Colombia on Tuesday began the sterilization of hippopotamuses, descendants of animals illegally brought to the country by late drug kingpin Pablo Escobar in the 1980s.

Two male hippos and one female underwent surgical sterilization, environmental authorities said. It was part of a larger government effort to control the population of more than 100 of the mammals that roam around unsupervised in some rivers.

The plan includes sterilizing 40 hippos each year, transferring some of them to other countries and, possibly, euthanasia.

The hippos, which spread from Escobar's estate into nearby rivers where they flourished, have no natural predators in Colombia and have been declared an invasive species that could upset the ecosystem.

A group of hippos was brought in the 1980s to Hacienda Nápoles, Escobar's private zoo that became a tourist attraction after his death in 1993. Most of the animals live freely in rivers and reproduce without control.

Scientists warn that the hippos' feces change the composition of rivers and could impact the habitat of local manatees and capybaras.

Independent journalist Audrey Huse, who has lived in Colombia for eight years, told CBS News that because the hippos roam freely, they end up killing fish and threatening endemic species like manatees, otters and turtles.

"Because they have no natural predators here, as they would in Africa, the population is booming an it's affecting the local ecosystem," Huse said. "Because they are such large animals, they consume considerable amounts of grassland and produce significant waste, which then poisons the rivers."

Sterilization takes time, because spotting and capturing the territorial, aggressive three-ton animals is complicated, David Echeverry López, chief of the environment office in charge of the plan, said in a video distributed to the press.

Rain events around the area have complicated efforts to capture the animals. More grass means "they have an oversupply of food, so baiting them to capture them becomes even more complicated," Echeverry said.

The government estimates there are 169 hippos in Colombia, especially in the Magdalena River basin, and that if no measures are taken, there could be 1,000 by 2035.

When the plan was first announced, the environment ministry said the procedure is expensive — each sterilization costs about $9,800 — and entails risks for the hippopotamus, including allergic reactions to anesthesia or death, as well as risks to the animal health personnel.

Experts say sterilization alone is not enough to control the growth of the invasive species, which is why the government is arranging for the possible transfer of hippos to other countries, a plan that was announced in March. But the cost of deporting the hippos is also expensive — an estimated $3.5 million.

In:
• Colombia
• Pablo Escobar
• Hippos