Goldfish: Pet or Pest?
SNP Restoration Efforts
Goldfish Infest Wallin Ponds
by Nell Larson, SNP Land & Education Co-Manager
Most of us have had a pet goldfish at some point, so we know they make good pets. When released into the wild, however, goldfish are a terrible pest! They are a non-native and invasive fish that seriously alters habitat and outcompetes native species. They reproduce rapidly and grow quickly, spending most of their time on the bottom of the pond or stream, foraging for food. They’ll consume all available food resources, including aquatic plants, invertebrate larvae, and the eggs of other fish. When these resources run out, they will even eat the smaller goldfish. Because of these characteristics, goldfish often eradicate all other aquatic organisms that live in the areas they infest, making the water bodies they inhabit very unhealthy and unsightly.
 The two small ponds at Wallin Farm recently became infested with goldfish. These goldfish came from the Silver Springs Lake, which is just upstream. A $30,000 effort was carried out there last year to eradicate the fish, and three dumpsters full of them were removed. The goldfish in Silver Springs probably came from just a few pet fish innocently released into the pond. This summer, it became clear that the Wallin Ponds were also becoming densely populated with goldfish, and we at Swaner realized that they needed to be eradicated before they became an even larger problem and traveled further downstream.
Therefore, the options for goldfish elimination were considered. The pond could be partially drained, in hopes that the remaining water and mud would freeze solid during the winter months. Unfortunately, this option would not remove the substantial nutrient load tied up in the goldfish, and it appeared to be risky: if the pond did not freeze, the remaining goldfish would multiply, and make their way downstream in the spring high water. Traps were also placed in the pond, and they proved to be completely ineffective.
The option decided upon was the use of rotenone: rotenone is a chemical immediately toxic only to fish and organisms that breathe through gills. It takes effect in a day or two, and its half-life is  approximately five days or less. Application at the end of summer was an ideal time: the ponds had a low water level and an elevated temperature. The warm water reduced the temperature-dependant half-life of the rotenone as well as the concentration of rotenone necessary for it to be effective. There was no flow into or out of the ponds, so the rotenone did not affect other areas of the Preserve and was not diluted by incoming water. Preparatory work for the project was done by DWR Native Aquatics Project Manager Michael Mills and the chemical was successfully applied by DWR rotenone expert Charlie Thompson. SNP staff members then used nets to remove the expired fish.
Conclusion: Goldfish are pets when they’re safely inside a fish tank, and pests when they’re in the wild!
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