How to Get a Fish to Cross a Dam

By Sarah Zhang on at

Imagine you're a herring, maybe about a foot long, swimming merrily upstream when you suddenly hit a giant concrete wall. How do you cross it? Engineers are now designing new fishways disguised as broad, rocky pools that help migrating fish make their way through dammed up rivers.

You have probably heard of fish ladders that already exist— or "narrow concrete, metal, or wood contraptions that look a bit like flooded pedestrian highway overpasses," as Rebecca Kessler puts it in her piece for Yale Environment 360. The artificial and frankly bizarre-looking contraptions have some success in getting fish through, but we could probably do better.

In Europe, engineers have been building more natural fishways, and it's catching on in the US, too. Kessler visited one such dam on the Pawcatuck River in Rhode Island, where large boulders had been carefully placed to create large pools of water that slowly ramped up and over the dam. "The new fishway looked downright pretty, much like a natural swoosh of river, albeit bound on one side by a stone retaining wall and ribbed by a rather orderly series of rapids," she writes.

And how does one design a natural fishway? By starting with a fake river, it turns out. USGS ecologist Alex Haro uses a 120-foot-long (36 metre) hydraulic flume, essentially a mini lab-sized river, to test different species of fish at different water speeds and depths. A well-designed fishway, he says, would be able to get fish through at a 94 per cent success rate.

Fishways in river dams might be analogous to highway crossings on land. After building out infrastructure all over land and water, we've come to realise how utterly disruptive massive pieces of concrete can be. So now we have infrastructure specifically engineered for animals built on top of our existing human infrastructure. [Yale Environment 360]

Top image: A traditional fish ladder via Carsten Medom Madsen/Shutterstock