One day in 2014, a female eel set off from Nova Scotia on a long and hazardous journey to her spawning grounds. This was no ordinary eel. Scientists had released her with a tracker to learn more about her migration. After 45 days, the eel had traveled no less than 2,400 kilometers (1,500 miles) and reached the northern edge of the Sargasso Sea. There, contact was lost, but it is assumed she spawned and released thousands of eggs.
It was the first time such a migration had been tracked in detail. The experiment, conducted by the Ocean Tracking Network at Dalhousie University, Nova Scotia, showed that migrating fish like eels have complex lives, combining journeys through rivers, down coasts and across open seas. For the eel, the unity and integrity of these ecosystems is a matter of life and death, as it is for other migratory fish—from New England river herring and California steelhead to the Mekong catfish, salmon species in many parts of the world, and even tuna, which take long migratory routes across the world’s oceans. All depend on regular migration and in turn, millions of people around the world rely on them as both a food source and income. In the Mekong River basin alone, migratory fish are an important source of food and income for tens of millions of people.
Yet populations of these and other migratory fish, especially those that migrate between river and ocean, have collapsed over the last century or are collapsing. Europe’s population of eels has tumbled to just 1% of the numbers of a few decades ago. Some fish are on the Red List of threatened species. What has caused this precipitous decline in once abundant stocks of migratory fish? What can be done about it?
The critical state of many fish migrations between rivers and oceans has a host of causes, mostly the unintended consequences of human activity. Unregulated fishing and destructive fishing practices, bycatch at sea which scoops up unwanted fish in the search for more profitable or targeted species, and the construction of dams and levees which cut rivers off from their natural floodplains, all harm the ecosystems essential to fish migration.
Dams in particular often block migratory fish from the stretches of rivers they need to thrive and reproduce. The proliferation of dams on the Amazon, Irrawaddy and Mekong, for example, threatens the remaining fish migrations on those rivers. Remedial measures such as fish bypasses and ladders can only partly compensate for the massive obstacle to swimming fish. Eels cannot negotiate fish ladders at all, although there are other types of fish passages they can use.
The migratory cycle is the rhythm of nature. It is intricate and sensitive, vulnerable to shocks from outside. If it is disrupted, fish stocks fall fast. The economic consequences can be serious, and poor people are often the hardest hit. The decline in fish stocks in the Mekong, one of the world’s richest river fisheries, is threatening supplies of animal protein for millions who rely on it. Fisheries directly employ over 58 million people worldwide and are valuable for sport and recreation. Fishing is the most popular sport in most developed countries, with about 50 million participants in the United States alone.