Canadian wildlife officials reported a haul of absolutely massive goldfish last week from Hamilton Harbour, a bay on the western tip of Lake Ontario. The jumbo-sized catches came as part of a research project that uses acoustic transmitter tags to track the movements of goldfish, which are a relative of the common carp. Water managers hope the research project will provide them with information that could help them control the remarkably invasive species.

“Ever wonder what happens to pet goldfish when they end up in our waterways?,” tweeted Fisheries and Oceans Canada, which is the government department that manages the country’s oceans and inland waters. “By tracking these goldfish, we’ve learned that they’re breeding in Hamilton Harbour and targeting key spawning sites for native species like Northern Pike, tearing up aquatic plants for food, and clouding the waters with their waste.”

The organization provided several photos of bright orange goldfish with enormous bellies but didn’t provide length, girth, or weight data. Still, the post vividly highlights just how large the typically home aquarium-dwelling species can grow if illegally released into public waters. Hamilton Harbour is such an ideal environment for goldfish because of decades of pollution that have reduced the bay’s water quality.

According to Canadian Sportfishing, sewage water and industrial byproducts have been polluting the Hamilton Harbour waters since the 1800s, exerting a devastating toll on the area’s native fish and plant life. Today, the sewage water that enters Lake Ontario near the port is treated, but it still contains a significant amount of nutrients that fuel algae growth. Currently, much of the water near the harbor has low oxygen levels, which makes it a tough place for native fish like walleye and yellow perch to survive—but not goldfish.

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Goldfish thrive in low-oxygen environments, and the way that they aggressively bottom feed, which stirs up sediment and uproots plants, can further degrade water conditions for other species. Fisheries and Oceans Canada is pursuing high-tech solutions, such as the tagging program used in Hamilton Harbour, to limit their impact in this case. However, the agency says the best way to truly combat the problem of invasive species is by keeping them out of public waters to begin with. Officials urge anglers and pet owners not to move species, organisms, or water from one body of water to another, and to keep any aquatic plant or animals, such as pets from aquariums, water gardens, or ponds, out of the natural environment. It is illegal everywhere in Canada to introduce any species into a body of water where it is not native.