When a fisherman cast a line in a duck pond in San Francisco, he was expecting to pull out the usual kind of fish. Instead, he caught the pacu fish, the cousin of piranhas.

The freshwater pacu fish is not native to California. In fact, the fish is actually endemic in the Amazon, making it an example of an invasive species.

Scientists are baffled as to how exactly the Amazonian fish got into San Francisco's Petaluma Pond, but this incident has happened before.

For instance, in 2015, two fishermen in New Jersey also pulled out the non-native pacu fish from the Swedes Lake.

One possible explanation is that the fish was sold illegally and kept as a pet.

Lawrence Hanja, a spokesperson from New Jersey's Environmental Protection department, said the fish might have outgrown its aquarium and so its owner released it to Swedes Lake.

How Invasive Species Affect Ecosystems

Invasive species are becoming more and more common.

According to The Christian Science Monitor, invasive species can become highly disruptive to their new ecosystem.

One example would be invasive Burmese pythons and Nile crocodiles that have recently invaded Florida.

On May 21, it was reported that Nile crocodiles have been found in the Everglades, or the tropical wetlands in the state's southern region.

Scientists confirmed that one of the three crocodiles discovered in Florida was found relaxing on the porch of a house in Miami.

Unfortunately, Nile crocodiles are extremely dangerous and reportedly kill about 200 people annually in Africa.

Outside Florida, invasive species are also affecting areas in the Great Lakes region, which now contains a large population of goldfish called golden carp.

Fortunately, the golden carp is an ambivalent invasive species as it appears that it does not cause any harm to the environment. The golden carp actually helps fishermen earn profit.

The Asian carp, which is the relative of the golden carp, is a different story. This fish, which probably travels to the Great Lakes through the Mississippi river, is seen as a great threat to the ecosystem and the fishing industry.

Marc Gaden, spokesperson for Great Lakes Fishery Commission, says Lake Erie — the fourth largest of the Great Lakes — will not be as valuable if 1 out of every 3 pounds of fish was the Asian carp.

"It's very sobering," says Gaden.

Dealing With Invasive Species

Because species such as the Asian carp and pacu fish pop up in areas they are not needed, some establishments are offering them as delicacy. Whole Foods is offering the invasive lionfish as a snack.

Alternatively, Indiana dealt with its Asian carp crisis with a wall that is two miles long and 7.5 feet high. This wall keeps Asian carp from finding their way into the marsh.

Photo : Pia Waugh | Flickr

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