January 28, 2022

How do city roads trap migrating fish?

Most people never think about fish migration or the underground waterways in which they move. Cities all over the world are built over small rivers and streams without people realizing it. Urban planners eventually found out they need to come up with more sustainable solutions to protect fish migration. Here’s a look at how roads over waterways reduce fish migration.

Decline of fish migration

Since 1970 fish migration has declined by 76% globally, according to the World Fish Migration Foundation. Since then a movement to protect marine life has grown to share ideas for sustainability. A report called “The Living Planet Index for Migratory Freshwater Fish” provides a comprehensive view of the current status of migratory fish and how they have been impacted by human intervention.

One of the reasons for the steady drop in fish migration has been the rise of human-made culverts, which are tunnels built over small rivers, designed to connect underground waterways. They’re typically found under roads and rail lines. The problem culverts pose to fish is they limit the movement of water and access to nutrients. Many old-fashioned culverts create barriers that fish cannot cross because they weren’t designed with fish in mind.

The way a culvert is constructed can make it extremely difficult for fish to move from one body of water to another. An old or degraded culvert creates small downstream waterfalls, which blocks fish from advancing. Other ways fish migration can be limited are when the water in a culvert is too shallow or the opening is too narrow. Sometimes debris can build up at a narrow opening, preventing the fish from passing.

Chemicals combine to pollute

Another harmful effect from human intervention has been how pollutants from busy roads run off into streams, creating a toxic environment for fish. Other toxins that are collected through water add to unhealthy conditions. A recent EU Water Framework Directive assessment has been able to identify pollution hotspots in Greater London. Analysts have pinpointed where the most runoff pollution exists in the city’s rivers.

Many large cities sit near the mouth of a river, where migratory fish are expected to pass upstream and downstream. In the 1990s Seattle scientists tried to figure out the mystery why 90% of upstream-migrating Coho salmon died after rainstorms. They determined it had to do partly with road density and traffic volume. But didn’t learn until 2020 the massive deaths were mainly due to a toxic chemical from tire particles called 6APPD-quinone.

Removing death traps for fish

Instead of allowing fish species to get wiped out by pollution, many local governments are focusing on sustainability and social responsibility issues. Since fish can get trapped in culverts, cities are moving toward removing culverts and coming up with new solutions that are more fish-friendly. Culverts and chemicals are the two main reasons attributed to falling fish migration the past several decades.

The point where a stream and culvert converges creates rapidly-flowing water that young fish are unprepared to handle. Managers of fisheries around the world are now widely aware of the negative impacts culverts and chemicals have on fish. New solutions must begin with a mindset beyond the traditional premise of focusing only on humans when it comes to building infrastructure.

One solution has been to modify culverts so that fish have easy access through them. Adding fish screens helps direct underwater traffic, as fish screens prevent fish from entering pipes or canals. Putting screens on dead ends is another effective strategy to promote fish migration. It will further be helpful for local officials to make sure culverts are not surrounded by turbulent areas that create rigid barriers.

The future of how culverts are constructed will take wildlife more seriously into account, as governments in general are moving more toward environmental protection. Luckily, enough scientists have observed the decline of fish migration to care about the need for promoting more sustainable solutions.