Size structure of illegally harvested and surviving caiman Caiman crocodilus yacare in Pantanal, Brazil

ELSEVIER

0006-3207(95)00076-3

Biological Conservation 75 (1996) 261-265 Copyright © 1996 E b e s ~ Science Limited Printed in Great Britain. All rights reserved 0006-3207/96/$15.00+.00

SIZE STRUCTURE OF ILLEGALLY HARVESTED A N D SURVIVING CAIMAN Caiman crocodilus yacare IN PANTANAL, BRAZIL Guilherme Mour~o,* Zilca Campos, Marcos Coutinho CPA-Pantanal/EMBRAPA, Laborat6rio de Vida Selvagem, CP-109 Corumbd, MS 79320-900, Brazil

&

Clarence Abercrombie Box 13, Wofford College, Spartanburg, SC 29303, USA

(Received 18 November 1994; accepted 10 March 1995)

Although some illegal hunting probably occurs throughout the Pantanal, the ease of access and the facility with which caiman may be taken vary greatly among habitat types. The economic incentive for illegal caiman harvest is provided by the international market of exotic leather, and consequently the intensity of hunting may be affected by the prices paid by intermediary hide buyers, which fluctuate in response to fashion trends. In general, there is little enforcement of wildlife laws in the Pantanal, although this can vary, and at least five poachers were killed in the police actions that confiscated the caiman skins used in our study. Investigating the effect of illegal hunting may provide insight for the design of a management programme that could eventually include legal harvest of Yacare caiman. The objectives of our study were (1) to estimate the size of harvested caiman, using data from confiscated skins and from skulls obtained at illegal hunting camps; (2) to compare our reconstructed harvest among years and against size-structure estimates for free-living caiman populations; (3) to evaluate the impact of illegal hunting; and (4) to suggest considerations for establishing legal harvest.

Abstract

The Yacare caiman Caiman crocodilus yacare is protected by law in Brazil but is hunted illegally. Until now, few authorities have had access to data that might allow analysis of the effect of hunting. We used measurements from 2978 confiscated hides and 1358 skulls found in hunters' camps between October 1987 and September 1990 to reconstruct the sizes of harvested caiman. Hunters preferentially took large caiman and probably harvested mostly males. Some reproductive-sized animals remained in residual populations in heavily hunted areas, and often successful reproduction was known to have occurred in these areas.

INTRODUCTION Although the Yacare caiman Caiman crocodilus yacare is protected by law in Brazil, it is subject to extensive illegal hunting. Numerous authorities have commented on the exploitation of this caiman (Brazaitis 1989; David, 1989; Crawshaw, 1991; Thorbjarnarson, 1992), but there is little scientific information on hunting activity and its effects on caiman populations. For example, David (1989) recognized that, although statistics on illegal trade are difficult to confirm, it is generally accepted that about one million yacare skins were exported from the Pantanal annually during the late 1980s. Male-biased sex ratios (Brazaitis et aL, 1990), altered population structure (Thorbjamarson, 1992), and decreased nest attendance by females (Crawshaw, 1991) have been postulated as results of the overharvest, because females would be more susceptible to hunting than males. However, this assumption has not been tested. The few data available (Cintra, 1989) on hunters' selectivity suggest that the hunters prefer caimans larger than 80 cm snout-vent length, but there is no register of preference by sex.

STUDY AREA

The Pantanal is a seasonally flooded plain in western Brazil, encompassing 140,000 km: of complex wetlands such as perennial rivers, intermittent streams, annually flooded grasslands, braided deltas, and forest wetlands. Although the confiscated skins we analysed had been collected throughout the southern two-thirds of the Pantanal, our field research was restricted to an area of 270 km 2 in the central region of the Pantanal (Fig. 1). This area is dominated by small lakes, remnant ponds, and seasonally flooded grasslands interspersed in a mosaic of forest.

*To whom correspondence should be addressed. 261

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Fig. 1. Map showing location of 20 hunter camps, in the Pantanal, Brazil. Area 'A' is occupied mainly by open grasslands subject to flooding; area 'B' is dominated by dense vegetation. Camps used by hunters annually between 1987 and 1990 are numbered (1-5).

METHODS We measured 2978 salted flanks (latero-ventral sections of hides) confiscated by the Forest Police of Mato Grosso do Sul between October 1987 and September 1990. Hides were confiscated directly from hunters and, except for 736 seized in 1987, had not been sorted by intermediate buyers. The skins taken in 1987 may have been purchased by hide dealers who may or may not have graded them by size. Hides were measured from their anterior tip to the posterior margin of the cloaca. These measurements do not encompass the entire snout-vent length because skinning commences immediately posterior to the mandibular symphasis. Hides are stretched during preservation and may also shrink slightly while drying (Rebelo & Magnusson, 1983). We could not assess the possible effect of these factors; therefore, we exercised caution in extrapolating our data to living animals. Some hunter camps were found by ground search, but most were located through interviews with ranch personnel and other persons familiar with local illegal hunting. Because hunters process animals near harvest sites, we assumed that skeletons found in the camps were from animals taken nearby. We used measurements from 406 living animals to predict the snout-vent length (SVL) in centimeters from length of the cranial plate (C) by linear regression: SVL = -17.16

+ 11.88 * C (r 2 = 0.96, p < 0.01). We then employed this equation to estimate the sizes of harvested caiman for skulls found between June 1987 and November 1990. We used analyses o f covariance (ANCOVA) to test the effects of year and data source (skulls versus skins) employed to reconstruct sizes of harvested caiman. We also tested the interaction between these factors. We located five areas known to have been hunted annually between 1987 and 1990; in 1990, size distributions of caiman populations surviving in these areas were estimated visually within 2 to 4 weeks after the hunters had left each area. Regression of measured SVL of 16 caimans on visually estimated SVL established a correction factor for our visual estimates of population size structure (r = 0-96; p < 0.01). To examine harvest impact on a local scale we compared the size structures of harvested and surviving populations at each site by Kolmogorov-Smirnov test (Siegel, 1956). We also combined these independent tests (Winer, 1971) to determine whether the overall harvested and surviving distributions differed. The size structure and sex ratio of a population from a densely forested area were estimated from a sample of caimans captured by hand, pole noose, or nets. Female minimum reproductive size was estimated from a sample of nesting females measured in the same area. RESULTS Location of hunter camps We located a total of 20 hunter camps between 1987 and 1990. Fifteen were in the 'open' portions of our study area, and 5 in the densely forested area (Fig. 1). We measured a total of 1358 skulls found in those camps. Five of the 20 camps were used by hunters every year between 1987 and 1990. At one of these sites (site 3) we located an active caiman nest in the breeding season of 1990. No additional camps were located after 1991. Size and sex of live caimans We captured 1019 caimans in a densely forested area (Fig. 2(a)). Mean SVL (~) was 58.6 cm, with a standard deviation (SD) of 30 cm. The overall sex ratio was 59% male. Males (R = 75.9 cm, SD = 18.8, n = 417) were larger than females (~ = 70 cm, SD = 12.5, n = 285) (t = 4.626, d.f. = 700, p < 0.001). Maximum female size was 92.3 cm SVL, whereas males reached 121.9 cm SVL. The sex ratio of the 115 animals larger than 90 cm was 94% male and, for animals between 50 and 90 cm SVL, the sex ratio was almost unbiased (51% males). Size and sex of harvested animals Snout-vent lengths of harvested caimans estimated from confiscated hides (R = 89 cm, SD = 19 cm) were similar to those estimated from skulls (R = 85 cm, SD = 15 cm) found in hunters' camps. Mean SVL of harvested caiman was larger (t-test; p < 0.01) than that of live caiman (Fig. 2). Approximately 34% or 40% (estimated from skulls or skins respectively) of the harvested caiman were animals

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Fig. 2. Size structure of (a) 1019 live-captured caiman from a densely forested area; (b) illegally harvested animals as measured from 2978 confiscated skins; and (c) as reconstructed from 1358 skulls found in hunter camps. [-], male; 1~, female; I1, unknown. > 90 cm in snout-vent length. Yacare caiman of this size were nearly all male (94%) in our sample of live caiman. If the sex ratio of the harvested caimans between 50 and 90 cm SVL was unbiased as in our sample o f live caiman, then about 65-68% of harvested caimans would be males.

Maturity of harvested females We measured 22 female caiman on nests. The smallest individual had SVL of 70 cm. We assumed this size was approximately the minimum reproductive size for females in the area. Based on our data only 22% or 26% (from skins or skulls respectively) o f all harvested caiman were smaller than this.

Temporal changes in size of harvested animals Apparently the size of harvested animals changed little among the years (Fig. 3). For the response variable of SVL, we found no interaction between years and data source (skins or skulls) (F = 0.718; d.f. = 1,4; p = 0.44). Also, year alone did not affect mean sizes of harvested caiman (F = 0.18; d.f. = 1,5; p = 0.69).

Fig. 3. Average sizes of illegally harvested animals and standard deviations for each year of study (a) from 2978 confiscated skins; (b) from 1358 skulls found in hunter camps.

Populations surviving in heavily hunted areas We estimated the size structures for harvested animals (based on skull measurements) and for surviving animals at five heavily hunted sites (Fig. 4). Overall, size structures of harvested and surviving caiman were different (X2 = 33.62; d.f. = 10; p < 0.005). Harvested animals tended to be larger; nevertheless, reproductivesized females remained at each site. Furthermore, the presence of caiman < 25 cm SVL at three sites (Fig. 4), and the presence of a nest at another site, suggested that reproduction had occurred at at least four of the five sites. D I S C U S S I O N AND C O N C L U S I O N S Interpreting results from non-random samples of hunting data does not allow unequivocal inferences regarding effects o f current exploitation. Moreover, we cannot derive guidelines for future harvest with certainty. Nevertheless, we offer two conclusions. (1) Substantial illegal harvest had been occurring in the Pantanal during 1987-90, apparently following no other criteria than those determined by the international market of exotic leather. The market may determine aspects of the harvest such as the preference

264

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Fig. 4. Size structure of illegally harvested animals as reconstructed from skulls (solid bars), and estimated sizes of living animals (open bars) in areas hunted over a period of years.

of the hunters for caiman size and locale of harvest, and also decide a temporary moratorium of illegal hunting for years. For example, hunters preferentially exploit larger, economically more valuable caiman. Three illegal hunters captured by the Forest Police of Mato Grosso do Sul State declared to us that the intermediary buyers paid about US$5 for each skin > 70 cm length, and half of that price for smaller skins. Also, our data indicate that the harvest has been concentrated in areas that are relatively easy to access and to hunt, suggesting that in some hard-to-hunt areas the exploitation is not economically feasible. Finally, we successfully found hunter camps from 1987 to 1990. After 1990, we were unable to locate new hunter camps and there was no register of confiscation of caiman hides by the Forest Police of Mato Grosso do Sul. It is significant that this period coincides with a collapse of the world trade in crocodilian skins, accompanied by a drop in the prices of the hides (Van Jaarsveldt, 1992) by as much as 50% (Woodward & David, 1993), with traditional buyers operating at very low levels (Van Jaarsveldt, 1992). Possibly, the hunting activity will begin again as soon as the world skin market recovers. (2) Although illegal harvest can impact local caiman populations by changing size structures, this impact is probably ameliorated by the hunting practices: hunters preferentially exploit large caimans, and our results indicate that about 65-68% of the harvested caimans would be males. The proportion of males may be higher, since most caiman populations in the Pantanal are more heavily male-biased than our live sample (Brazaitis et al., 1990). Because these most heavily har-

vested size classes are predominantly male, and because a single male can fertilize several females, the demographic effects of harvest may be minimized (Joanen & McNease, 1987, but see Lang, 1987). In addition, perhaps because of the wariness which hunted crocodilians can develop (Hutton et al., 1987) or perhaps because some caiman live in microhabitats that cannot be effectively hunted, hunters leave at least some animals that are large enough to reproduce. In the wet season, animals from hard-to-hunt refugia could disperse into hunted areas. Furthermore, Rebelo and Magnusson (1983) have argued that where hunting reduces population densities subadult caiman grow rapidly to reproductive size. In any case we have observed that caiman reproduce in heavily hunted areas. Although the illegal harvest of caiman populations in the Pantanal does not appear to be as excessive as stated before (e.g. Brazaitis, 1989), it is still a serious problem. Presently, it is impossible to regulate the harvest or even to assess its exact magnitude. Obviously, the economic benefits of this harvest are directed exclusively to persons who violate wildlife protection laws. The advantages of replacing illegal hunting with a legal harvest would appear compelling. Management authorities could monitor the size of harvest as they gained control over its seasons, bag limits, and hunting localities. A coordinated system of hide preparation, grading, and merchandizing could increase revenue from the exploited resource. This is particularly important since a continuing decline in ranch size (Cadavid Garcia, 1981) will increasingly force owners to maximize revenue per area. We hope that profit derived from hide sales may provide incentive for ranch owners to preserve caiman habitat, which is also habitat for many other wildlife species. This concept of 'value-added conservation' is well established in the literature of crocodilian management (Hines & Abercrombie, 1987), and it apparently enjoys significant success in some areas (Hollands, 1987). It is also important that profits derived from caiman harvest be shared with the people who will actually harvest the animals. Unfortunately, the problems associated with the management of a legal harvest are quite substantial. Brazilian federal law (5.197/67) presently prohibits direct commercial use of wildlife, so implementing a legal hunt would require fundamental changes in national wildlife policy. Technical details of establishing seasons, licences, quotas, and enforcement procedures are obviously complex, and no Brazilian governmental authority presently has the requisite personnel or expertise. Currently operational programmes, for example in the United States and Papua New Guinea, demonstrate that problems involved with legal harvest of crocodilians can be solved (HoUands, 1987; Woodward, 1987). However, these successful programmes clearly indicate the importance of two additional factors: (1) the status of surviving populations must be monitored, at least by means of a density index; and (2) further management-oriented research will be needed to assess the demographic effects of harvest and to evaluate the impact of environmental changes.

Illegal caiman harvest ACKNOWLEDGEMENTS This research was supported by PNP-Pantanal/ E M B R A P A and FINEP. W. Tom,is and T. Barros kindly helped us find some hunter camps. We thank W. Magnusson, P. Bayliss and Stephen Hamilton for their reviews of draft manuscripts.

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New Guinea. In Wildlife management of crocodiles and alligators, ¢d. G. J. W. Webb, C. Manolis & P. J. Whitehead. Surrey Beatty and Sons, Chipping Norton, NSW, pp. 73-89. Hutton, J. M., Loveridge, J. P. & Blake, D. K. (1987). Capture methods for the Nile crocodile in Zimbabwe. In Wildlife management of crocodiles and alligators, ed. G. J. W. Webb, C. Manolis & P. J. Whitehead. Surrey Beatty and Sons, Chipping Norton, NSW, pp. 243-7. Joanen, T. & McNease, L. (1987). The management of crocodilians in Louisiana, USA. In Wildlife management of crocodiles and alligators, ed. G. J. W. Webb, C. Manolis & P. J. Whitehead. Surrey Beatty and Sons, Chipping Norton, NSW, pp. 33-42. Lang, J. W. (1987). Crocodilian behavior: implications for management. In Wildlife management of crocodiles and alligators, ed. G. J. W. Webb, S. C. Manolis & P. J. Whitehead. Surrey Beatty and Sons, Chipping Norton, NSW, pp. 273-94. Rebelo, G. H. & Magnusson, W. E. (1983). An analysis of the effect of hunting on Caiman crocodilus and Melanosuchus niger based on the sizes of confiscated skins. Biol. Conserv., 26, 95-104. Siegel, S. (1956). Nonparametric statistics. McGraw-Hill Kogakusha Ltd., Tokyo. Thorbjarnarson, J. (1992). Crocodiles: an action plan for their conservation. IUCN, Gland. Van Jaarsveldt, K. (1992). Trade group report. Crocodile Specialist Group Newsletter, 11(3), 2-3. Winer, B. J. (1971). Statistical principles in experimental design. 3rd edn. McGraw-Hill Kogakusha Ltd., Tokyo. Woodward, A. (1987). Alligator ranching research in Florida. In Wildlife management of crocodiles and alligators, ed. G. J. W. Webb, C. Manolis & P. J. Whitehead. Surrey Beatty and Sons, Chipping Norton, NSW, pp. 363-7. Woodward, A, R. & David, D. N. (1993). The rise and fall of classic crocodilian skin prices: where do we go from here? Second Regional Conference of the Crocodile Specialist Group, SSC, IUCN. Darwin, NT (unpublished report).