TAVARE, 2008. C. limbatus

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Occurrence, Diet and Growth of Juvenile Blacktip Sharks,Carcharhinus limbatus, from Los Roques Archipelago National

Park, Venezuela

RAFAEL TAVARES

1Instituto Nacional de Investigaciones Agrcolas (INIA), Ministerio de Ciencia y Tecnologa (MCT). La Asuncin6311, Isla de Margarita, Estado Nueva Esparta, Venezuela.

Telephone: +58 295 2422220. Fax: +58 295 2420252. E-mail: [email protected] para la Investigacin de Tiburones (CIT). Av. Don Bosco, Qta ABC, No. 10,

La Florida, Caracas 1050, Venezuela.

ABSTRACT.Distribution, occurrence, diet and growth of juvenile blacktip sharks, Carcharhinus limbatus,

were examined based on specimens caught between 1995 and 2002 by the artisanal shark fishery from LosRoques Archipelago. Fishery monitoring revealed that C. limbatus was the species most frequently caught,accounting for 48.5% of the shark catch. Capture of neonate and juvenile sharks was concentrated in the

central lagoon of the archipelago, indicating that this area corresponds to a nursery area for the species. Birth

season was approximately from mid June to the end of August. Average length at birth was 63.5 cm TL, based

on neonates with an open umbilical scar. The smallest mature male and female sharks measured 152 and 161

cm TL, respectively. Diet was composed of teleost fishes, with the most important prey being Eucinostomusargenteus (13.3 %IRI), Opisthonema oglinum (6.9 %IRI) and Gerres cinereus (5.5 %IRI). Monthly length

frequencies revealed that juveniles remain within the nursery area for 14-16 months after birth, reaching a

length of about 130 cm TL during this period. The growth of the juvenile population is characterized by a

linear function, with sharks almost doubling their birth length during the first year of life. This growth was

more rapid than the estimated rates reported from the Gulf of Mexico, western Florida and South Africa. The

results suggest that growth rate is negatively correlated with latitude or positively correlated with tempera-ture. The rapid growth shown by these juveniles will have positive implications for the recovery of the adult

populations of this species.

KEYWORDS.Carcharhinus limbatus, diet, growth, nursery area, sharks, Los Roques

INTRODUCTION

The blacktip shark, Carcharhinus limbatus,is a widespread species found on or adja-cent to the continental and insular shelves

of tropical and subtropical regions. In thewestern Atlantic, it is distributed fromMassachusetts to southeastern Brazil, in-cluding the Gulf of Mexico and CaribbeanSea (Compagno 1984). Carcharhinus lim-batus is one of the most important sharkspecies caught by commercial longline andgillnet fisheries in the southeastern UnitedStates of America and Gulf of Mexico(Castro 1996; Castillo-Gniz et al. 1998).Likewise, on the northeastern coast of

South America, between French Guyanaand Brazil, this species is commonly caughtin the commercial shark fishery conducted

by the Venezuelan gillnet fleet (Tavares2005a). In the Caribbean region, it was re-ported as abundant around Venezuelan is-lands, where it was frequently captured by artisanal fisheries targeting sharks

(Cervign 1966). Currently, the blacktipshark is rarely observed among the sharkscaught by diverse commercial fisheries op-erating in Venezuelan waters (Tavares2005b; Tavares and Arocha 2007). Despitethe commercial importance of this sharkspecies in the western Atlantic, biologicaland fisheries information is scarce or non-existent. The migratory behavior of C. lim-batus is a factor that has impeded the studyof its biology and ecological. Tag-recapture

information has revealed a migratory routefrom the eastern coast of the USA to theGulf of Mexico, and there is probably also a

Caribbean Journal of Science, Vol. 44, No. 3, 291-302, 2008Copyright 2008 College of Arts and SciencesUniversity of Puerto Rico, Mayaguez

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connection between these two regions andthe Caribbean Sea (Castro 1996; Kohler etal. 1998).

Studies on the biology traits of Carcharhi-nus limbatus, such as distribution, diet, re-production and growth have been carriedout in several regions of the Atlantic Oceanin order to generate the information re-quired for stock assessment and for the de-sign of management strategies. The dietcomposition and hence the feeding habitshave been studied in the Los Roques Archi-pelago by Tavares and Provenzano (2000),and in the Gulf of Mexico by Barry (2002)and Bethea et al. (2004). The reproductivebiology was described by Castro (1996) in

the southeastern USA and by Capap et al.(2004) on the western and northern coastsof Africa. Growth biology was examined byBranstetter (1987) in the Gulf of Mexico,Killam and Parsons (1989) on the west coastof Florida and Wintner and Cliff (1996) onthe east coast of South Africa. More re-cently, life history traits were analyzed inorder to determine the likelihood of sepa-rate stocks between two geographic areas,the eastern Gulf of Mexico and South At-

lantic Bight (Carlson et al. 2006).Contributing to the reduction in globalshark abundances is the high proportion of juvenile sharks caught by commercial fish-eries and the degradation of nursery areas(Camhi et al. 1998). Shark nursery areas aregeographically discrete zones where thegravid females give birth to their pups, andwhere the young spend their first weeks,months or years (Castro 1993). Nurseriesare usually located in highly productiveshallow water areas as coastal marshes, es-

tuaries and lagoons which offer abundantfood to the young, as well as protectionagainst predators. The identification anddelimitation of the areas inhabited by juve-nile sharks are prerequisites for the designof conservation strategies, particularly forspecies heavily exploited by commercialfisheries (Rechisky and Wetherbee 2003). Inrecent years, biotelemetry has been used inextensive studies of the juvenile popula-tions of Carcharhinus limbatus within their

nursery areas (Heupel and Hueter 2002;Heupel and Simpfendorfer 2002, 2005).These studies have generated significant in-

formation regarding movement patterns,aggregation behavior and habitat use.Other studies have sought to characterizethe genetic structure of juvenile popula-tions of the blacktip shark in nursery areasdistributed along the southeastern coast ofthe USA and the Gulf of Mexico (Keeney etal. 2003).

MATERIALS AND METHODS

The Los Roques Archipelago NationalPark is located in the Caribbean Sea, 160km directly north of the central coast ofVenezuela (1143-1158 N, 6635-657 W)

(Fig. 1). The archipelago occupies a totalarea of 2,251 km2 and comprises more than40 small and low islands that are distribut-ed around a main central lagoon. Thissemi-closed central lagoon covers an areaof about 210 km2 with a maximum depth of8 m, and it contains a large number of sandand coral banks. The water temperatureranges between 25 and 30 C, with theminimum values occurring in January-February and maximum values in June-

October. Los Roques Archipelago sustainsan important marine biodiversity, with itsichthyological fauna comprising about 370species, inc luding 21 shark species(Ramrez and Cervign 2004; Tavares2005b).

The data used in this paper were ob-tained from the local commercial fisherytargeting sharks and during the followingperiods: April 1995-April 1996, September1998-August 1999, January-November 2001and March-August 2002. The Los Roques

shark fishery employs artisanal bottomlonglines of monofilament with 100 to 400hooks (J-type, size # 3-5). A common char-acteristic of these longlines is the use of asection of stainless steel between the lineand hook to avoid the escape of the sharks.The fishing vessels are mainly smallwooden and fiberglass boats with outboardmotors, ranging between 6 and 8 m long.Data collected during 2001 are the mostcomplete, because they were obtained dur-

ing intensive fishing monitoring as part ofthe shark program. Two relative abun-dance measures were used, the number of

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individuals/100 hooks and the number ofindividuals/fishing set. In both cases, therelative abundance was calculated for eachfishing operation and then averages wereestimated by length classes of sharks.

A tagging experiment of juvenile Carcha-rhinus limbatus was also conducted withinthe central lagoon of the archipelago withthe assistance of the local fishermen to fa-

cilitate shark catches during 2001. Fishingsites were located around several coralbanks located in the western end of the cen-

tral lagoon which is a common fishing areafor C. limbatus. Fishing operations were al-ways conducted during daylight hours, be-tween 6:00 and 9:00 AM, and between 4:00and 6:00 PM. On the basis of initial fishingresults, it was determined that the optimalset time to avoid the death of the blacktipsharks was one hour. Two common sharktags were used (M dart-tag and plastic

dart-tag), attached near the base of the firstdorsal fin. To avoid injury and additionalstress on captured sharks, the dart head of

FIG. 1. Map of the study area, Los Roques Archipelago, showing the catch distribution of Carcharhinuslimbatus during the whole study period, 1995-2002. Map colors: black = islands or keys, dark grey = eastern coralreef barriers, light grey = shallow-water zones maximum depth 8 m. Symbols: CL central lagoon, + neonates and

juveniles, adult males, adult females, fishing sets with positive catches for other shark species during thefishing monitoring in 2001.

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the tag was inserted between the skin andthe muscle through an incision made bysurgical scalpel.

The data recorded for each specimencomprised sex, size (cm), weight (kg), andcatch location (by GPS). The size measuresused were the total length (TL) and forklength (FL) according to Compagno (1984).Neonates were identified by the presenceof an open or partially closed umbilicalscar. This ontogenetic stage lasts four to sixweeks, until the umbilical scar is com-pletely healed (Castro 1993). For discussionpurposes, neonates were included in a ju-venile group as a single stage prior to sex-ual maturation. Sexual maturity was as-

sessed by the general development andcharacteristics of the reproductive organsin both sexes (Castro 1996). All biologicalmeasurements of the sharks were recorded by the same researcher. Regression tech-niques were applied between the most im-portant biometric variables (FL vs. TL andTL vs. W), in order to obtain conversionequations. Differences between line regres-sions by sex were tested by analysis of co-variance, ANCOVA (Zar 1996). During the

period January-November 2001 period,stomachs were collected for analysis of dietcomposition. The prey items contained inthe stomachs were identified to the lowesttaxonomic level possible. The diet was ana-lyzed quantitatively by applying the im-portance percentage indices by number(%N), by occurrence (%O) and by weight(%W). The index of relative importance(%IRI), which incorporates the previousthree indices and facilitates comparisonsamong the contributions of different prey,

was estimated (Corts 1997).Growth analysis of Carcharhinus limbatus

was based on the juvenile population andemployed methods that required lengthand age information. Juvenile length struc-ture (years combined) is graphically pre-sented through histograms of monthlylength frequencies. On the basis of theselength frequency distributions, the relativeages (in months) were then assigned to thedistinct groups. Since blacktip sharks are

born mainly during July, this month wastaken to be the arbitrary initial birth date(age-0). Therefore, neonate sharks observed

during June were excluded from the analy-sis. Juvenile growth rates by sample periodand by sex were examined by regressionanalysis between the assigned ages and ob-served lengths. The resultant growthcurves were statistically compared by anF-test (Zar 1996).

A method based on tag-recapture infor-mation (Francis 1988) was also employed toexamine the growth rates of the juvenilepopulation. Because the data obtained fromthe tag-recapture procedure were insuffi-cient for fitting the growth models with thismethod, analogous information was usedbased on the average length for the differ-ent age groups. For this approach, average

lengths for the age groups (0-14 months)were estimated by applying a bootstrap re-sampling (n = 2000) routine of non-parametric statistics (Manly 1997). Thevariables required in this growth analysisare length at release (LM), length at recap-ture (LR) and period at liberty (T). It wasassumed that these variables provideanalogous information related to the differ-ence between the monthly average lengthof the age groups and the time period be-

tween these groups (where T

2 months).For example, the monthly average lengthsfor the age groups of 0 and 2 months com-prise the lengths at release (LM) and at re-capture (LR), where T is 2 months. Simi-larly, the monthly average lengths of theage groups of 0 and 3 months are taken torepresent the lengths at release and at re-capture of the next observation, where T is3 months. All possible combinations amongthe average lengths of the age groups gen-erated a total of 91 data, which are equiva-

lent to the observations.This method (Francis 1988) incorporates

a regression analysis by a maximum likeli-hood routine to fitting growth models, andinvolves estimates of growth parametersand measurement error. The expected indi-vidual growth increment (Li), consideringthe length at release (Li), and the time atliberty (Ti), is calculated by the followingequation:

Li = g gg g Li1 1 +g g

Ti

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the year 2002 were captured in August andhad a recently closed umbilical scar, sothese specimens were not considered to beneonates. On the basis of this temporalcatch distribution of the neonates, it wasdeduced that the birth season of C. limbatusis approximately from mid June to late Au-gust, with most pups being born during

July. The birth size estimated from the av-erage lengths of the neonates was 63.5 4.1cm TL.

All adult blacktip sharks were recordedduring the year 2001 and these comprisedseven males (152-188 cm TL) and five fe-males (161-218 cm TL). Two femalesshowed no sign of ovulation or recent mat-ing, and the other 3 females were pregnantand carried embryos at advanced stages ofdevelopment. These three pregnant fe-

males were caught in the months precedingthe putative birth season; one female (173cm TL) carrying 3 embryos was caught on

18 March, a second female (180 cm TL) car-rying 5 embryos (38.0-38.6 cm TL) wascaught on 20 March, and the third female(193 cm TL) carrying 4 embryos (43.4-45.0cm TL) was caught on 4 April. Except forthe pregnant female caught on 4 April,which was found at the southwestern edgeof the central lagoon, all the adult sharkswere observed in the northwest area of thearchipelago (Fig. 1). The two smallest adultmales examined (152 cm TL) were caughton 4 April and 9 August, and their claspercalcification and length, as well as their tes-tis development, suggested that thesespecimens had only recently reached sexu-al maturity. Similarly, the smallest adult fe-

male (161 cm TL) examined on 19 Marchwas classified as recently mature, on the basis of the development of the ovary,uterus and oviductal gland.

Of 184 specimens (62-218 cm TL) exam-ined for diet analyses in 2001, 71.73 % hadempty stomachs. The prey items foundwere all teleost fishes, comprising 16 iden-tified taxa together with unidentified tel-eost material (Table 1). The most importantprey were the species Eucinostomus argen-

teus (13.32 %IRI), Opisthonema oglinum (6.88

FIG. 3. Biometric relationships and conversionequations between the variables selected (FL vs. TL,TL vs. Weight) for Carcharhinus limbatus from the LosRoques Archipelago. TABLE 1. Diet composition of Carcharhinus limbatus

in the Los Roques Archipelago, expressed as percent-age by numbers (%N), frequency of occurrence (%O)and percent weight (%W). Also shown is the index ofrelative importance (%IRI), which incorporates thethree previous indices.

Prey items %N %O %W %IRI

TeleostsEucinostomus argenteus 18.75 13.46 7.80 13.32Opisthonema oglinum 9.38 9.62 9.81 6.88

Gerres cinereus 6.25 5.77 19.36 5.51Haemulon spp. 9.38 7.69 4.30 3.92Albula vulpes 6.25 7.69 5.70 3.43Calamus bajonado 1.56 1.92 8.72 0.74

Archosargus rhomboidalis 3.13 3.85 1.96 0.73Acanthurus chirurgus 1.56 1.92 8.51 0.72Pomacentrus spp. 3.13 3.85 1.12 0.61Lutjanus synagris 1.56 1.92 4.96 0.47Sparisoma viride 1.56 1.92 2.41 0.28Lutjanus spp. 1.56 1.92 0.94 0.18Caranx hippos 1.56 1.92 0.84 0.17Sparisoma rubripinne 1.56 1.92 0.73 0.16Chaetodon striatus 1.56 1.92 0.65 0.16

Ablennes hians 1.56 1.92 0.51 0.15Teleosts non-identified 29.69 32.69 21.67 62.58

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%IRI) and Gerres cinereus. (5.51 %IRI).Other prey with relatively important val-ues were Haemulon spp. (3.92 %IRI) and Al-bula vulpes (3.43 %IRI). The teleosts that re-mained unidentified because of theiradvanced stage of digestion representedthe highest proportion of the diet (62.58%IRI).

The monthly length frequencies of juve-nile Carcharhinus limbatus showed a bimo-dal distribution during June-November,leading to the recognition of two cohorts(Fig. 4). Cohort-1 comprises the new sharksof that year and cohort-2 constitutes thesharks that were born in the previous year.The sharks of cohort-2 were not observed

after November because they had left thenursery area. Hence, after birth, juvenilesremain within the central lagoon for about14-16 months. Therefore, the ages of thegroups were assigned as follows: cohorts 1and 2 during July were aged 0 and 12months, respectively; during August theywere aged 1 and 13 months, respectively;and so on. Regression analysis of ageagainst length revealed that juvenilegrowth was described by a linear model

(Fig. 5). Differences between sample peri-ods and between sexes were not significant(F-test, p > 0.05). Growth rate obtained bycombining years and sexes was 4.12 cm/month. Average lengths estimated by a bootstrap routine for the distinct agegroups observed in the juvenile populationare also shown in Fig. 4. In relation to theFrancis (1988) approach, the results fromthe likelihood ratio test (LRT) indicatedthat the appropriate model contained theparameters g, g, v and s (Table 2). Growth

rates estimated for individuals of 65 and120 cm TL were 5.00 and 3.10 cm/month,respectively. Hence, this approach detecteda slight decrease in growth rate along thelength range examined.

Between 26 January and 25 July 2001, atotal of 31 juvenile blacktip sharks weretagged, comprising 16 males (64.5-95.6 cmTL), 14 females (62.5-95.0 cm TL) and onespecimen of unidentified sex (81.0 cm TL).During the same year, seven recaptures

(82.0-108.0 cm TL) were recorded throughthe monitoring of the shark fishing. The in-tervals in time and distance between

tagged and recapture sites were 40-139days and 0.7-3.0 km, respectively. Sinceeach recapture was comparatively near thetagging site, juvenile individuals of thisspecies appear not to travel far within thenursery. Individual growth rates obtainedfrom tag-recapture data varied from 4.57 to5.13 cm/month, with an average of 4.82cm/month.

DISCUSSION

The monitoring of the local shark fisherydeployed in the Los Roques Archipelagoduring 2001 indicated that Carcharhinus lim-

batus was the species most frequentlycaught, and its occurrence in the study areais limited to juveniles captured in a specificarea of the archipelago. Considering theabundance of juvenile blacktip sharks inthe archipelago, it might be expected that asignificant number of adults would becaught by this local fishery, as occurs forexample with the species C. perezi. How-ever, a low number of C. limbatus adultswas caught in the study area and this did

not appear to be related to fishing effort orgear selectivity. The monitoring of otherfishery activities targeting sharks off sev-eral Venezuelan oceanic islands has shownthat adult C. limbatus were infrequentlycaught on the insular shelf of the LosRoques Archipelago (Tavares 2005b). Tak-ing into account these findings and the mi-gratory behavior of this species, it is prob-able that adult individuals are not commoninhabitants of the study area. Furthermore,the fact that three of the five adult females

were at similar stages of pregnancy duringthe months preceding the parturition sea-son suggests that these females had arrivedat the archipelago in order to give birth.

The absence of captures of other sharkspecies (juveniles or adults) within the cen-tral lagoon indicated that this shallow-water zone is a specific nursery area forCarcharhinus limbatus. This strategy associ-ated with habitat use by juvenile sharksmight reduce predation and competition,

and might also ensure food availabilityduring these early life stages. Other studiesconducted with C. limbatus on the south-



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FIG. 4. Monthly length frequencies and relative abundance estimates by length classes of juvenile Carcharhinuslimbatus within the nursery of the Los Roques Archipelago (years combined). Average lengths (AL, in cm LT)estimated by a bootstrap re-sampling (n = 2000) method are presented for each age group.

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eastern coast of the USA and Gulf ofMexico have reported multispecific sharknurseries (Castro 1993; Heupel and Hueter2002; Hueter et al. 2006). Juvenile popula-tions of C. limbatus on the Gulf coast ofFlorida showed the highest rates of naturalmortality (61-91%) caused mainly by pre-dation, and an absence of correlation be-tween neonate distribution and prey den-

sity suggested that predator avoidancewould be an important factor in the selec-tion of nurseries (Heupel and Hueter 2002;Heupel and Simpfendorfer 2002). In the

case of the Los Roques Archipelago, it isprobable that the juvenile population of C.limbatus experiences a low level of preda-tion and competition within the nursery,since few if any adult individuals or othershark species are caught there.

The birth season of Carcharhinus limbatusin the study area is approximately frommid June to late August, with a peak in thenumbers of neonates seen in July. Similarly,new-born blacktip sharks were observedmainly during July in the tropical region offthe western African coast (Capap et al.2004). However, off the southeastern coastof the USA and Gulf of Mexico, births oc-curred during May and June (Branstetter

1987; Castro 1996; Castillo-Gniz et al.1998). This temporal variation in birth sea-son among geographic zones could be prin-cipally conditioned by water temperature.In sub-tropical and temperate regions influ-enced by temporal climatic changes, the birth of sharks during spring months willallow the young to remain within the nurs-eries for a period of several months charac-terized by warm water. Castro (1993) statedthat in temperate zones, winter tempera-

tures force the young sharks out from thenurseries into deeper waters or to other ar-eas. The birth size observed for C. limbatusin the Los Roques Archipelago is similar to

TABLE 2. Log-likelihood values () resulting from the distinct growth models fitted to juvenile Carcharhinuslimbatus from the Los Roques Archipelago and using the Francis (1988) method. Model 6 was selected throughthe likelihood ratio test (LRT).

Model LRT

# Parameters Models 2 p

1 g65, s 307.701 vs. 2 9.59 19.19 0.000

2 g65, s, v 317.302 vs. 3 16.01 32.01 0.000

3 g65, s, v, m 301.293 vs. 4 5.75 11.51 0.000

4 g65, s, v, m, p 307.044 vs. 5 43.43 86.87 0.000

5 g65, g125, s 263.615 vs. 6 9.87 19.74 0.000

6 g65, g125, s, v 253.746 vs. 7 0.10 0.20 0.652

7 g65, g125, s, v, m 253.64

7 vs. 8 0.67 1.33 0.2478 g65, g125, s, v, m, p 254.30

FIG. 5. Relationship between age and length formales (n = 459) and females (n = 390) of juvenile Car-charhinus limbatus sampled between 1995 and 2002from the Los Roques Archipelago. Plotted line is linearregression fitted to data.



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those recorded from other areas of the At-lantic Ocean (Castro 1996; Castillo-Gniz etal. 1998; Capap et al. 2004). The sizes of thesmallest adults recorded in the study area(152-161 cm TL) are comparable with thelengths at maturity reported for the north-western Atlantic (130-150 cm TL; Branstet-ter 1987; Castro 1996; Castillo-Gniz et al.1998; Carlson et al. 2006), but differ fromthose reported from African coasts (170-185cm TL; Capap et al. 2004).

The diet of sharks of the genus Carcharhi-nus is typically dominated by teleost fishes(Wetherbee and Corts 2004). Previousstudies of C. limbatus have shown that clu-peids such as herring and menhaden are

among the most important prey groups: O.oglinum in the Los Roques Archipelago (Ta-vares and Provenzano 2000), Brevoortia ty-rannus in the southeastern USA (Castro1996) and Brevoortia patronus in the north-eastern Gulf of Mexico (Barry 2002; Betheaet al. 2004). In the present study area, twomain groups dominated the diet of the ju-venile C. limbatus: the Gerreidae throughconsumption of E. argenteus and G. cinereus,and Clupeidae through consumption of O.

oglinum. The high percentage of emptystomachs observed, combined with the fre-quent occurrence of prey items in advancedstages of digestion, was probably a conse-quence of the fishing method used. Accord-ing to Wetherbee and Corts (2004), thesharks attracted with bait (e.g. linehooks)were primarily those that had relativelyempty stomachs. The use of gill nets wouldbe the fishing method most appropriate fordiet analysis in sharks.

The growth biology of sharks is usually

examined through non-linear models suchas the von Bertalanffy relationship and itsderivative functions. These procedurescommonly require information on lengthsand ages that includes individuals with as-ymptotic lengths, as well the use of ageingtechniques based on analyses of calcifiedstructures. Therefore, analysis of thegrowth of juvenile shark populations mightrequire the use of other approaches. In thepresent study, it was possible to assign ages

to the distinct size groups, owing to theclear pattern in the juvenile length struc-ture and its evolution in relation to time.

Results showed that growth of juvenileCarcharhinus limbatus was best described bya linear model. Other studies conductedwith Carcharhinus obscurus in southwesternAustralia (Simpfendorfer 2000) and N.brevirostris in the Bahamas (Barker et al.2005) also showed that juvenile growthduring the first years of life was character-ized by a linear function. Although regres-sion analysis showed a constant growthrate, the use of the Francis (1988) methoddetected a slight decrease in growth ratebetween lengths of 65 and 120 cm TL, indi-cating that this approach can be a usefultool to detect smallest differences withinthe length range examined. A decrease in

growth rate with ontogenetic developmentis biologically the more likely. Growth rateestimates for juveniles were supported bydata from recaptured tagged individuals,although these recaptures were limited.Taking into account the growth rates ob-tained from all methods employed, youngblacktip sharks appear to grow at about 5cm/month during the first months of life, arate that decreases to about 3 cm/monthafter the first year of life. This is more rapid

juvenile growth than the rates reported forother geographic areas. In the Gulf ofMexico, juvenile blacktip sharks had agrowth rate of about 20 cm/year (1.7 cm/month) for the first 2 years of life, and ofabout 10 cm/year (0.8 cm/month) throughadolescence (Branstetter 1987; Killam andParsons 1989). In waters off South Africa,the rate was about 32 cm/year (2.7 cm/month) during the first year of life andabout 16 cm/years (1.3 cm/month) fromthen until attainment of sexual maturity

(Wintner and Cliff 1995).Considering that Carcharhinus limbatus

reaches 130 cm TL during a period of 14-16 months, and the smallest mature indi-viduals with unknown ages measured 152-161 cm TL, it is reasonable to assume thatthis species in the study area would attainsexual maturity in about 2 years. Thegrowth rate estimate (3.1 cm/month) fromindividuals of 120 cm LT supports this hy-pothesis. These findings contrast signifi-

cantly with the age at maturity recorded forthis species. Previous aging studies havedetermined that C. limbatus, reaches sexual

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maturity on the southeastern coast of theUSA and Gulf of Mexico after 4-7 years(Branstetter 1987; Killam and Parsons 1989;Carlson et al. 2006), and in South Africaafter 6-7 years (Wintner and Cliff 1995).

In fisheries science, geographic variationin biological parameters of the species canindicate separate stocks, and this feature isessential for resource assessment. Never-theless, what happens with migratory spe-cies of sharks? It is reasonable to assumethat at least the growth rate in these speciesvarying according to geographic region orchanges in latitude. Hence, variability ingrowth estimates for these migratory spe-cies (principally in juvenile populations)

between regions would not necessarily in-dicate distinct stocks. The present studyhas demonstrated that significant differ-ences exist in growth rate and age at matu-rity for blacktip sharks between tropicaland subtropical-temperate regions in thenorthwestern Atlantic. The results suggestthat growth rate of Carcharhinus limbatus isnegatively correlated with latitude (or posi-tively correlated with temperature). Con-versely, a study conducted by Lombardi-

Carlson et al. (2003) reported a positiverelationship between growth and latitudefor bonnethead shark, Sphyrna tiburo, alongthe eastern coast of the Gulf of Mexico.Those authors concluded that faster growthrates in individuals at high latitudes wouldbe a mechanism to compensate the shortergrowth season by growing faster during ashorter time period. In the case of C. lim-batus, Carlson et al. (2006) detected no dif-ference in life history traits of this speciesfrom the western coast of Florida and south

Atlantic Bight. In any case, it is necessary toconsider that a significant separation in lati-tude (of 15-25 degrees) exists between theLos Roques Archipelago and the Gulf ofMexico and southeastern coast of USA.

To conclude, the results obtained in thepresent study reveal that the Los RoquesArchipelago is an important habitat for ju-venile Carcharhinus limbatus where parturi-tion occurs and young sharks spend the ini-tial phase of their life cycle. Furthermore,

the rapid growth and sexual maturation ex-hibited by this species would have a posi-tive implication for the recovery of the

adult population in the area. In general,sharks are primarily characterized by slowgrowth; consequently, the populationshave a lowest rate to increase in size whenconfronted with high levels of fishing pres-sure. Considering the fast growth and thehighly migratory behavior of blacktipsharks, it can be concluded that the LosRoques Archipelago may function as an ef-ficient producer of recruits for the adultpopulations of blacktips distributedthroughout the Caribbean Sea and prob-ably other areas of the western Atlantic. Animmediate conservation measure thatshould be considered is the protection ofthe nursery area and the juveniles by estab-

lishing size limits and a closed season for C.limbatus in the Los Roques Archipelago.

Acknowledgments.This study was par-tially supported by the Oficina Nacional deDiversidad Biolgica (Ministerio del Ambi-ente) through projects No. 2000-279 and2001-0074. Research permits were given bythe Instituto Nacional de Parques and theInstituto Nacional de la Pesca y Acuicul-tura. The shark tags used were provided

through the collaboration of the Coopera-tive Shark Tagging Program/National Ma-rine Fisheries Service of the USA. Thanks tothe benefactor members of the FundacionCientifica Los Roques, which contributedwith the flights to transport personnel andequipment during the research. Specialthanks to L. Castillo-Gniz, A. Arangurenand A. Grant, who made invaluable revi-sions and suggestions to improve themanuscript. This research was possiblethanks to the full cooperation of the fisher

community of Cayo Fernando, Los Roques.

LITERATURE CITED

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Documents

TAVARE, 2008. C. limbatus