Management and Conservation

A Retrospective Analysis of Sea Turtle NestDepredation Patterns

RACHEL L. WELICKY,1 Department of Biological Sciences, Florida Atlantic University, 777 Glades Road, Boca Raton, FL 33431, USA

JEANETTE WYNEKEN,2 Department of Biological Sciences, Florida Atlantic University, 777 Glades Road, Boca Raton, FL 33431, USA

ERIK G. NOONBURG, Department of Biological Sciences, Florida Atlantic University, 3200 College Avenue, Davie, FL 33314, USA

ABSTRACT Nest predation can significantly reduce hatchling recruitment in sea turtle populations. Weexamined 20 yr of data from Canaveral National Seashore, Florida, which has pristine and altered beaches.We used chi-squared test to determine if secondary predation events were related to the nests’ primarypredation events, and proportional hazard regression analysis to determine the relative risk of individual nestpredation. To determine if human beach use and nest predation risk were spatially or temporally linked, weranked human beach use and examined predation frequency across all screened and marked nests. We foundthat once a nest has experienced predation it has an increased likelihood of experiencing a subsequentpredation event when compared to other nests on the beach. Primary and multiple predation events occurredwith greater frequency in limited beach use areas and with lowest frequency in moderate use areas. Predationrisk decreased by an average of 29.5% from 2000 to 2008, relative to the initial year of study. Nests depositedmid-season were 9.8% more likely to be predated than early or late season nests. We conclude that whereanti-nest predator management efforts are required but restricted (e.g., by funds, available personnel, ormultiple management goals) resources should be concentrated to protect mid-season nests and those inlimited access areas, if enhancing hatchling numbers is a management goal. � 2011 The Wildlife Society.

KEY WORDS Caretta caretta, Florida, loggerhead, marine turtle, nest, predation, raccoon, survival.

The recovery and survival of imperiled sea turtle populationsare affected by oceanic and terrestrial factors that occur atvarious life history stages. Threats faced by sea turtles includenatural losses (e.g., predation, tidal inundation of nests,beach erosion, disease; Magnuson et al. 1990) as well asanthropogenic effects (e.g., poaching, incidental capture,hunting, boat strikes, and habitat degradation; Seminoff2004, National Marine Fisheries Service and U.S. Fishand Wildlife Service 2008), which have contributed tothe global decline in sea turtle populations. Magnusonet al. (1990) estimated that if none of the anthropogenicnesting beach risks were present, approximately 80% ofhatchling loggerhead turtles (Caretta caretta) would hatchsuccessfully, and the remaining 20% would fail as a result ofmicrobial infection and environmental conditions. Althoughthe majority of a sea turtle’s life is spent in the water, manythreats to their survival occur on nesting beaches and depletethe earliest life stages (eggs and hatchlings). The nestingfemale secures and camouflages the eggs by filling the cham-ber with sand and disguising the nest site (Hailman &Elowson 1992). Yet the incubating eggs may be vulnerableto overheating, cooling, and tidal inundation, which are

increasingly common events that are a result from extremeweather events and climatic changes (e.g., National MarineFisheries Service and U.S. Fish and Wildlife Service 2008).Nests are also susceptible to poaching and predation (Milleret al. 2003). These vulnerabilities lead to decreased recruit-ment of hatchling turtles; such mortality also reduces laterage or stage classes (Congdon et al. 2001).Predation risk can vary across time and space (Abrams

1987), and predators can influence the range of prey species,alter their energy budgets, and change their role in theecological community (Lima 1998). Predation risk forturtle nests is influenced greatly by the spatial and temporalvariability of nest deposition (Leighton et al. 2008) as wellas the behavior of nearby predators. Predation upon seaturtle eggs results when nests are easy to detect (Leightonet al. 2009) and where predators learn to exploit the resourcefrom conspecifics (Krebs and Davies 1997, Ditchkoff et al.2006).Moreover, if predators can locate prey and successfullyforage without large energetic costs or risks, the benefitsmake the resource particularly attractive (Leighton et al.2009).Proximity to an urbanized region can influence the popu-

lation size of predators found on a nesting beach. Urbanizedareas have aggregations of raccoons (Procyon lotor) becausefood resources (i.e., trash) are readily available (Prange et al.2003). Urbanized areas often are fragmented and may lacksufficient corridors to facilitate predator dispersal (Prangeet al. 2003, 2004). Thus, raccoon densities can become high,

Received: 28 January 2011; Accepted: 30 June 2011;Published: 7 November 2011

1Present Address: Department of Biological Sciences, Arkansas StateUniversity, PO Box 599 State University, AR 72467, USA.2E-mail: jwyneken@fau.edu, jwyneken@gmail.com

The Journal of Wildlife Management 76(2):278–284; 2012; DOI: 10.1002/jwmg.255

278 The Journal of Wildlife Management � 76(2)

which can lead to increased probabilities that alternative andseasonally available food sources such as turtle nests will beexplored and exploited. Sea turtle egg predators, such asraccoons, reach peak densities during summer (Prangeet al. 2004), concurrent with sea turtle nesting season.Predators may identify a nest during nest deposition and

incubation. Sea turtles do not tend or defend their nests, butbury their eggs in sand, which acts as an anti-predatorstrategy. Visual and chemical cues remain following eggdeposition (Stancyk et al. 1980). Once a nest is partiallydepredated, it alerts other predators to the nest’s location(Hitchins et al. 2004).Predator control may be one of the most direct ways to

increase hatchling production in sea turtle populations, butmethodologies are as diverse as success rates. Wire cagesprotected nests from mammalian predators at Key Island,Florida (Addison 1997), but similar structures attractedraccoons and foxes to nests at Boca Raton, Florida(Mroziak et al. 2000). At Canaveral National Seashore(CNS), Florida, buried wire screens have been used effec-tively since 1985, when nest predation was estimated to be97% (McMurtray 1986, Ratnaswamy et al. 1997, Antworthet al. 2006). A predator removal system targeted at specifictimes and at sections of nesting beaches with high nestpredation, reduced predation (Engeman et al. 2003).However, a different predator control regime that removedhalf of the known nest predators then was discontinued didnot effectively reduce overall predation (Engeman et al.2003). Barton and Roth (2008) suggested that the declinein population size of one predator creates an open niche thatcan be occupied by another predator so that a shift in species-specific predation may occur. For example, when raccoonremoval was used as a management strategy at CNS, theghost crab population increased 2-fold and caused a 50%increase in nest predation that resulted in a 30% egg loss(Barton and Roth 2008). The ecology and behavior of nestpredators of the CNS sea turtle rookery influenced oneanother; therefore, the impacts were coupled (Barton andRoth 2007, 2008).To minimize the losses of imperiled species to non-

imperiled predators, resource managers must understandlocal predator–prey interactions and the ecosystem leveleffects of predator control. Strategies that rely on under-standing how, where, and why predation occurs may be themost robust in promoting recruitment of threatened andendangered species. We analyzed sea turtle nest predationpatterns in a long-term data set from CNS, which hosts alarge sea turtle rookery within both urbanized and pristineregions. We examined spatial and temporal variation in nestpredation as well as anthropogenic effects (beach use) onpredation risk. The objectives of our study were to 1) identifyif a predation event to a particular nest affects the probabilityof later (secondary) predation events to the same nest, 2)spatially and temporally quantify individual nest predationrisk, and 3) assess the relationships of varying levels of humanbeach use on nest predation risk. We apply our results tosuggest management approaches to enhance hatchling pro-duction from in situ nests.

STUDY AREA

Canaveral National Seashore is located on Florida’s centraleast coast (Fig. 1A). The 38 km of beaches have varyingwidth from dune to shoreline. This site encompassed ap-proximately 19 km of public access beaches and 19 km ofbackcountry beaches that were accessible only by permit.Loggerhead turtles were the most abundant species nestingon this beach. A smaller number of green turtles (Cheloniamydas) also nested here. Nest protection via screening hadbeen ongoing since the 1980s (Ratnaswamy et al. 1997,Antworth et al. 2006) and occurred throughout the studyperiod. In the absence of nest screens, nest loss to predators,primarily raccoons (P. lotor), was as high as 97% (McMurtray1986).

METHODS

Nesting SurveysNational Park Service (NPS) staff collected and provided alldata from sea turtle nest surveys conducted from 1989 to2008. The CNS beach was divided into 100 sequential linearsegments of 0.4 km each aligned parallel to the dune andfrom north to south (Fig. 1A). All segments were monitoredconsistently and managed for sea turtle nest protection.Nightly nesting surveys began mid-April and ended in thefall. All nests in this study were screened with self-releasingwire mesh to reduce losses to predators. Occasional experi-mental studies at CNS left some nests unscreened through-out incubation; we excluded these nests from our study. Weincluded each nest that was screened and monitored from thenight of deposition to its known fate (hatching and emer-gence, loss to predation, inventory in the absence of anobserved emergence, washout from inundation and/or ero-sion if known, etc.). Data for each nest used in the analyses(n ¼ 40,441) included date laid, nest number, segment inwhich the nest was laid, if and when predation occurred,predator species (if known), and total days the nest wasobserved. We considered an act of predation to be when anest was tampered with (i.e., surface signs such as digging) oreggs were destroyed (can be surface or subterranean in thecase of crab predators).

Nest, Season, and Beach Use CategorizationWe divided the nesting season into thirds based on 20-yrranges. We categorized nests by date of deposition into early(Mar–May), middle (Jun–Aug), or late (Sep–Nov) seasonnests. We identified nest locations by the segment withinwhich they were deposited. We assigned a level of humanbeach use to each segment (Fig. 1A) by its proximity topublic accesses (boardwalks). Segments near public beachaccesses receiving the most frequent use, based upon irregularsurveys, were categorized as heavy use; segments near lessused boardwalks were considered moderate use; and those inrestricted access areas, lacking boardwalks, paths, and/orroads were termed limited use (J. Stiner, NPS, unpublisheddata). Physical disturbance in these areas included footprintsfrommoderate or heavy foot traffic, NPS All Terrain Vehicle(ATV) tracks, trash in and around trashcans, trash removal,

Welicky et al. � Sea Turtle Nest Predation 279

debris that washed ashore, beach toy and beach furniture use,and recreational sand excavation (sand castles). Limitedaccess beach areas had a maximum of 25 visitors/day, butwere rarely visited other than by NPS personnel (approx.6–8/day). Physical disturbance in the limited access includedfew footprints, NPS ATV tracks, and any debris that washedashore. Overall, beach use probably increased from 1989 to2008, but access has changed little (J. Stiner, NPS, personalcommunication). Somewhat lower use of more easilyaccessed areas in the early years of this study would reduceactual differences among use levels and therefore make ourresults more conservative.

Statistical Analysis

Primary and secondary predation analyses.—We defined aprimary predation event as the first time a nest was depre-dated and secondary predation events as any subsequentoccurrences of depredation on the same nest. If nests weredepredated at random, the number of predation events pernest would follow a Poisson distribution (with parameter lequal to the total number of predation events divided by thenumber of nests). Alternatively, if primary predation in-creased the probability of secondary predation, we expectedgreater frequency of secondary predation events compared tothe Poisson distribution. We used a chi-squared test toidentify deviations from the null expectation of a Poisson

distribution of predation events among nests (McDonald2008).Predation risk analyses.—We used proportional hazard re-

gression analyses to compare predation risk by year, season,beach use level, and turtle species. Proportional hazard re-gression estimates predation risk relative to an arbitrarybaseline for each of the predictor variables. This relativerisk, termed the hazard ratio, expresses the difference inpredation risk due to a change in each of the variables.Because time of predation was recorded at daily intervals,predation events could occur at identical times in the data;hence, we used Efron’s method, a nonparametric maximumpartial likelihood approach that allows for tied and non-tiedevent times (Allison 1995). This analysis allowed us toestimate predation risk from the number of nests depredatedeach day after nest initiation, given the number available on abeach segment. Risk, therefore, measured the instantaneouspredation pressure on nests, rather than simply counting thefrequency of primary and secondary predation events.To determine if loggerhead and green turtle nests differed

in risk levels, we initially analyzed the species separately. Thespecies did not differ significantly in risk level (see Resultsbelow), and were pooled for the remaining analyses.We thencalculated the hazard ratios using the limited access, earlyseason, and 1989 data as baseline parameters (after Allison1995). We examined the temporal variation in predation risk

Figure 1. Map of Canaveral National Seashore (CNS), Florida (A). CNS is part of a narrow barrier island located along Florida’s east coast (2885503900N,8084902600Wto 2883905300N, 8083802100W) and is bordered on the east by the Atlantic Ocean and on the west by theMosquito Lagoon (shaded area). This site isa sea turtle nesting beach that hosts a large number of loggerhead turtles (Caretta caretta), a much smaller number of green turtles (Chelonia mydas), and anoccasional leatherback (Dermochelys coriacea). We analyzed nest fate data for C. caretta and C. mydas, from 1989 to 2008. The study area, approximately 38 kmlong, was divided into 100 sequentially labeled segments, each 0.4 km in length with northern and southern margins that run perpendicular to the dune line.L, limited; M, moderate; H, heavy human use categories. The numbers of consecutive segments per use level are in parentheses. Graphical summaries of themean numbers of nests (B) and predation events (C) per segment (�1 SD; black portion of bars) describe the site.

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by testing for correlations between hazard ratios and nestabundance, number of predation events, and percent nestsdepredated. We used Pearson’s or Spearman’s Rank correla-tion tests where appropriate (Zar 1999).Environmental correlates analyses.—Hazard ratios calculat-

ed for the variable ‘‘year’’ in the proportional hazard regres-sion analysis correspond to year-to-year variation inpredation risk that is not attributable to the fixed valuesof season and human use level. We performed a correlationanalysis to determine if there were relationships between thistemporal variation in predation risk and 1) predator die-offsor 2) tropical storm/hurricane occurrence near CNS. Wecalculated the correlation between the hazard ratio for eachyear and the number of raccoon rabies cases (a surrogatemeasure of raccoon die-offs) in Florida, using rabies datagleaned from Bourne (2006). We also calculated the corre-lation between the numbers of days with hurricanes andtropical storms, in the years with occurrences and the yearlyhazard ratio.We used SAS v.9.2 to analyze all data (SAS Institute, Inc.,

Cary, NC).

RESULTS

Approximately 2,000 nests were laid, on average, eachyear at CNS. Average nests per segment and predationevents by segment (Fig. 1B,C) showed spatial variation.Approximately 22% of nests were depredated at least once(Fig. 2); the majority of documented nest predatorswere raccoons, although not all predators were identifiedin the data set. From 1989 to 2008, predation percentageswere highly variable, but we found a significant decline inpredation risk (x2

19, P < 0.018). Nest abundance variedgreatly across early, middle, and late season; however, thefraction of nests depredated varied relatively little acrossseasons (Fig. 3). The heavy and moderate beach use regionsreceived similar percentages of total nests (13% and 10%respectively), experienced similar percentages of primary andsubsequently depredated nests (moderate use, 1.01%; heavyuse, 2.28%), and covered similar numbers of segments.Annual nest abundance/segment was similar across limitedand heavy use areas, and greater in moderate use areas

(Fig. 4A). The percentage of nests depredated/year/segmentwas highly variable but tended to be lower in limited use areas(Fig. 4B).Primary predation events made a nest more likely to be

depredated subsequent times. The distribution of numberof predation events deviated from a Poisson distribution(x2

8 ¼ 75:479, P < 0.0001), with greater frequency of sec-ondary predation than expected. Subsequent predationevents occurred even though all depredated nests werereburied, rescreened, and had physical cues such as brokeneggshells removed from the nest site.Predation risk did not differ by species (x2

1 ¼ 1:511,P ¼ 0.219) in the survival analysis, and nest abundanceand annual predation risk were not correlated (r ¼�0.143, P ¼ 0.559). The percent of nests depredatedeach year was correlated with the percent change in predationrisk of that year relative to the baseline year 1989, (r ¼ 0.511,P ¼ 0.026), demonstrating that the relative risk of nestpredation may be more closely related to nest distribution,predator density, and predator behavior than nest abundance.Middle season nest predation risk was greater (9.8%) than

early season nest predation risk (x21 ¼ 11:683, P ¼ 0.001).

We calculated late season predation risk that was lower thanearly season nest predation risk, but not significantly so(x2

1 ¼ 0:417, P ¼ 0.519).Nest predation risk differed at moderate and heavy access

areas when compared to limited access areas (moderate vs.

Figure 2. The total number of sea turtle (Caretta caretta andChelonia mydas)nests (light gray) and number of nests depredated (dark gray) from 1989 to2008 in Canaveral National Seashore, Florida.

Figure 3. The average (�1 SD) annual number of sea turtle (Caretta carettaand Chelonia mydas) nests deposited per season (A) and the annual average(�1 SD) fraction of nests depredated per season (B) in Canaveral NationalSeashore, Florida, 1989–2008. We categorized seasons as early (Mar–May),middle (Jun–Aug), or late (Sep–Nov).

Welicky et al. � Sea Turtle Nest Predation 281

limited access, x21 ¼ 14:116, P ¼ 0.001; heavy vs. limited

access, x21 ¼ 10:380, P ¼ 0.001). The predation risk (hazard

ratio) for nests in moderate access areas was 17.7% less thanlimited access areas. The hazard ratio in heavy access areaswas 11% less than limited access areas. Thus, predation ismore likely to occur in limited access areas than moderate orheavy access areas.Predation risk at CNS was not correlated with our envi-

ronmental variables (rabies cases, r ¼ 0.212, P ¼ 0.413; thenumber of days a hurricane or tropical storm occurred andpredation risk by year (1995, 2004, 2006, 2008),r ¼ �0.4000, P ¼ 0.6) using Spearman’s rank correlationanalyses. We note that the resolution of the different meas-ures is not equivalent; nest depredation was recorded daily,predator mortalities were recorded annually, and stormevents had impacts on the scale of days.

DISCUSSION

We found that sea turtle nest predation risk at CNS isgreatest in the middle of the nesting season and in areaswith little human presence. Predation patterns did not differbetween loggerhead and green turtle nests, despite differ-ences in nest depth (which has been shown to influencepredation risk on other beaches; see Leighton et al. 2009).A primary predation event increased the probability of a nestbeing depredated subsequent times. When predators suc-cessfully invade a nest, chemical cues from the nest including

albumin, yolk, embryos, and odors associated with decom-position, as well as odors from the predators such as saliva,urine, and feces are left behind on and within the sand(Stancyk et al. 1980). In addition, physical or visual cues,such as an open nest, eggshells, tracks, and digging may alertor attract predators (conspecifics as well as other taxa) to theresource (Stancyk et al. 1980, Hitchins et al. 2004).The frequency of predation events/year on CNS varied over

the past 20 yr and fluctuated annually (Figs. 2 and 3A). Thenumber of years with high fraction of nests depredateddeclined between the first 10 yr and the next, so overallpredation risk declined. This decrease in predation riskwas likely a result of consistent nest protection by screening.Although screening represents an imperfect solution andmay pose a challenge for hatchling orientation (Irwinet al. 2004), it is an effective predator deterrent on manynesting beaches (e.g., Ratnaswamy et al. 1997; Yerli et al.1997; Engeman et al. 2003, 2005; Antworth et al. 2006).Factors such as epizootic outbreaks, reduced food resources,habitat, and recruitment can contribute to reduced popula-tion size and density in many mammalian species, such asraccoons, which in turn may decrease predation risk for seaturtle nests. However, rabies outbreaks were not recognizedinside CNS and distemper was not monitored (J. Stiner,personal communication). On the other hand, limited habi-tat decreases raccoon dispersal, which causes the local popu-lation density to increase (Prange et al. 2003).The numbers of nests depredated and the number of

individual predation events also varied seasonally (Fig. 3).Within a nesting season, predation risk increased from earlyto mid-season by 9.8% then declined in the late season; nestslaid in early and late seasons did not significantly differ inrisk. An increase in predation risk during the middle season islikely due in part to increasing abundance of predators,particularly raccoons. Raccoon density is highest duringsummer months (i.e., Jun, Jul, Aug; Prange et al. 2004),when predators may be drawn to high density nestingbeaches because of the combination of both old and newphysical and chemical cues from the density of early and mid-season nests. The high nest numbers may have inundatedpredators with chemical cues thereby decreasing their abilityto identify individual nests (Eckrick and Owens 1995). As aresult, individual nests may experience protection simply bytheir density, but predators still are alerted to an area rich innests. Nests laid early in the season are fewer and becomecamouflaged by the loss of location cues over time and henceare less susceptible to predation by mid-season when preda-tor densities are highest; in mid-season new nests laid will bethe ones with physical and visual cues present and may bemore easily targeted by predators (Leighton et al. 2009).Human activity often affects predation patterns and fre-

quency. At CNS, predation risk in each use level varied bythe level of human disturbance, despite similar nest numbersper beach segment (Fig. 4A). Predation risk was highest inlimited use areas, possibly because of the low abundance ofhuman-associated food. Limited use areas at CNS also mayhave greater vegetation coverage (although this variable wasnot quantified in the present study), which is preferred by

Figure 4. The annual average (�1 SD) number of sea turtle nests (Carettacaretta andChelonia mydas) per segment (A) and the annual average (�1 SD)fraction of nests depredated per segment (B) among 3 classes of human use(limited access [n ¼ 78], moderate access [n ¼ 10], heavy access [n ¼ 13])in Canaveral National Seashore, Florida, 1989–2008.

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some raccoons (Beasley et al. 2007). In areas with highdensities of experienced nest predators and high turtle nest-ing densities (e.g., arribadas), nest predation tends to increasewith vegetation (Eckrick and Owens 1995, Bernado andPlotkin, 2007).The heavy use regions of CNS, with roads and many

boardwalks, may have 2 indirect consequences for sea turtlenests and raccoons: 1) ecological traps and 2) alternative foodresources that attract and retain predators. Ecological trapsare regions where predators cannot disperse to higher qualityareas (Schlaepfer et al. 2002). As a result of fragmentation inurbanized areas and losses of their predators, sea turtle nestpredators such as raccoons may aggregate at high concen-trations (Smith and Engeman, 2002, Prange et al. 2004).The distribution of nests and their proximity to a group ofecologically trapped predators increases predation frequencyand decreases hatchling success (Leighton et al. 2008).Predation frequency, rates, and locations are determined by

the energetic trade-offs a predator is able to endure(Stephens and Krebs 1986). Optimal foraging maximizesforaging events while maintaining a low level of costs(risk and energy expenditure) to the predator. The lowestlevels of nest predation occurred in the moderate use regions,which were few. Thus, together there are few nests availablefor predators to exploit and they may not acquire sufficientcalories to counter the costs of searching and interruptedfeeding. High use areas were also relatively few and so offeredfewer nests. Nevertheless, the high use segments are easilyaccessed (or fled) and there is supplemental human-relatedfood from the ample trash. Although disturbance can behigh, beach use at night is limited when other nocturnalpredators are most active, and thus the costs of feeding uponnests at night may be low. The combined resource availabilityand high caloric value of sea turtle eggs and hatchlings(described by Kraemer and Bennett 1981) may make thesites cost effective. Finally, the abundant limited use siteshave neither the expectation of anthropogenic interferencenor supplemental food. Available calories in turtle nests arehigh; risks of human interference are low. High nest num-bers, particularly in themiddle season, make it not surprising,from an optimal foraging perspective, that these areas’ nestsexperience the highest risk from predators.

MANAGEMENT IMPLICATIONS

Sea turtle nest depredation patterns at CNS are spatially andtemporally linked, with greatest predation risk in the middleof the nesting season and in areas with little human presenceand associated supplementary food resources. Where fund-ing and personnel are highly limited, management of seaturtle nesting beaches should focus on areas with higherpredation risk and the greater densities of nests. In orderto have the greatest impact on increasing hatchling numbersat CNS, management resources should be concentrated toprotect mid-season nests (Jun–Aug) and focus on limitedaccess areas (e.g., Segments 30–75 of CNS) as long as thedistribution of nests remains similar to our observations.Management should continue to include nest screening be-cause of the demonstrated decline in successful predation.

Our study identified inconsistent monitoring of predatorpopulations. Metrics that identify and track nest predatordensities and diversity as well as associated epizootic out-breaks and die-offs would increase understanding and betterinform management decisions. Finally, once a nest is depre-dated, if viable eggs remain, the nest should receive addedprotection because its risk of secondary predation events isincreased.

ACKNOWLEDGMENTS

We thank J. Stiner, Head of Resource Management atCanaveral National Seashore for supplying the data forthis project. E. Dougherty, L. Macesic, E. McMichael,and T. Meredith provided discussion that enhanced thequality and scope of this article.

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Associate Editor: Bret Collier.

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