Aerial surveys of caiman, marsh deer and pampas deer in thePantanal Wetland of Brazil

Guilherme MouraÄ o a,*, Marcos Coutinho a, Rodiney Mauro a, Zilca Campos a,Walfrido Toma s b, William Magnusson c

aCPA-Pantanal/EMBRAPA, LaboratoÂrio de Vida Selvagem, CP-109 CorumbaÂ, MS 79320-900, BrazilbCENARGEN/EMBRAPA, SAINÐParque RuralÐFinal W/5 Norte, BrasõÂlia, DF 70770-900, Brazil

cINPA, CP-478, Manaus, AM 69011-900, Brazil

Received 16 March 1998; received in revised form 2 October 1998; accepted 16 February 1999

Abstract

The yacare caiman (Caiman c. yacare) was illegally hunted in the Pantanal during the 1970s and 1980s at levels that may havereached one million skins per year. The possibility that yacare caiman had been over-exploited generated pressure for a monitoring

programme for caiman populations. The marsh deer (Blastocerus dichotomus) and pampas deer (Ozotocerus bezoarticus) are listedas endangered in Brazil and need the protection of e�ective management programmes. Ground surveys are di�cult for the extensiveand inaccessible Pantanal Wetland, south-western Brazil, but aerial surveys provided information that allowed re-evaluation of

conservation priorities. Caiman and marsh deer have larger populations than was believed. Preliminary data indicates that thepampas deer density decreased at a rate of about 30% per year from 1991 to 1993. This indicates the need for detailed ground-levelstudies for the pampas deer population in the Pantanal. We recommend a long term monitoring program using standardizedcounting procedures for wildlife populations in the Pantanal. # 2000 Elsevier Science Ltd. All rights reserved.

Keywords: Yacare caiman; Marsh deer; Pampas deer; Pantanal; Aerial surveys

Resumo

O jacare -do-pantanal (Caiman c. yacare) foi cacË ado ilegalmente no Pantanal durante as de cadas de 1970±1980, a nõ veis quepodem ter chegado a um milhaÄ o de peles por ano. A possibilidade de o jacare -do-pantanal estar sendo sobre-explotado gerou

pressoÄ es para que fosse desenvolvido um sistema de monitoramento para as populacË oÄ es de jacare s. O cervo-do-pantanal (Blas-tocerus dichotomus) e o veado-campeiro (Ozotocerus bezoarticus) saÄ o listados como ameacË ados no Brasil, e precisam da protecË aÄ o deprogramas de manejo e®cazes. Levantamentos aÁ nõ vel do solo saÄ o di®cultados pela extensaÄ o e difõ cil acesso do Pantanal, no

sudoeste do Brasil, mas levantamentos ae reos forneceram informacË oÄ es que permitiram a reavaliacË aÄ o de prioridades de conservacË aÄ o.O jacare e o cervo-do-pantanal apresentaram populacË oÄ es mais vigorosas do que se acreditava. Dados preliminares indicaram que adensidade do veado-campeiro diminuiu a uma taxa de cerca de 30% ao ano, de 1991 aÁ 1993. Isto indica a necessidade de estudosdetalhados a nõ vel do solo sobre as populacË oÄ es de veado-campeiro no Pantanal. Recomendamos que um programa de monitor-

amento usando procedimentos padronizados de contagens ae reas seja iniciado para estas espe cies. # 2000 Elsevier Science Ltd. Allrights reserved.

Palavras-chaves: Yacare -do-pantanal; Cervo-d-pantanal; Veado-campeiro; Pantanal; Levantamentos ae roes

1. Introduction

The Pantanal Wetland is the largest continuous¯oodplain of South America, with about 140 000 km2 inBrazil. The wildlife of the Pantanal is diverse and

abundant, and the potential bene®ts of sustainablewildlife use programs are attractive. Species such as theyacare caiman Caiman crocodilus yacare were exten-sively harvested before the ban imposed by the BrazilianFederal law no. 5197. From 1964 to 1969, about 1.5million caiman skins were produced in the former MatoGrosso state (IBGE, 1965±1970), the majority probablytaken from the Pantanal. Since the 1970s, the caiman

0006-3207/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.

PI I : S0006-3207(99 )00051-8

Biological Conservation 92 (2000) 175±183

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* Corresponding author. Fax: +55-67-2311011.

E-mail address: gui@cpap.embrapa.br (G. MouraÄ o).

harvest has continued clandestinely. The levels of theexploitation are uncertain, but were estimated at 1 millionskins per year (David, 1989). As a consequence, manyauthorities claimed that the yacare caiman had beenover-exploited in the Pantanal (Brazaitis, 1989; Brazaitiset al., 1998; Thorbjarnarson, 1992), generating pressurefor a monitoring programme for caiman populations.The marsh deer (Blastocerus dichotomus) and pampasdeer (Ozotocerus bezoarticus) also should be monitoredin the Pantanal because they are listed as endangered inBrazil (Fonseca et al., 1994) and need the protection ofe�ective management programmes.To manage wildlife populations, either for production

or for protection, it is necessary to develop survey methodsto initially quantify and then monitor their distributionand abundance. A practical and cost-e�ective means ofmonitoring wildlife abundance over extensive and remoteareas is by aerial surveys (Caughley and Grigg, 1981;Bayliss and Yeomans, 1989), which have been used toprovide information on patterns of distribution andabundance of large vertebrates and habitats in the Pan-tanal wetland (MouraÄ o et al., 1994; Mauro et al., 1995).In this study, we present information on distribution

and abundance of yacare caiman, marsh deer and pam-pas deer, estimated by aerial surveys across the Pantanalwetland. We use this information to suggest researchand/or management priorities for those species.

2. Study area

The Pantanal is a seasonal ¯oodplain located near thegeographic center of South America at approximately100 m elevation. It is drained to the west by the tributariesof the Paraguay river, which then ¯ow southwards alongthe western border of the Pantanal. The land is ¯at witha slope of about 6±12 cm kmÿ1 east±west and 1±2 cmkmÿ1 north±south (Adamoli, 1982). Summers (November±March) are hot and rainy while winters (April±October)are warm and dry, except for occasional cold frontsfrom the south which cause abrupt drops in air tem-perature. During the rainy season, water levels on the¯oodplains are >1 m and most of the Pantanal is sub-merged. In the dry season only some perennial riversand remnant pools, creeks and lakes persist, and wildlifeconcentrate at these water bodies. Although the Pantanalis a distinct ecosystem, it contains subregions whichdi�er in hydrology and vegetation physiognomy (Fig. 1).

3. Methods

3.1. Animal counts

Aerial surveys were conducted in the dry season(September to October), because wildlife concentrate at

permanent wetlands and are more visible from the airthan during the rainy season. Altitude and ¯ight speedwere standardized at 60 m above the ground and 200km/h. Flight sessions were between 07:00 and 11:00 h orbetween 13:00 and 16:00 h, and usually comprised 3±5transects, frequently of unequal size. We used a GlobalPositioning System (GPS), model Magellan Pro Nav5000, to navigate along east±west oriented transects.Rods ®xed on the wing struts delineated 200-m widetransects. Counts were tabulated at the end of each unitof six longitudinal minutes (�10 km or 189 s of ¯ight).In 1991, transects were systematically placed 6 latitu-dinal minutes apart to obtain a uniform coverage of allof the Pantanal between the 16�S and 21�S [Fig. 2(a)],with a sample intensity of 1.7%.In the following years, the exact latitude of each

transect was randomly allocated within latitudinalstrata that were 6 latitudinal minutes wide. In the 1992survey, we excluded a 1-degree-square block, known to bepredominantly covered by trees [Fig. 2(b)]. The sampleintensity was 1.4%. In 1993 we surveyed transects in 22randomly-chosen 0.5-degree-square blocks [Fig. 2(c)],

Figure 1. Pantanal map showing major rivers and boundaries of

Hamilton's subregions [after Hamilton et al. (1996)]. Subregions

are: CORI=Corixo Grande; CUIA=Cuiaba; PIQU=Piquiri/SaÄ o

Lourenco; PARA=Paraguay; TAQF=Taquari Fan; TAQR=Ta-

quari River; AQUI=Aquidauana/Negro; MIRA=Miranda; NABI=

Nabileque.

176 G. MouraÄo et al. / Biological Conservation 92 (2000) 175±183

with a sample intensity of 0.7%. The air temperaturewas measured by a probe placed into the air entranceduct of the aircraft, at the beginning and at the end ofeach transect in the 1992 and 1993 surveys, but only atthe beginning and the end of each ¯ight session in the1991 survey. Therefore, transects and ¯ight sessionswere confounded for the analysis involving air tem-perature in the 1991 survey.During the 1991 survey the two observers counted

caiman, marsh deer and pampas in 135 transects in 33¯ight sessions, with no interval between contiguousunits of counts. During the 1992 survey, three observers

counted as in the 1991 survey on 63 of the 135 transects.In the remaining transects, the observers had an intervalof 9 s between contiguous units of counts. Transectsusing each of these methods were alternated in thenorth±south direction. There was no signi®cant di�er-ences in densities estimated by these two methods(F1;133 � 0:599;P � 0:440 for caiman, F1;133 �0:015;P � 0:903 for marsh deer, and F1;133 �0:046;P � 0:831 for pampas deer) and the observersconsidered the new procedure less exhausting. There-fore, we included the 9-s interval procedure in the 1993survey for all of the 108 transects. The two observershave not interchanged information about the countsduring the ¯ights. The counted entities were non-hatchling caimans, individual marsh deer and groups ofpampas deer. To maximize time of counting, the obser-vers did not record the group size, except for the 1992survey when group size of pampas deer was recorded.Occasionally caiman occurred at high densities, necessi-tating counting in units of 10 animals.

3.2. Observed densities and abundance indices

We estimated the observed densities using the equa-tions suggested by Caughley and Sinclair (1994, p. 202),for sampling transects of unequal size without replace-ment. The input data were the uncorrected transectcounts. However, counts from aerial surveys are likelyto be negatively biased (Caughley, 1977, p. 35). There-fore, we used a variation of the Petersen's mark-recap-ture model, called the `double count method' (Caughleyand Grice, 1982; Bayliss and Yeomans, 1989; Caughleyand Sinclair, 1994, p. 213) to improve the accuracy ofthe counts of the marsh deer and pampas deer. By thismethod, two tandem observers scan the same transectand make their counts independently, thus allowing anestimate of the following parameters:

S1 number of animals (or groups) sighted by observer 1and missed by observer 2

S2 number of animals sighted by observer 2 and missedby observer 1

B number sighted by both observers.

The probability of observer 1 sighting an object in thetransect is given by

P1 � B

B� S2�1�

and the multiplicative correction factor (CF ) to thisobserver by

CF1 � 1

P1�2�

Fig. 2. Areas surveyed: (a) in 1991Ðall of the Pantanal between 59�

W and 55� W and 16� S and 21� S; (b) in 1992Ðthe same area, except

for a 1-degree-square block; (c) in 1993Ð22 randomly chosen 0.5-

degree-square blocks.

G. MouraÄo et al. / Biological Conservation 92 (2000) 175±183 177

As in many other studies (e.g. Caughley and Grice,1982; Bayliss and Yeomans, 1989; MouraÄ o and Cam-pos, 1994), we used the counting unit to de®ne whetherthe target was seen by one or both observers. When thelength of the counting unit is large, as it is in our study,the resulting correction factors are underestimates, andreturn conservative corrected density estimates (Caugh-ley and Grice, 1982). Because time-of-day can a�ect thevisibility of some large Pantanal vertebrates (MouraÄ o etal., 1994), we estimated time-of-day speci®c correctionfactors for marsh deer and pampas deer (Table 1).Yacare caiman is not a good model for the double-

count method because it occupies habitats very irregu-larly, frequently occurring in concentrations of 10s or100s of animals (MouraÄ o et al., 1994). Therefore, weused the correction factor for open habitats estimatedby MouraÄ o et al. (1994) to correct the caiman counts,instead of the double-count model. To calibrate thecounts of caimans by di�erent observers, we comparedtheir counts from the same side of the aircraft. We useda 2-factor ANOVA to examine possible e�ects of time-of-day on caiman density in transects, including yearand its interactions in the model. Both factors were®xed and the input data were the logarithms of thedensities in transects, plus 0.001.To index the abundance for each species we used the

maximum corrected estimated abundance during thethree years of study.

3.3. Variation in population size

We examined possible e�ects of temperature betweensurveys using ANCOVA, where year was a ®xed vari-able and temperature was the covariate. We used 1-fac-tor-ANOVA to compare the corrected estimates of each

species among years (Caughley and Sinclair, 1994, p.206). If this analysis indicated a signi®cant e�ect, wesearched for a consistent trend over years and tested ifwe had enough power to reject the null hypothesis of nochange (Gerrodette, 1987, 1993). For these analyses weused the larger estimated coe�cient of variation of thethree surveys and assumed that our species follows anexponential growth model and that the coe�cient ofvariation is proportional to the inverse of the squareroot of the estimate. According to Marsh (1995), this isthe expected relationship when the sample is made bystrip transects.The log-transformation of the observed densities of

yacare caiman sighted from the air is linearly related towater level (Coutinho and Campos, 1996). Therefore, tostandardize for the e�ects of changes in the water levelacross years, we calculated the residuals of the regres-sion of the natural logarithm of the observers correcteddensities against the total inundation area in the Panta-nal in each year, and used the anti-log of the residuals asa second index of caiman density. We estimated thetotal inundated area in the Pantanal in each year, basedon the height of the Paraguay river at the city ofLada rio, using equation 4 from Hamilton et al. (1996).

3.4. Distribution

We plotted distribution maps of caiman, marsh deerand pampas deer using the corrected densities from the1991 survey. Also, we examined the distribution of cor-rected animal densities for each subregion. For this, weuse the survey that returned the highest overall densityfor each species.

4. Results

4.1. Caiman

In the 1992 survey, the observers M.C. and Z.C.counted caiman in the same transect strip on 63 of the135 transects. There was a signi®cant di�erence betweentheir counts (t62 � 4:749;P < 0:001), with M.C. sighting1.73 times more caimans than Z.C. Temperature andyear were signi®cantly related to the log-transformation ofcaiman densities (F1,272=11.089, P=0.001 and F2,272=12.291, P<0.001, respectively), but since the proportionof variability explained by the model was small(r2 � 0:093), we did not attempt to correct the countsfor temperature e�ects. The estimated mean correcteddensities of non-hatchling yacare caiman ranged from4.53 caiman/km2 in the 1992 survey to 28.34 caiman/km2 in the 1993 survey (Table 2), with an apparentl92ÿ93 � 6:3. The caiman density in the 1993 surveyreturns a corrected abundance index of about 3.9million non-hatchling caiman. Natural logarithms of

Table 1

Estimates of time-of-day speci®c correction factors (CF), used to

improve the accuracy of the counts of marsh and pampas deers

between 1991 and 1993. S1 is the number of marsh deer or groups of

pampas deer sighted by observer 1 and missed by observer 2, S2 is the

number of animals or groups sighted by observer 2 and missed by

observer 1, and B is the number sighted by both observers

Species Year S1 S2 B Time-of-day CF

Marsh deer 1991 168 32 159 AM 1.2

88 24 81 PM 1.3

1992 40 11 15 AM 1.4

56 13 48 PM 1.3

1993 31 37 53 AM 1.7

24 17 69 PM 1.2

Pampas deer 1991 154 22 75 AM 1.3

151 6 62 PM 1.1

1992 37 13 19 AM 1.7

41 6 22 PM 1.3

1993 13 7 20 AM 1.4

22 11 30 PM 1.4

178 G. MouraÄo et al. / Biological Conservation 92 (2000) 175±183

corrected caiman density were negatively correlatedwith the estimated total inundation area in the Pantanalin each year and suggested a linear trend. The anti-logarithms of the residuals of the observed values inrelation to the expected for a linear trend, returned asecond caiman density index, standardized for di�er-ences in the ¯ooded area among years (Table 2). Thoseindices were not signi®cantly di�erent among years(F2;365 � 1:422;P � 0:243), and power analyses sug-gested that we would need 11, 7 or 5 surveys (i.e. years)to detect a decrease of 10, 20 or 30% in corrected cai-man densities, respectively.At the time of the 1991 survey caimans were widely

dispersed in the Pantanal. Units with corrected densitiesof more than 10 caiman/km2 were frequent near theTaquari river, in the Taquari river and Taquari Fansubregions, and in the vicinity of the Negro river, in theNhecolaà ndia subregion (Fig. 3). Nhecolaà ndia andAquidauana/Negro subregions had the highest cor-rected densities, with more than 50 caiman/km2, whilethe Piquiri subregion had about one-tenth of this den-sity (Table 3).

4.2. Marsh deer

Log-transformation of marsh deer densities tended tobe related to air temperature (F1;272 � 6:418;P � 0:065),but air temperature explained only a small proportionin the variability of the dependent variable (r2 � 0:06).Therefore, we did not correct the counts for tempera-ture e�ects.The time-of-day speci®c correction factors ranged

from 1.2 to 1.7 (Table 1). The mean corrected densitiesof marsh deer did not di�er among years(F2;365 � 0:588;P � 0:556), and power analyses sug-gested that we would need 9, 6 or 5 surveys to detect a

decrease of 10, 20 or 30% in corrected densities,respectively. The highest mean corrected density was0.32 marsh deer/km2 in the 1992 survey (Table 2), whichreturns an abundance index of about 44 000 (SE�7700)marsh deer for the whole Pantanal.The marsh deer occupied mainly the northwest por-

tion of the Pantanal and parts of the southern Pantanal(Fig. 4). The Paraguay and Corixo Grande subregionsin the north of the Pantanal and the Aquidauna/Negrosubregion, in the south, had high mean densities (0.98±0.57 marsh deer/km2), while the central and north-eastern subregions had comparatively low mean marshdeer densities, at the time of the 1992 survey (Table 3).

4.3. Pampas deer

Log-transformation of pampas deer densities wasrelated to air temperature (F1;272 � 18:240;P < 0:001),

Table 2

Observed densities (Dobs) and density corrected for observers, time-of-

day and visibility bias (Dcor) for caiman, marsh deer and pampas deer

in the Pantanal. Icor is a second density index for caiman that provides

correction for di�erences in ¯ood levels among years. The densities of

caiman and marsh deer are expressed as individuals/km2. Data for

pampas deers are expressed as groups/km2. SEobs, SEcor and SEI are

the estimated standard errors of the observed density, corrected den-

sity, and the ¯ooding-corrected caiman-density index, respectively

Species Year Dobs SEobs Dcor SEcor Icor SEI

Caiman 1991 1.72 0.23 6.62 0.89 0.72 0.1

1992 0.68 0.10 4.53 1.13 1.19 0.3

1993 7.36 1.24 28.34 4.77 1.17 0.2

Marsh deer 1991 0.21 0.03 0.25 0.05 ± ±

1992 0.23 0.04 0.32 0.06 ± ±

1993 0.17 0.03 0.25 0.05 ± ±

Pampas deer 1991 0.18 0.02 0.25 0.05 ± ±

1992 0.12 0.01 0.18 0.02 ± ±

1993 0.08 0.02 0.11 0.03 ± ±Fig. 3. The distribution of caiman density in the Pantanal Wetland

(September 1991).

Table 3

Corrected densities of caiman, marsh deer and pampas deer (from

1993, 1992 and 1991 surveys, respectively), post-strati®ed by the sub-

regions of the Pantanal Wetland. Subregion limits follow Hamilton

et al. (1996)

Subregion Area Caiman SE Marsh

deer

SE Pampas

deer

SE

Corixo Grande 11 479 20.93 16.80 0.78 0.34 0.16 0.10

Cuiaba 14 406 16.90 7.08 0.26 0.10 0.03 0

Piquiri 15 996 5.00 2.66 0.10 0 0.03 0

Taquari Fan 39 344 15.62 5.01 0.12 0 0.43 0.10

Nhecolaà ndia 8623 61.34 15.20 0.38 0.18 0.57 0.16

Paraguay River 16 258 8.82 6.49 0.98 0.32 0.13 0.10

Taquari River 2927 16.66 3.27 0.35 0.12 0.25 0.13

Aquidauana/

Negro

9197 53.85 18.50 0.57 0.10 0.14 0

Miranda 5035 9.67 5.41 0.25 0.10 0.05 0

Nabileque 13 662 14.17 2.16 0.18 0.10 0 ±

G. MouraÄo et al. / Biological Conservation 92 (2000) 175±183 179

but explained little of the variation in the dependentvariable (r2 � 0:14). Therefore, we did not correct thecounts for temperature e�ects.Time-of-day speci®c correction factors ranged from

1.1 to 1.7 for groups of pampas deer. The mean cor-rected densities of pampas deer di�ered across years(F2;365 � 3:683;P � 0:026), and the logarithm of thecorrected pampas deer densities approximates a linear ®t(r2 � 0:99;P � 0:073). However, we do not have enoughpower (1-�=0.57) to detect a constant decrease of about34% per year, suggested by the slope of the regressionline, when we compute power using the larger coe�cientof variation of the survey series (CV1993 survey=0.27).Power analyses suggested that we would need about 12,8 and 6 surveys to detect constant decreases of 10, 20 or30% in pampas deer density, respectively.The maximum corrected density of pampas deer

groups in the whole Pantanal was 0.25 groups/km2 inthe 1991 survey, which returns a corrected abundanceindex of about 34 800 (SE�7600) pampas deer groupsfor the whole Pantanal. Mean group size of the pampasdeer was 1.67 (SD � 0:85; n � 224) during the 1992survey, with groups ranging from 1 to 5 individuals.The pampas deer occupied mainly the central areas of

Pantanal (Fig. 5), where the mean observed densitieswere as high as 0.57 groups of pampas deer/km2,whereas no pampas deer were seen in the Nabilequesubregion in the south of the Pantanal (Table 3).

5. Discussion

5.1. Caiman

The visibility problems associated with aerial surveys ofyacare caiman were specially critical. Caiman occupying

open habitats are easily seen from the air but those indensely vegetated habitats are di�cult to see. MouraÄ o etal. (1994) showed that about 26% of the non-hatchlingcaimans were sighted from the air in open habitats ofthe Pantanal, but in a densely vegetated habitat onlyabout 2% were sighted. If all caimans sighted in the1993 survey were in habitats as open as those of thesurvey of MouraÄ o et al. (1994), then the appropriatecorrection factor for visibility bias would be 3.85, whichreturns an abundance index of about 3 900 000(SE�670 000) non-hatchling caimans for the wholePantanal. However, considering the proportion of openhabitats (33.5%, Coutinho et al., 1997) and dense vege-tated habitats (66.5%, Coutinho et al., 1997) andaccepting that in dense vegetated habitats only about2% of the caimans could be sighted, the correctedabundance index would be about 35 000 000 non-hatchling caiman for the whole Pantanal. The estimateof several million individuals is consistent with densityestimates available for small areas. For example,MouraÄ o et al. (1994) and Campos et al. (1995) estimateddensities of about 7 million and 10.6 million adultyacare caiman, respectively, when extrapolated for thewhole Pantanal. Three and a half to 35 million indivi-duals may seem an amplitude too large to provide use-ful information for ecological studies or to supportmanagement decisions. However, for a population untilnow believed to be over exploited (e.g. Brazaitis et al.,1998; Brazaitis et al. 1996; Brazaitis, 1989), just thefact that the abundance estimates are in the millions isrelevant.Groombridge (1987) considered the status ofC. c. yacare

as indeterminate and noted that it has been proposed toshift the species from Appendix 2 of CITES (Conven-tion on International Trade in Endangered Species ofWild Fauna and Flora) to the more restrictive Appendix 1,

Fig. 4. Distribution of marsh deer (Blastocerus dichotomus) densities

in the Pantanal Wetland (September of 1991).

Fig. 5. Distribution of pampas deer (Ozotocerus bezoarticus) densities

in the Pantanal Wetland (September of 1991).

180 G. MouraÄo et al. / Biological Conservation 92 (2000) 175±183

while biological and population studies are conducted.Also, the USA. listed C. c. yacare as endangered underthe US Endangered Species Act (Groombridge, 1987),imposing restrictions on trade of yacare caiman pro-ducts within the USA. However, the US harvests alli-gator (Alligator mississippiensis), which has an estimatedtotal abundance of about 800 000 of all ages (Groom-bridge, 1987).The population of yacare caiman is widely distributed

in the Pantanal (Fig. 3). The density index quadrupledfrom 1991 to 1993. This increase was undoubtedly anartifact of the visibility conditions rather than realpopulation growth. Water level a�ects caiman sight-ability. During low water the exposed banks betweenthe water-bodies and neighboring vegetation enlarge,exposing basking caimans. Coutinho and Campos(1996) showed that the observed densities of yacare cai-man were logarithmically related to the water levelindex. The logarithm of the observer-corrected densitywere negatively correlated to the inundated area in thePantanal among years and the ®t was apparently linear.The anti-log of the residuals of this function, used as anindex of yacare caiman density which take into accountdi�erences in ¯ooding area, show no changes in caimandensities across years (F2;365 � 1:422;P � 0:243).High abundance, widespread distribution and the

absence of evidence for negative net trends in the den-sity index reinforce the idea of a vigorous population,despite the extensive illegal harvest of the 1980s (seeBrazaitis, 1989; David, 1989; Crashaw, 1991; Thorbjar-narson, 1992; but also MouraÄ o et al., 1996). Somemechanisms that could be contributing to reduce thenegative e�ects of the illegal harvest are discussed inMouraÄ o et al. (1996), but a mechanism based on spatialstructure of the population and harvest intensity(McCullough, 1996) is also plausible.The presence of an abundant, vigorous and wide-

spread population, despite illegal hunting, makes theoption of a legal harvest programme attractive. Pre-sently, it is impossible to regulate the harvest or even toassess its exact magnitude. With a legal harvest pro-gramme, the management authorities could monitor thesize of harvest, as they would gain control over its sea-sons, bag limits and hunting localities. A co-ordinatedsystem of hide preparation and merchandising couldincrease revenue from the resource. The pro®ts derivedfrom hide and meat sales may provide incentives forranch owners to preserve caiman habitat, which is alsohabitat for many other wildlife species. This concept of`value-added conservation' apparently enjoys success inother countries (Hollands, 1987). However, the pro-blems associated with the management of a legal harvestare substantial and require fundamental changes innational wildlife policy, including revision of the Fed-eral law no. 5197/67 (MouraÄ o et al., 1996; Magnussonand MouraÄ o, 1997).

5.2. The cervids

The observed marsh deer densities were 0.21 marshdeer/km2 in 1991 and 0.23 and 0.167 marsh deer/km2 inthe 1992 and 1993 surveys, respectively, which returnuncorrected abundance estimates of 23 000 to 32 000marsh deer and an abundance index corrected for visi-bility bias of about 44 000 (SE�7700) marsh deer in thePantanal. Those numbers contrast with the estimate ofSchaller and Vasconcelos (1978) of a total of 7000marsh deer in the Pantanal. The study of Schaller andVasconcelos was undertaken in September of 1977, alsousing aerial survey techniques. They did not correcttheir results for visibility bias and concluded that themarsh deer populations had been impacted by diseasestransmitted by domestic cattle and made a prediction ofa rapid reduction in the marsh deer population numbersin the Pantanal. However, the marsh deer populationsdid not decrease. Using the observed population densityobtained by Schaller and Vasconcelos (1978) and theones in this study as population indices, we found agrowth of about 10% per year from 1977 to 1993. Thehypothesis of a constant population growth of thisamount cannot be discarded, since ®nite rates of popu-lation growth (lm) as large as 1.63 has been reported forneotropical cervids (Robinson and Redford, 1991, p.422) and the expected lm for a herbivorous mammalof this size [�80 kg, Schaller (1983)] is 1.20 (Caughleyand Sinclair, 1994, p. 41). The fact that the period1963±1973 was relatively dry, with the inundated areain the Pantanal almost always less than 20 000 km2 andnever reaching the historic mean of 34 190 km2 from aseries of 95 years (Hamilton et al., 1996), favors thishypothesis.Since 1974 the weather has been wetter with inunda-

tion areas exceeding the historic mean annually andfrequently reaching surfaces as large as 80 000 km2

(Hamilton et al., 1996). It is possible that in 1973 themarsh deer population was depressed, and that it hasexperienced fast growth in response to the increase ofhabitat availability after the 1974 ¯oods. However,Schaller and Vasconcelos (1978) report a small propor-tion of yearlings and young at foot in the surveyedpopulations, which is not expected for a quicklyincreasing population. Alternatively, MouraÄ o et al.(1997) suggested that discrepancies in the marsh deerabundance estimates could be due to di�erences ininterpretation of data. Schaller and Vasconcelos (1978)sampled 4 strata of di�erent sizes in the Pantanal, in atotal of 25 800 km2 and used di�erent sampling inten-sities for each stratum. MouraÄ o et al. (1997) calculatedthe weighted mean of the observed densities from theSchaller and Vasconcelos (1978) data, which was 0.164marsh deer/km2, returning an observed abundance ofabout 23 000 marsh deer in the 140 000 km2 of Pantanaland, thus, of the same magnitude as the estimates of this

G. MouraÄo et al. / Biological Conservation 92 (2000) 175±183 181

study. In any case the rapid population decrease pre-dicted by Schaller and Vasconcelos (1978) did not hap-pen and the marsh deer populations either increased orhave been stable during the last 16 years.In general, marsh deer and pampas deer inhabit dif-

ferent areas in the Pantanal, re¯ecting di�erent habitatuse. According to Mauro et al. (1995), the marsh deer isa generalist in terms of use of plant communities and aspecialist in respect to water depth, consistently pre-ferring sites with about 70 cm water depth. The marshdeer occupied preferentially the northwest of the Pan-tanal (Fig. 4), which is a region of frequent ¯ooding,containing ¯oodplains with marshes, ¯ooded ®elds and¯oating grass mats with water of the depth preferred bymarsh deer.The pampas deer characteristically uses open habitats

(Toma s, 1995), and occupied principally the centralareas of the Pantanal (Fig. 5) where grasslands andopen savanna dominate. The pampas deer abundanceindex was 34 800 groups in the 1991 survey. Estimatingmean group size from simultaneous counts of numbersof groups and numbers of individuals in the groupsresults in problems of non-independence of the countsplus the usual visibility problems. Therefore, the meanof 1.67 pampas deer per group must be regarded as anunderestimate. Applying this mean to the 1991 surveyresults indicate that there are at least 60 000 pampasdeer inhabiting the Pantanal. Considering the meanbiomass of 80 kg for marsh deer and 40 kg for pampasdeer (Schaller, 1983), the biomass of marsh deer andpampas deer is of the same magnitude (�26 and 17 kg/km2, respectively).The marsh deer has been an object of concern for

researchers and conservationists because its distributionarea is decreasing and because there was a prediction ofrapidly decreasing population in the Pantanal, themajor refuge of its largest population (Thornback andJenkins, 1982). The pampas deer has attracted lessattention. Of the three subspecies recognized, only theone that occurs in Argentina (Ozotocerus bezoarticusceler) is listed as endangered by IUCN, while O. b. leu-cogaster and O. b. bezoarticus are listed as indeterminate(Thornback and Jenkins, 1982). Like marsh deer thepampas deer has a decreasing and fragmenting dis-tribution besides being threatened by hunting, diseasestransmitted by cattle and other domestic species (Jun-gius, 1976), and the Pantanal probably holds its largestremaining population. However, while the data show avigorous population of marsh deer in the Pantanal, thedensity index of the pampas deer decreased at a rate ofabout 30% per year from 1991 to 1993. The hypothesisof opposite population responses of marsh deer andpampas deer to the multiyear hydrologic changes isattractive, but cannot be properly examined with theavailable data. This result indicates the needs fordetailed ground-level studies for the pampas deer

population in the Pantanal, and for aerial surveys inother areas of its distribution.

5.3. General conclusions

Three years of aerial survey cannot be used to makede®nitive conclusions about trends in wildlife popula-tions. However, the results of this study show that aerialsurveys are useful to obtain information on patterns ofdistribution and abundance of yacare caiman, marshdeer and pampas deer. A monitoring programme of 10years of a single annual survey would have a 95% ofchance to detect constant yearly decreases of 20% ormore in target populations and a series of 12 surveysmay detect decreases of 10% per year. The surveysindicated vigorous populations of yacare caiman, sug-gesting that this species could be suitable for harvestprogrammes. The surveys do not indicate problems formarsh deer which, until now, were assumed to be dis-appearing from the Pantanal, but indicate the need forintensive studies on the demography and ecology ofpampas deer.

Acknowledgements

The aerial surveys were supported by the WorldWildlife Fund (WWF/USA), FundacË aÄ o ConservationInternational do Brasil (CI-Br), Sociedade de Defesa doPantanal (SODEPAN) and Empresa Brasileira de Pes-quisa Agropecua ria (EMBRAPA). CAPES provided ascholarship funding to G.M. We are grateful to theowners of the Porto Jofre, Corguinho and SantanaRanches, and to Hotel Sta. Rosa for the use of itsfacilities. We thank Dilson Franco for ¯ying preciselyand safely.

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