Over one million dams and culverts (tunnels that encircle rivers passing under roads) block the movements of fish and other wildlife in Europe. Scientists estimate that less than 1% of catchments in the UK are free of obstruction. A report released in 2020 showed the effect this trend is having worldwide: a more than 75% decline in the abundance of 247 migratory fish species globally since 1970.
Brown trout, for example, must swim upriver into streams to spawn. Adult European eels meanwhile need to make their way downriver and out to sea to do the same. Even small dams like weirs that are a metre tall can prevent fish from swimming upstream because they are higher than most fish can jump.
Suitable spawning areas aren’t found everywhere in a river: every fish species has its own needs. If a dam prohibits fish from reaching those habitats, the shortfall in offspring can cause populations to decline. Species living on the edge of suitable habitats are more vulnerable to droughts and pollution and are therefore at greater risk of extinction.
One solution is to remove dams. The EU plans to reconnect at least 25,000km of river across the continent by 2030, restoring the pathways migratory fish take to reproduce or feed. Completely removing a dam isn’t always possible, though, particularly where they are still used to help boats navigate. Instead, fish passes – also called fishways or ladders – can be built alongside dams to help fish swim up and around. So how do fish passes work – and what can they do to help river ecosystems recover?
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