Sunday 4 July 2021

Understanding negative Human-Crocodile interactions in the La Encrucijada Biosphere Reserve, Mexico.

There are 24 extant species of Crocodilians, of which three can be found in Mexico; the Spectacled Caiman, Caiman crocodilus, and Morelet’s Crocodile, Crocodylus moreletii, both of which are considered to be of Least Concern under the terms of the International Union for the Conservation of Nature's Red List of Threatened Species, and the American Crocodile, Crocodylus acutus, which is considered to be Vulnerable. All of these are subject to Special Protection under Mexican law. Both the Spectacled Caiman and American Crocodile were previously heavily hunted for their skins, but this practice was banned in the 1970s, since when their populations have recovered somewhat, although they are still targeted illegally in some parts of the country.

A Morelet’s Crocodile, Crocodylus moreletii. Gautier Poupeau/Wikimedia Commons.

As Crocodilian populations have recovered, negative interactions with Humans have increased, something fuelled apparently by both the rise in Crocodilian numbers, and that of Humans, combined with changes in land use, and some widespread flooding incidents. Crocodiles are prone to pre-emptively defending both themselves and their territories, and females are particularly agressive during the nesting season, which can increase negative encounters with Humans, and American and Morelet's Crocodiles are calculated to be responsible for 2.9 and 0.5 fatalities per year respectively, across the Americas as a whole, but this is probably an underestimate, as not all incidents are recorded. 

Detailed recording and analysis of negative Crocodile-Human interactions can help scientists to understand the causes of such incidents, and come up with ways to minimise the number of such events. The Mexican State of Chiapas would be predicted to be a high risk one for such attacks, but the Worldwide Crocodilian Attack Database lists only four incidents for the state, making it likely to be an underestimate.

In a paper published in the journal Orynx on 21 May 2021, Giovany Arturo González-Desales of the Centro de Investigación en Ciencias Biológicas Aplicadas at the Universidad Autónoma del Estado de México, Luis Sigler of the Dallas World Aquarium, Jesus García-Grajales of the Universidad del Mar, Pierre Charruau of the Centro del Cambio Global y la Sustentabilidad A.C. Calle, and Martha Mariela Zarco-González, Ángel Balbuena-Serrano, and Octavio Monroy-Vilchis, also of the Centro de Investigación en Ciencias Biológicas Aplicadas at the Universidad Autónoma del Estado de México, investigate Crocodile attacks in the El Hueyate estuary in Chiapas State, as well as the wider situation in Mexico, focussing on whether such attacks are more common in the nesting season, and whether they increase with rising Human or Crocodile populations.

González-Desales et al. compiled monthly records of negative interactions between Humans and Crocodiles in Mexico from the Worldwide Crocodilian Attack Database, for the period 15 August 1993 to 8 June 2018. Using this data they calculated the distance from each attack to the nearest Crocodile nesting site for which data was available, as well as calculating the relationships between attack frequency and the density of the Crocodile and Human populations. They also look at a number of socio-economic factors in areas of Mexico where adverse Human-Crocodile interactions have been recorded, including economic activities, male education levels, Human population density, proportion of males in the Human population, and the proportion of those males who are economically active. González-Desales et al. concentrated on the male part of the Human population, as these are significantly more likely to be involved in negative interactions with Crocodiles.

The El Hueyate Estuary forms part of the La Encrucijada Biosphere Reserve on the Pacific coast of Chiapas State, Mexico. It comprises a 1086.5 km² buffer zone surrounding two core areas, El Palmarcito and La Encrucijada, which have a combined area of 316.2 km². Two Crocodilians can be found within this area, the Chiapas Caiman, Caiman crocodilus chiapasius, a subspecies of Spectacled Caiman, and the American Crocodile, Crocodylus acutus. These are found both in natural waterways, and Human-created environments such as dams, cattle ponds, fishery ponds and pampas, but are more common in the core zones than elsewhere. Both species nest and breed within the reserve. The reserve also contains 64 Human settlements, with a total population of 26 992 people; 28.5% of this population having no formal education of any type. The main sources of income for this Human population are fishing, agriculture and livestock breeding.

La Encrucijada Biosphere Reserve, with the locations of the six negative interactions (cases) between people and crocodilians recorded during 2015-2018, five with American Crocodiles, Crocodylus acutus, and one (case 5) with a Spectacled Caiman, Caiman crocodilus chiapasius, and nests recorded in 2014. González-Desales et al. (2021).

González-Desales et al. carried out unstructured interviews with residents of settlements on the El Hueyate estuary between October 2013 and September 2014. They interviewed seventeen men and five women, all between 22 and 51 years old, out of a population of 543 people. All of those interviewed had either been involved in a Crocodile-related incident, or were related to someone who had. González-Desales et al. concentrated on gaining information about attacks, notably the date and location of any interactions, what people were doing at the time of the incidents, and any actions subsequently taken against the Crocodiles.

During March–October 2014 (the nesting season of both the Chiapas Caiman and the American Crocodile), González-Desales et al. carried out a series of surveys in the El Hueyate estuary, using a 6 m boat with a 15 horsepower outboard motor. They used a white LED light to search for Crocodiles at night, using the reflectiveness of the Animals' eyes to detect them, then carefully moving closer to identify their species. 

The Worldwide Crocodilian Attack Database contains 121 records of attacks on people by American Crocodiles in Mexico between 15 August 1993 and 8 June 2018; there are no records of any attacks by Caiman. These attacks were significantly more likely to occur close to nesting sites. Attacks were most common in municipalities where the Human population was below average, the economy dominated by service industries, and the male Human population had an average of 7.4 years of education.

Negative interactions by month involving Crocodylus acutus in Mexico, showing that the incidents are more frequent in nesting season. González-Desales et al. (2021).

Surveys of American Crocodiles in the El Hueyate estuary carried out in the years 1997, 1998, 2002, 2008, 2009, and 2010 found their population density varied between 1.14 and 7.11 individuals per km, with the 2014 survey carried out by González-Desales et al. finding an average of 1.5 Crocodiles per km. The previous surveys also found between 0 and 2.42 Caiman per km, with González-Desales et al. finding an average of 2.71 Caiman per km.

The interviews carried out by González-Desales et al. revealed six Crocodilian-related incidents in the El Hueyate estuary between 2005 and 2014, two of which resulted in loss of Human life, two in major injury (defined by González-Desales et al. as limb loss or permanent motor impairment), and two in minor injuries. Five of these incidents involved American Crocodiles, and one involved a Caiman.

(a) Relative abundance index (Crocodilians observed per km, during 1997-20014) of Crocodylus acutus and Caiman crocodilus chiapasius, and (b) the number of negative Human–Crocodilian interactions per year (during 2005-2018), in El Hueyate estuary, La Encrucijada Biosphere Reserve, Mexico. González-Desales et al. (2021).

The interviews carried out by González-Desales et al. also found that Crocodile attacks on domestic Animals (mainly Dogs) and livestock (Cattle and Pigs) were common in the El Hueyate estuary. As well as Crocodile attacks on Humans, incidents of Humans killing Crocodiles were common, with 30 reported Crocodile killings in 2011-2012, most of which which were in response to Crocodile attacks on Humans, domestic Animals, or livestock. Attacks on Humans were also more common near nesting sites, with five of the recorded incidents happening at such locations.

Previous studied have shown that most attacks on Humans by Saltwater Crocodiles, Crocodylus porosus, in Australia and Sri Lanka were associated with females defending nests, and González-Desales et al.'s work, the first detailed study of Crocodile attacks in Mexico, suggests that this also might be the case here. However, the Sri Lankan and Australian studies found that the number of attacks went up with increasing Crocodile population density, but that does not appear to be the case in Mexico.

The limited scale of the study makes it impossible to make to many inferences about the Crocodile population from it, but the report of Crocodiles being killed in response to attacks is of concern, as this appears to be on a large enough scale to have an impact on the Crocodile population, particularly if this involves females of reproductive age being killed in the breeding season. Previous studies have noted that Crocodiles are frequently killed in response to attacks on Humans in other parts of Mexico; this was also found to be a factor in Sri Lanka.

The records of Crocodile populations available for La Encrucijada suggest that the population underwent a dip following each recorded attack on a Human, supporting the idea that Human retaliatory action is having an impact on the Crocodile population. The population in the area is currently as low as it was before protection of these species was introduced in the 1990s, and it may be necessary to repeat some of the actions taken in the 1990s to protect the species, notably the removal of egg clutches and rearing of Crocodiles in captivity before releasing them back into the reserve. Actions such as this have been employed more recently in other parts of Mexico, with some success.

A Chiapas Caiman, Caiman crocodilus chiapasius, reproductive female in Isla La Concepcion, La Encrucijada Biosphere Reserve. Giovany Arturo González-Desales/Orynx.

González-Desales et al. report that, since 2011, people living in the El Hueyate estuary have abandoned spear fishing, for fear of being attacked by Crocodiles, and that they have begun to destroy Crocodile eggs as a way of controlling the Crocodile population. This appears to indicate that both Humans and Crocodiles are currently suffering as a result of negative interactions between the species. González-Desales et al. also note that while Crocodile attacks have obvious impacts on the Human population, both as a result of death and injury to Humans, and the loss of valuable livestock, the loss of Crocodiles can cause problems, as it is linked to a rise in the populations of other Animals (mostly small Mammals, Birds, and Arthropods) which cause problems for agriculture.

González-Desales et al. believe this makes it important to properly monitor Human-Crocodile interactions in areas such as the El Hueyate estuary, and develop strategies facilitate coexistence, such as ecotourism or even sustainable harvesting. To properly conserve Crocodile populations it is not enough to simply pass laws protecting them; conservation efforts should involve local stakeholders such as governmental and non-governmental agencies, local businesses, particularly those involved in tourism, the media, and the general public.

The majority (78.5%) of Crocodile attacks in the El Hueyate estuary happen between February and September, the breeding season of the American Crocodile. This is similar to the situation seen with the Mugger Crocodile, Crocodylus palustris, in India, where 71.8% of attacks occur during the breeding season. The main economic activity in the El Hueyate estuary is fishing, with most attacks by Crocodiles happening when people are fishing close to nests in the breeding season. The population of the estuary believe that attacks are more common when Crocodile numbers are high, but González-Desales et al. could find no evidence to support this; if anything, attacks were more common when Crocodile numbers were low, possibly suggesting that people were lass wary at these times, and therefore more at risk.


American Crocodile, Crocodylus acutus, hatchlings in Isla La Concepción. Humberto Yee/Orynx.

The size of the Crocodiles appears to be another important factor, with the average size of Crocodiles attacking Humans in Mexico being 2.8 m, and 36 of the recorded Crocodile attacks on the Worldwide Crocodilian Attack Database for which size was available being by Animals in excess of 2 m. All of the attacks in the El Hueyate estuary involved Crocodiles between 1.5 and 3.7 m in length, and all of the incidents during the breeding season, when Crocodiles were either breeding, nesting, or looking after neonates. 

For a variety of other Crocodile species, it has been demonstrated that attacks on Humans occur when Human activities take them into the Crocodile's territories, and this appears to be the case for the Mexican Crocodiles. González-Desales et al. found a strong correlation between Crocodile attacks, the Crocodile breeding season, and Human economic activities, something which has previously been demonstrated on the Mexican Pacific coast. There does not appear to be any direct corelation between Crocodile population sizes and frequency of attacks. However, this probably needs to be monitored more closely, in order to find the best ways to protect both local populations and tourists. Local economic activities and the locations of Crocodile nesting sites should also be taken into account when planning Crocodile management strategies.

Negative interactions between Humans and Crocodiles appear to be increasing in Mexico, and the reasons for this need to be studied in order to develop good strategies to manage Human-Crocodile activities. Conservation policies need to take into account both Crocodile behaviour and Human economic activities in order to prevent the unwanted killing of Crocodiles; simple prohibitions on such activities are not enough. Public education campaigns are also very important, giving people the tools to beter avoid Crocodile attacks, and protecting Crocodiles by emphasising the ecological, and therefore economic, importance. A diverse range of stakeholders need to be involved in conservation efforts.

Finally, González-Desales et al. make a number of recommendations for Crocodile management in Mexico, including the monitoring of tourism activities close to Crocodile nesting sites during the breeding season, the provision of better public information on Crocodile behaviour, the protection of eggs by artificial rearing, and the development of strategies for Crocodile management that also take into account the economic needs of local Human populations. Some of these strategies have already been implemented in the El Hueyate estuary, and do appear to have prevented further Crocodile attacks; these include the provision of signs warning people of the presence of Crocodile nests, and the active involvement of local people in protecting those nests. 

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