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How To Measure Alligators

  |   Wildlife & Flora

The Historic Amazon River Cruise ships have as their mission 'biological conservation'. This takes two forms.

Firstly, the careful restoration of iron river boats that date from the late 19th and early 20th Century, such as the Ayapua and the Clavero, provides a living connection to the Rubber Boom period in the Amazon, when fortunes were made (and lost) from the collection and sale of rubber found in the rainforest. 


Secondly, these ships are used as a base, by students, naturalists and researchers, from which to conduct biological investigation in Peru's vast and ecologically-important Pacaya Samiria National Reserve. But what form does this research take, what conclusions were achieved, and why is it important?

South American Alligator- The Caiman

Of the many species studied, we provide a report relating to the censuses done on caiman (informally known as 'South American Alligators') in early 2012 to serve as a model. This census was part of an overall study into the impact of climate change - with greater extremes of high and low water seasons - on wildlife conservation. NB. No caiman were hurt in the process! 

Measuring a Caiman

Measuring a Caiman

'To assess the population and ecology of caiman species in the ecosystem it is necessary to gain an understanding of their population size. Aquatic transects were used, traveling upstream or downstream on the Samiria River and in nearby channels or lakes. A GPS was used to determine the distance surveyed each night.

All caimans seen were identified to the species level as best as possible and size of the caiman and location were noted. These data, along with data collected from captured caimans, were used to analyze the caiman population size. Caiman surveys and captures were conducted from a small boat fitted with a 15-horsepower engine. Caimans were located by their eye reflections using a 12-volt spotlight and approached to a distance where the engine was silenced and the boat paddled closer. 

Noosing was used to capture caimans. The noose was made of a long pole about 2m in length with a loop of rope that can be pulled tight over the caiman’s neck. The caimans were secured with rope tied around the jaw behind the nostrils and around the neck. Total body length was measured from the tip of the snout to the tip of the tail, while head length was measured from the tip of the snout, to the posterior edge of the orbital (the vent). The sex was also determined. Weight of the caiman was recorded in kilograms. A measuring tape and weighing scales was used.

The population abundance of each species was calculated using the formula N/L, where N= the number of individuals and L= the distance traveled in kilometers. The results indicate the number of individuals per kilometer.

Measuring a Caiman

Measuring a Caiman

Three species of caimans occur in the Pacaya-Samiria National Reserve: the most abundant is the Spectacled Caiman (Caiman crocodylus); the Black Caiman (Caiman niger) is the second most abundant, but is recovering in number after the overhunting of the 1950s to 70s and looks likely to overtake the spectacled caiman in numbers; and the Smooth Fronted Caiman (Paleosuchus trigonatus) is the rarest.

The caimans have not shown any lasting impacts from the recent climate change events. However, during the drought of 2010, there were short term impacts on the common caiman population.

How Caiman Interacted and Adapted

The spectacled caiman appeared to be impacted by the extreme low water levels, whereas the black caiman appeared to be less affected. The spectacled caiman had an overall lower abundance in the Samiria River during the drought than their six year average, with the upper section having 56% fewer, the mid section having 27% fewer, and the lower section having 40% fewer. It appears that the drop was due to spectacled caimans moving to more isolated habitats, because after the drought the caiman populations returned to the more stable numbers.

The black caiman abundances were more similar and did not show general declines, with the upriver section having very similar numbers to previous years, the mid section having slightly fewer and the lower section having greater numbers than previous years.

The habitat use of the caiman also differed in the Samiria, according to the species. The common caiman had the greatest abundance in the river habitat, followed by the lake habitat and was least abundant in the channels. The black caiman showed a similar pattern and had the greatest abundance in the river habitat and similar abundances in the lake habitat and channels. The smooth fronted caiman was only found in the river habitat.

There was a clear relationship between the black caiman, common caiman and the white piranha (Serrasalmus rhombeus). The greater the densities of white piranha, the more abundant the caiman species, indicating the correlation between food resources and caiman numbers. A reverse trend was found with the smooth fronted caiman.'

This sort of information is invaluable in understanding how species interact and adapt, which in turn informs policy-makers at a local, national and international level when trying to maintain healthy wildlife populations, and involve the local populations in this goal. If you would like to be involved in this sort of research - which does not necessitate any previous experience or skills - please get in touch to find out about the latest departures. 

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About Rainforest Cruises

Rainforest Cruises is a boutique travel company specializing in Amazon river cruises and tours, providing you with the finest collection of cruises in Peru, Brazil, Ecuador, the Galapagos Islands and Panama. As South American travel experts we have all the advice you need to help you find and book your dream cruise and an unforgettable adventure of a lifetime.


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