Black Caiman

[Dark Jaguar]

SIZE OF CAIMANS KILLED BY HUMANS AT A HYDROELECTRIC DAM IN THE MADEIRA RIVER, BRAZILIAN AMAZON.

https://www.zobodat.at/pdf/HER_28_1_2_0101-0104.pdf

Crocodilians face conflicts with humans where they use aquatic environments to seek food, leisure and housing. The Nile Crocodile, Crocodylus niloticus LAURENTI, 1768, and the Saltwater Crocodile, Crocodylus porosus SCHNEIDER, 1801, are among the species that   most frequently attack humans  (BOYLE et al. 2009; WEBB et al.2010), but also other species are killed by local people, who believe that the crocodilians compete with them for food, principally fish. Killing of crocodilians has intensified with habitat loss caused by fishing, sand mining, hydropower, tourism and housing in areas close to rivers and other aquatic environments  (FERGUSSON 2010). This has caused an increase in the likelihood of attacks on humans  (WALLACE et al.2012).

Four species of crocodilians occur in the Amazon region, and  one, the Black caiman, Melanosuchus niger (SPIX, 1825), can reach 6.0 meters in length (MEDEM 1983). There are   many reports of attacks on humans during activities, such as fishing, onthe shores and   rivers ( DA SILVEIRA & THORBJARNARSON 2010;  HADDAD & FONSECA 2011 ). Fishing with fixed hooks is not prohibited under Brazilian law and fishermen make use of this practice   in many Amazonian Rivers ( BARTHEM 1987; BAYLEY et al. 1989 ). Crocodilians are attracted by fish caught on the lines. Hydroelectric dams change river shorelines and often force caimans into closer contact with people. In this study, the author recorded crocodilians killed by humans in the newly flooded Santo Antônio dam, Madeira River, Amazon Basin.

*This image is copyright of its original author

From September 2010, to May 2013, dead caimans floating near the banks of the Madeira River and its tributaries were recorded (Fig. 1). The reservoir area of the Santo Antônio   dam (8°47’S,  63°57’W), Madeira River, was formed in October/November 2011 and extends to 100 km at inundation periods. Each dead  caiman was inspected to determine   the cause of death based on physical evidence, such as bullet wounds or the presence of a   hook. The length of the head (tip of snout to posterior end of cranial plate, HL cm) of dead individuals was measured using a measuring tape. This measure was used to estimate snout-vent length (SVL cm) since decomposition negatively affects correct measurement of   trunk and tail. The relationship between HL and SVL for the four species was calculated by   measuring  numerous individuals captured in the area and released at the site of capture immediately after measuring. The following equations were computed: for Melanosuchus niger SVL = -4293 + 3927 *HL (N = 94, r2 = 0997, p<0.001); for Caiman yacare (DAUDIN, 1802) SVL = -3126 + 4033 * HL (N = 118, r2 = 0993, p< 0.001); for Paleosuchus palpebrosus (CUVIER, 1807) SVL = -3350 + 4121* HL (N = 97, r2 = 0.995, p< 0.001); and for Paleosuchus trigonatus (SCHNEIDER, 1801) SVL = 0986 + 3504 * HL (N = 121, r2 = 0882, p< 0.001). Analyses were done in the SYSTAT Program (Systat Software Inc., San Jose, california, USA).

Over the period of two years, the author encountered the following dead crocodilian   individuals (estimated snout-vent length [SVL] in parentheses):  42 M. niger (58.6 - 247.0 cm), six C.yacare (68.4 - 89.6 cm), three P. palpebrosus (74.9 - 82.4 cm) and three P. trigonatus (36.0 - 78.1 cm). The caimans had been killed by gun shots (36), hooks (9), water turbulence near the dam walls (4) and unidentified causes (5). Most dead Black caimans were longer than 2.0 m SVL (Fig. 2) and had gun-shot wounds. The estimated maximum SVL of the Black caimans was 247.0 cm.

*This image is copyright of its original author

Accordingly, all four species of Amazonian crocodilians suffer from contact with people where they share aquatic environments, even though most caiman species do not pose threat to humans. In particular Melanosuchus niger, which occupies large rivers and lakes, is hunted by people in the region of the Madeira River. They frequently regard these biggest  Amazonian caimans as an imminent danger and kill them, as is known for two large Old World crocodilians, C. porosus (WHITAKER 2008) and C. niloticus (WALLACE et al. 2011). There are however only marginal reports of illegal hunting of M. niger in the Amazon region (RÊBELO & MAGNUSSON 1983; DA SILVEIRA & THORBJARNARSON 1999).

In the Bolivian Mamoré River, which is a tributary of the Madeira River, the maximum total length of M.niger was estimated to be 4.94 m based on a head length of 58.8 cm (THORBJARNARSON & MCINTOSH 1987). In this latter study, 52 % of the killed M.niger individuals had an estimated total length of more than 4.00 m (i.e., SVL> 2.00 m). In conclusion, with regard to the region of the Madeira River, the conflict between humans and crocodilians is focused on large individuals of M.niger.

Most dead caimans had gun-shot wounds, however no meat had been taken from the body.  One M. niger and one C.yacare were lacking tails, indicating that the individuals had been killed (also) for food. Many caimans died because they swallowed hooks or drowned in gill nets. Flooding by the dammed river makes the crocodilians come into closer contact with fishermen andothers, which will probably increase conflicts in the future.  For the benefit of all parties involved, downstream translocation is thus recommended for caimans approaching hydroelectric dams to prevent the reptiles’ deaths both in the area of the turbines and floodgates. Reply