If you’ve ever heard the urban legend about released pet alligators growing to colossal sizes in sewer systems, you might be surprised to learn that there are indeed released aquatic pets thriving in the wild in Ontario — just think smaller and less bitey.
The very goldfish you often see swimming around in fishbowls and small aquariums (Carassius auratus) can grow to enormous sizes when released into the wild — topping out at a maximum length of 45 centimetres and weighing up to two kilograms.
And, unfortunately, it’s a frequent means of irresponsible pet owners ditching unwanted fish into ecosystems where they can cause harm.
Researchers commonly believe that goldfish were the first foreign fish species to make the trans-oceanic leap into North America, with human assistance, of course.
The provincial government notes that there is a prevalent practice of fish owners releasing their no-longer-wanted store-bought goldfish into ponds, lakes, or streams, where they can grow to immense sizes relative to your typical pet store specimen.
In the wild, introduced goldfish will typically make their homes in areas with slow or no current, like vegetated ponds. While not native to Ontario, goldfish are able to tolerate large fluctuations in water temperature, and even survive in water with minimal available dissolved oxygen.
Between their surprising resilience in these harsh conditions and their diet consisting of fish eggs, larvae, and aquatic plant life, these seemingly harmless aquarium species can do their fair share of damage to local ecosystems.
According to the province, the threat of this introduced species is not severe “In healthy ecosystems, [where] goldfish don’t appear to compete well with some native fish.”
“However, they are quite tolerant of poor water quality, and may threaten some native species in degraded ecosystems.”
While goldfish are not treated as seriously as other introduced species, provincial officials stress that “The ecology and impacts of goldfish introduced to North American waters have not been widely studied. However, the behaviour of Ontario populations indicates they may reduce the province’s biodiversity through their effects on native species.”
They can also carry diseases that can harm local fish populations. One example is koi herpesvirus (KHV), which is exactly what it sounds like. Fish herpes. It’s fish herpes.
While KHV does not present a risk to human health, it has the potential to spread between fish species and cause additional damage to local ecosystems.
According to the province, KHV is an invasive virus that affects only carp, koi, and goldfish. It first spread to North America in 1999 with the first cases in the U.S., and experts believe it was likely introduced to Ontario waters via the hobby fish trade.
Goldfish can be easily mistaken for decorative fish like koi, which share several similarities due to their shared lineage in the carp family. Both fish are domesticated and often share colouring characteristics, though koi can be differentiated through their slender body shapes and prominent barbels that resemble whiskers.
However, platforms like biodiversity tracking website, iNaturalist, allow experts to weigh in and confirm the species in question.
Locally, the species is often found in parks and other partially-naturalized areas where well-intentioned but equally dim-witted fish owners look to give their pets a ‘better life’ at the expense of local wildlife.
Sightings of alarmingly large goldfish have already been recorded on iNaturalist this spring in Ontario, like a recent fist-sized specimen caught in a park in Oxford County.
michieldewit/iNaturalist Canada
Within Toronto, sightings are often clustered in public parks where the species has been introduced artificially. Some hot spots include Grenadier Pond in High Park and the former brick quarry-turned-pond at the Evergreen Brick Works.
The problem spans far beyond Ontario’s borders, affecting the entire Great Lakes system. Elsewhere on the Great Lakes, even larger goldfish have been pulled from waters, like one enormous specimen in Cleveland caught in 2021 by local parks staff.
In my own unscientific analysis of the situation, I’d be willing to wager that any areas with an overlapping presence of fresh water and idiots probably have an established population of feral goldfish.
