Oversized Goldfish Could Become Super Invaders

Just west of Toronto last summer, startled biologists counted more than 20,000 goldfish in a single urban stormwater pond the size of two basketball courts. And the fish, likely descended from abandoned pets, weren’t just thriving numerically – some had grown into three-pound behemoths. Cities across North America have increasingly built such ponds over the past 40 years to catch rain and runoff, and invasive goldfish are flourishing in thousands of them.

Ecologists from the University of Toronto and Fisheries and Oceans Canada (DFO) are currently studying if and how the harsh, polluted environments of these ponds select extra-tolerant fish – which could eventually succeed in out-compete native species in the nearby region. of Great Lakes. As Nicholas Mandrak, a conservation biologist from the University of Toronto at Scarborough who is working on the project, puts it: “Are we creating these ‘super-invaders’ that are likely to have ever-increasing impacts? more important in nature due to climate change?

The goldfish is native to East Asia. They likely first made their way via ships’ ballast water to North American rivers and the Great Lakes, where Mandrak estimates small, localized populations have survived for 150 years. They are a nuisance presence in any new habitat they enter, says Anthony Ricciardi, professor of ecology and invasive species at McGill University, who has worked with Mandrak in the past but was not involved in the new research. For one, goldfish are messy eaters. They swallow mouthfuls of fine sediment from the bottom of lakes and rivers, swirl it around, spit out the cloud of soil, and then suck up the falling food. This uproots the plants and makes the water cloudy. Less light then filters to the aquatic plants, which may eventually die as a result. Through this destructive behavior, goldfish alter their habitat in ways that make it worse for other species that catch prey on sight or rely on sunlight, Ricciardi says.

Although invasive goldfish have a long history in North America, their populations in stormwater ponds and some Great Lakes harbors have increased sharply over the past decade, along with a concurrent increase in pond construction. urban stormwater. Biologists suspect that most stormwater pond goldfish were originally introduced by humans; goldfish from the lake are unlikely to have made their way upstream into these isolated pools. Most species of fish cannot live in the harsh and unstable conditions of stormwater ponds, where water levels fluctuate frequently with rainfall. These ponds can also be low in oxygen and have relatively warm temperatures due to their shallow depth. But goldfish have developed a special metabolic system that can sometimes allow them to survive up to five months without oxygen.

One of the stormwater pools that scientists investigated for signs of goldfish adapting to the harsh water conditions. Credit: Christine Boston/Fisheries and Oceans Canada

Scientists fear the latter ability could give goldfish a competitive advantage over native species, as global warming leads to lower oxygen levels in lakes and rivers, says Christine Boston, a fish production biologist at the DFO. If this happens, and if fish from urban ponds enter natural wetlands, they could wreak even more havoc than existing off-pond goldfish populations. To learn more, Mandrak and his colleagues compare pond goldfish with wild Canadian goldfish populations under current conditions and those expected from climate change.

Last summer, the team tested the temperature tolerance of goldfish from two stormwater ponds. The researchers placed goldfish in the water and slowly increased the temperature until the fish could not maintain an upright position, indicating that they had reached their maximum heat tolerance. Mandrak is set to test goldfish from 24 other ponds this summer and compare the overall tolerances of pond goldfish with those of wild Great Lakes populations. Eventually, the team plans to identify specific genes that regulate temperature tolerance and determine whether they vary between wild and pond fish, which would be a sign that adaptation is underway.

The project also aims to characterize the environments of stormwater basins. These ponds are generally less than six feet deep and tend to be relatively warm. They are often very salty due to winter road salt runoff and often contain additional nutrients from fertilizers. Warm temperatures and high nutrient levels lead to low oxygen levels in the water, Boston says. She is also developing environmental DNA (eDNA) sampling methods to test goldfish genetic material in small water samples. Knowing pond characteristics can help DFO identify specific stormwater ponds as “high risk” goldfish habitat, and they can then quickly use eDNA samples to determine which species are present. If goldfish are detected, drainage into adjacent waterways could be blocked to reduce the chances of exceptionally tolerant fish entering the natural environment.

Future management of these potential super-invaders comes down to prevention, say experts including Mandrak, Boston, Ricciardi and others. For example, signs could be placed around ponds advising fish owners to take unwanted animals to the store or give them to a friend instead of throwing them away. Beyond that public messaging, Boston says real estate developers and engineers may want to reconsider the design of stormwater ponds to keep goldfish and other invasive species out. That could include building barriers between ponds and adjacent waterways or stocking ponds with goldfish predators such as largemouth bass (which are already native to affected areas), Boston says.

Boston and other biologists hope to better understand the threat before it’s too late for downstream native fish nurseries and wetlands. “Until we complete the risk assessment,” Mandrak says, “we need to do our best to make sure these goldfish don’t end up in the wild.”

About Ethel Nester

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