The fish that walked inland

When a new road or dam crosses a river, the fish on either side stop being one population and become two. This sounds like a small inconvenience. For some species it is the end.

Many freshwater fish migrate within a river system, often over short distances. They spawn in shallow gravel upstream and feed in deeper water below, and the journey between the two is not optional. A barrier a metre high can be enough to stop them. The fish do not die immediately; they simply cannot reach the place where their young can survive, and the population ages and disappears over a decade or two while the river still looks full of fish.

Engineers build fish passes to deal with this: stepped pools, or channels that slow the water enough to be swum. Some work well. Many work for strong swimmers such as salmon and poorly for everything else, because they were designed with salmon in mind and most river fish are not salmon. A pass that a trout crosses easily may be impassable for a small bottom-dwelling species that never jumps.

The cheapest solution is usually not to build the barrier in the first place, and the second cheapest is to remove one that is no longer needed. Thousands of small weirs across Europe serve mills that stopped turning a century ago. Several countries now run programmes simply to find and remove them, which is unglamorous work with unusually rapid results: fish return to stretches of river within a single season.