A common household goldfish, if released into a local lake, can fundamentally alter the ecosystem, degrade water quality, reduce biodiversity and weaken native fish populations. According to a new international study, the innocuous, ornamental fish has in it to do far more than survive in the wild. The research, published in the Journal of Animal Ecology, has found that goldfish (Carassius auratus) is invasive and can trigger a rapid ecological ‘regime shift’ in freshwater lakes, particularly those already enriched with nutrients.
Goldfish has long been viewed as symbol of prosperity and good fortune but scientists say their growing spread into rivers, ponds and lakes through deliberate or accidental release is becoming overlooked as a conservation threat.
Led by researchers from the University of Toledo, Rensselaer Polytechnic Institute and the University of Missouri, the study examined how goldfish affect freshwater ecosystems under both nutrient-poor (oligotrophic) and nutrient-rich (eutrophic) conditions using controlled experimental lake ecosystems.
The researchers hypothesised that goldfish would transform clear-water ecosystems into murkier, algae-dominated environments while negatively affecting native aquatic life. Their experiments confirmed many of those concerns.
The most dramatic impacts were observed in eutrophic systems. Goldfish increased suspended solids in the water by up to 81 per cent and reduced water clarity by as much as 65 per cent, turning the ecosystem turbid. Unlike native fish, goldfish disturb lake bottoms while feeding, re-suspending sediments and organic matter into the water column.
The study found that the invasive fish also reshaped aquatic food webs. Populations of snails, amphipods and several zooplankton groups declined sharply in tanks containing goldfish. It was caused by a combination of direct consumption and habitat degradation, particularly the loss of filamentous algae that provides food and shelter for many aquatic invertebrates.
It also found goldfish affect native fish indirectly. When shared the habitat with goldfish, physical condition of the native species deteriorated by around 12 per cent compared with populations living without the invader. This pointed at competition for food resources and researchers believe the species’ exceptionally high feeding rates allow goldfish to outcompete native fishes over time.
“Despite many viewing goldfish as signs of prosperity, luck and wealth, it is critically important to inform the public that their pets can grow into sizable pests that will harm freshwater ecosystems,” the authors wrote. They urged natural resource managers worldwide to include goldfish among invasive species requiring greater attention and management.
Interestingly, not every ecological change was unique to goldfish. The study found that increase in phytoplankton and decline in filamentous algae were largely driven by the overall density of fish rather than the invasive species alone. It indicated substantial dietary overlap between the invasive and the native species. However, goldfish were significantly more damaging in terms of sediment disturbance, habitat degradation and impacts on invertebrate communities.
The scientists said the ecological consequences of goldfish invasions depend partly on the nutrient status of the lake. Nutrient-rich waters escalated impacts, including poorer water clarity and higher suspended solids. The researchers observed said that goldfish remains undesirable in both oligotrophic and eutrophic lakes because of their ecological effects. Goldfish have been domesticated for more than a thousand years and introduced to Europe in the 17th century and North America during the mid-1800s. Today they occur in freshwater ecosystems across much of the world.
Individual fish can grow up to 45 cm long, survive for more than 30 years and tolerate a remarkable range of environmental conditions, making them particularly successful invaders. Despite their widespread establishment, researchers note that goldfish remain poorly recognised in invasive species management policies compared with other carp species. The findings also called for tighter regulation of pet releases and greater public awareness about the ecological consequences of abandoning ornamental fish in natural water bodies.