Just a few days ago I photographed goldfish in the lake of a nearby nature center. I googled for more info and read they got a grant of $5,500 USD three years ago in an attempt to eradicate them.
From an April 2017 article:
“The first couple of years … we didn’t notice a lot of differences in the marsh,” she said. “But now it’s really very, very obvious how detrimental they’ve been.”
The goldfish multiplied and began to compete with native sunfish and minnows for food. They sucked food off the bottom of the lake, disrupting plant life and churning the water.
“The water used to be so clear,” she said. “They eat, they defecate, they eat, they defecate. … It’s just unpleasant all the way around.”
Each spring, the nature center hosts a class that studies the biodiversity of the marsh.
“We were used to getting 15 different organisms,” she said. “Now we’re just happy to find one or two or three.”
Shragg said the problem of invasive goldfish has only gotten worse since [last year.]
(http://www.startribune.com/officials-warn-don-t-flush-your-goldfish/365792231/)
Obviously, if I took a photo of a goldfish there last week, it hasn’t worked. And the water was, in fact, really murky.
There wouldn’t be many people who would bemoan the removal of those goldfish - feral descendants of someone’s pet - from the lake. Similar to conversations about when deliberately cultivated plants become (over time) feral populations and thereby ‘wild’ and uncultivated, one could discuss how, once an animal is let loose from human care and transitions from pet to feral, it becomes part of the ecosystem to be managed as an invasive species.