegetation cover) and do not quantify its



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amount. Kamel and Mrosovsky (2005)evaluated the amount of vegetation cover, butunlike in our study, they evaluated the amountof cover by forest because at this Caribbean sitehawksbill females nest in the area between thebeach sand and the edge of the forest.

In our work, we followed Ackerman (1997),who suggested that the incubation environmentis influenced by factors acting as far as 50 cmfrom the nest’s center, and hence we evaluatedpercentage of vegetation cover in an area of onesquare meter around each nest based onquantification by digital pictures. Ourmethodology, based on Camacho et al. (2007),can be used to quantify the amount of groundvegetation covering the nests. However, it doesnot allow quantification of the taller mid-canopy or canopy vegetation for cases wherenests are located under trees (Kamel andMrosovsky 2005).

Many studies have evaluated nest siteselection by sea turtles and its relationship tohatching success (Mortimer 1982, Withmoreand Dutton 1985, Horrocks and Scott 1991,Hays and Speakman 1993, Wang and Cheng1999, Garmestani et al. 2000, Wood andBjorndal 2000, Ferreira-Junior et al. 2003,Kamel and Mrosovsky 2005). Those focusingon loggerheads and hawksbills generally did notfind any association between hatching successand beach zones (Garmestani et al. 2000, Kameland Mrosovsky 2005) or environmentalvariables, such as temperature, humidity,conductivity, or elevation (Wood and Bjorndal2000). However, Hays and Speakman (1993)detected a positive association between distanceto the sea and hatching success of loggerheadnests in Mediterranean beaches. Also Horrocksand Scott (1991) observed that hatching successof hawksbill nests in Barbados was highest onthe beach’s slope where most nests werelocated, and decreased above and below thislocation. These authors also detected that thehawksbill nests in Barbados were more frequentin the vegetation zone and that hatching successwas higher there. However, Kamel and

Mrosovsky (2005), at Guadeloupe, found nosignificant differences between the hatchingsuccess in vegetated and sand zones, althoughmost of the nests were associated with thevegetated zone. In Brazil, Ferreira-Junior et al.(2003) reported that, when the nests subject totide erosion were excluded from analyses, theloggerhead hatching success in Espírito Santo(Southeast Brazil) was higher for nests locatedin the open beach than for those located at theberm, which is normally covered with beachvegetation.

Similar to the results of Mortimer (1982) forgreen turtles at Ascension Island, we found norelationship between hatching success and thelocation of nests along the beach profile forhawksbills and loggerheads, nor did we observeincreased hatching success in the areas with thehighest concentration of nests. So we canconclude that in Arembepe there is no obviousrelationship between nesting sites and hatchingsuccess. However, our results apply only tonests located above the tide line, where they arefree from risks of erosion and flooding, becausenests that are at risk by flooding are moved(Table 1). The hatching success in nests locatedbelow or very close the tide line would tend tobe zero.

Similar to the results of Karavas et al.(2005), we found that beach vegetation coverhad no influence on loggerhead hatchingsuccess, despite their avoidance of vegetationzones. On the other hand, hawksbill hatchingsuccess tended to reduce with increasingpercentage of beach vegetation cover duemainly to the larger number of unhatched eggsin their nests. This significant result forArembepe hawksbills, however, should beinterpreted cautiously: it could represent eithera spurious pattern due to the low numbers ofnests in highly vegetated areas or a true pattern.If a true pattern, we should expect some levelof avoidance of highly vegetated areas byhawksbills, which does not seem to be the case.An experimental study based on the relocationof nests in areas with different percentages of

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beach vegetation cover could solve thisquestion. Several authors have reported thatroots surround eggs in vegetated zones(Whitmore and Dutton 1985, Witherington1986, Hays and Speakman 1993), a fact that wealso noticed in the vegetation zone at ArembepeBeach. These roots could in some way interferewith development as there is more rootinfiltration in vegetated areas than in open areas(Witherington 1986), and therefore, less watermay be available for use by the incubating eggs(Carthy et al. 2003). To our knowledge, therehave been no specific studies on the effects ofroots on the survival of sea turtle embryos. Inour study there was a tendency for theproportion of dead hatchlings to decline asvegetation cover increased suggesting that theprimary negative effect of vegetation is duringthe embryo development phase.

Loggerhead hatching success in both nestingseasons was higher for in situ nests than forthose translocated. However, even if nesttranslocation has a negative effects on hatchingsuccess (Eckert and Eckert 1990, Marcovaldiand Laurent 1996, Marcovaldi et al. 1999,Almeida and Mendes 2007), it may still beworthwhile to adopt the approach as aconservation strategy for nests at high risk oftide erosion, inundation, and thus, total loss.

Besides hatching success, other reproductiveparameters are relevant for nest management,and must be taken under consideration.Hatchling sex ratio is one of them. But in thisstudy we did not measure the effect of beachzones and vegetation cover on this parameter.

Useful conservation guidelines specific tothis nesting beach can be derived from ourresults: (1) loggerhead nests near the high tideline and at risk of erosion and inundation canbe translocated to the berm of the beach in orderto increase overall hatching success; (2) toachieve maximum hatching success, hawksbillnests at risk should not be translocated to areaswith dense beach vegetation cover; and (3)beach vegetation should be preserved as itinfluences nest distribution for both species.

Acknowledgements

We are indebted to Paulo Dias FerreiraJúnior, Paulo de Oliveira Mafalda Júnior andLuciano Soares e Soares for their helpfulcomments on a previous version of thismanuscript. We are also indebted to twoanonymous referees for their contributions tothe manuscript. This work was supported byFundação de Amparo à Pesquisa do Estado daBahia (FAPESB, financial support to TZS,process number APR0337/2005), ConselhoNacional de Desenvolvimento Científico eTecnológico (CNPq, research scholarship toPLBR, process number 304374/2006-7), andProjeto TAMAR-ICMBio (support to fieldwork).

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