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Just west of Toronto, startled biologists accumulated more than 20,000 goldfishOne urban stormwater pond was the equivalent of two basketball courts. And the fish, probably descended from dumped pets, were not only thriving numerically—some had grown into three-pound behemoths. These ponds have been built in North America by cities over the past 40 years to collect rain and runoff. Invasive goldfish thrive in thousands of these ponds.
Ecologists at the University of Toronto and the Fisheries and Oceans Canada (DFO) are now investigating if and how these ponds’ harsh, polluted environments are selecting for extra-tolerant fish—which might eventually manage to out-compete native species in the nearby Great Lakes. As Nicholas Mandrak, a University of Toronto Scarborough conservation biologist working on the project, puts it: “Are we creating these ‘superinvaders’ that are likely to have incrementally greater impacts in the wild under climate change?”
In the year 1898, Goldfish was created. East Asia. They probably first made their way via ships’ ballast water to North American rivers and the Great Lakes, where Mandrak estimates that small, localized populations have survived for 150 years. Anthony Ricciardi of McGill University is a professor of ecology, invasive species and ecology. He has worked with Mandrak before but was not involved with the new research. One reason is that goldfish can be messy eaters. They swallow fine sediment from the river and lake bottoms, swirl it around, and then take in whatever food they can. This causes plants to be rooted. Makes water cloudy. This causes less light to reach aquatic plants, which can eventually cause them to die. Ricciardi states that this destructive behavior makes it harder for other species, such as those that are dependent on sunlight or prey by sight, that goldfish have created their habitat.
Though invasive goldfish have had a long presence in North America, their populations in stormwater ponds and some harbors in the Great Lakes have sharply increased in the past decade—alongside a concurrent rise in urban stormwater pond construction. Biologists believe that stormwater pond goldfish were introduced by humans. It is unlikely that these lake fish made their way upstream to these isolated pools. Stormwater ponds can cause severe flooding, which causes water levels to fluctuate with rainfall. This makes it impossible for most fish species to survive. These ponds can also have low oxygen levels and relatively warm temperatures due to their shallow depth. However, goldfish have a unique metabolic system that allows them to sometimes be able to use this special metabolic system. You can survive for up to five months without oxygen.
Christine Boston, DFO fish production biologist, is concerned that this will give goldfish an advantage over native species because of global warming’s effect on oxygen levels. If this happens, and urban pond fish are allowed to enter natural wetlands, they can cause even more damage than non-pond goldfish populations. Mandrak and colleagues are comparing pond and wild Canadian goldfish populations under current conditions as well as those expected to change due to climate change.
The team tested the temperature tolerance in goldfish using two stormwater lakes last summer. The researchers placed the goldfish in water and slowly increased its temperature until the fish couldn’t stand upright. This indicated that the fish had reached their maximum heat tolerance. Mandrak will test goldfish from 24 additional ponds this summer to compare their tolerances with wild populations in the Great Lakes. Eventually the team plans to identify the specific genes that regulate temperature tolerance and to determine whether they vary between wild and pond fish—which would be a sign that adaptation is occurring.
The project also aims for the identification of stormwater pond environments. These ponds are typically less than six feet deep, and they tend to be quite warm. They are often very salty due to runoff from winter road salt and contain extra nutrients from fertilizer. The Warm temperatures and high nutrient levelsBoston states that low oxygen levels can lead to low water quality. She is also developing. environmental DNA(eDNA) sampling methods to detect genetic material in small water samples. Knowing pond characteristics can help the DFO identify specific stormwater ponds as “high risk” goldfish habitats, and they can then quickly use eDNA samples to determine what species are present. To reduce the chance of fish that are extremely tolerant, draining into nearby waterways could be done if goldfish are found.
Experts, such as Ricciardi, Boston, Ricciardi, and Mandrak, believe that the future management of these superinvaders will come down to prevention. Signs could be placed around ponds to remind fish owners to return their pets to the store, or to give them to a friend, rather than dumping them. Boston suggests that engineers and land developers should reconsider stormwater pond design in order to prevent the introduction of goldfish and other invasive species. This could include constructing barriers between ponds, adjacent waterways, or stocking ponds in largemouth bass (which are invasive species) to keep them out. Already nativeBoston states that it is important to consider the specific areas.
Boston biologists and other biologists hope to better understand the threat, before it is too late. “Until we have completed the risk assessment,” Mandrak says, “we should do our best to make sure that those goldfish don’t get into the wild.”
