BioEd Online

Urban planners could be set to gain a new weapon in the fight against flooding: the humble goldfish. Environmental engineers hope that the pets could help to make new open-air drainage systems more acceptable to the people who would have to live alongside them.

The new flood-defence approach, called a sustainable urban drainage system, or SUDS, aims to prevent rainwater from entering the regular underground drainage system and mixing with sewage. In the wake of a big storm such mixing can cause out-of-town water-treatment facilities to be swamped with contaminated water, often prompting workers to simply discharge the water into the nearest river rather than risk the effluent backing up into the city's drains.

SUDS aims to get round this by channelling rainwater into a network of culverts, ponds and artificially created wetlands. This would allow the water to soak slowly into the ground, preventing it from inundating the city's sewers.

The problem, however, is that the pools built to accommodate overflow water are likely to become overgrown with algae, points out Miklas Scholz, an environmental engineering researcher at the University of Edinburgh, UK. "Algae are perceived as a nuisance," he says. "People want to see something in the water that's attractive."

That's where the fish come in. Scholz and his colleagues added common goldfish (Carassius auratus) to experimental drainage ponds set up on their university campus. The ponds were fed by runoff from a nearby road. As reported in the International Journal of Environmental Studies¹, the fish kept the ponds free of surface algae. The find will be familiar to many who keep goldfish or other carp species in their garden ponds for just the same reason, to keep things clean.

Turning the tide

Scholz and his team have been consulting with urban planners in Glasgow and Edinburgh, and almost 200 sites are being considered for the SUDS approach. "But the system can only be built if the public is happy with it," Scholz says.

Fish could do the trick in shifting public opinion. So could an attractive display of plants around the ponds, adds Sholz, although this would have to be considered carefully: plants can also provide alternative food for the fish, which may divert them from eating the algae.

Rebecca Wade, an environmental engineering researcher at the University of Abertay Dundee, UK, agrees that encouraging SUDS systems is a good idea. But she adds that fish may not always be the answer. "It's not necessarily appropriate in all areas," she says. "It has to be a site-by-site decision."

Unwelcome invaders

The ponds might encourage all sorts of unwelcome plants to take root, points out David Knight, who advises the conservation group English Nature on urban issues. Planners should be wary of allowing the ponds to become havens for problem plant species such as New Zealand pygmy weed (Crassula helmsii), and water hyacinth (Eichhornia crassipes), he notes, both of which have a history of invading British aquatic habitats. But he adds that compared to the problems posed by overflowing sewers, concerns over introducing new species are relatively insignificant.

One threat that Scholz does take seriously is the danger of dog excrement being washed into the drainage pools by rain runoff, which could expose both people and fish to dangerous bacteria.

To evaluate this danger, he gave his students the unenviable task of retrieving dog faeces from the bins provided in parks, and washing them into the university's experimental SUDS system to monitor their effects on the fish. Luckily they found that the grass and wetlands around the ponds were able to filter out most of the contamination, saving the water from an excess of organics and the fish from nasty pathogens.