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Migrating Freshwater Fish Populations Have Declined 81% Since 1970, Report Finds

Ahead of World Fish Migration Day on May 25, a new Living Planet Index report has revealed major declines in migratory freshwater fish since 1970. According to the findings, migrating freshwater fish populations have declined 81% from 1970 to 2020.

The Living Planet Index Migratory Freshwater Fishes report focused on data for migrating freshwater fish, or fish that move from one habitat to another for breeding and non-breeding in a seasonal or cyclical pattern. The report was a collaboration among the World Fish Migration Foundation, Zoological Society of London (ZSL), International Union for Conservation of Nature (IUCN), The Nature Conservancy (TNC), Wetlands International and World Wildlife Fund (WWF).

On average, the index of 1,864 monitored populations of 284 migratory freshwater fish species from around the world revealed an 81% decline since 1970, leading to an average 3.3% decline per year. In Latin America and the Caribbean, the report noted an average decline of 91%, and Europe's migratory freshwater fish have declined by about 75%.

"The catastrophic decline in migratory fish populations is a deafening wake-up call for the world. We must act now to save these keystone species and their rivers," Herman Wanningen, founder of the World Fish Migration Foundation, said in a press release. "Migratory fish are central to the cultures of many Indigenous Peoples, nourish millions of people across the globe, and sustain a vast web of species and ecosystems. We cannot continue to let them slip silently away."

Although the report found smaller declines in North America, about 35%, and Asia-Oceania, about 28%, the authors explained they had deficient data for these areas, and they did not have enough data to produce an average for migratory freshwater fish in Africa.

Many factors have contributed to the declining populations. According to the report, habitat degradation, loss and alterations, such as building dams in rivers or clearing wetlands for agriculture, made up about half of threats to the fish. Overexploitation made up nearly one-third of the threats.

But in the past 30 years, other threats are becoming more prominent, such as the warming waters and other effects of climate change and increasing pollution in freshwater areas.

Aside from being important parts of their ecosystems, freshwater fish are also an important food source globally, particularly for areas that may face food scarcity.

"In the face of declining migratory freshwater fish populations, urgent collective action is imperative," Michele Thieme, deputy director of freshwater at WWF-US, said in a statement. "Prioritizing river protection, restoration, and connectivity is key to safeguarding these species, which provide food and livelihoods for millions of people around the world."

Not all species have experienced declines, though, and the report highlighted that managed habitats and fisheries helped minimize declines for some populations. Management activities including fishing restrictions, no-take zones and bycatch reductions helped reduce declining populations of freshwater fish. Some populations also experienced increasing numbers.

In addition to improved management and monitoring, the report authors suggested removing of barriers such as dams, preserving and restoring rivers, promoting public and political engagement on freshwater fish conservation and increasing international collaboration efforts to save migratory freshwater fish.

Migrating Clanwilliam sandfish in South Africa. Jeremy Shelton / World Wildlife Fund

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Report Reveals 'Catastrophic Decline' Of Migratory Fish

A migrating salmon in Finland. Petteri Hautamaa / WWF Finland

Populations of salmon, trout, eel, sturgeon, and other migrating freshwater fish have shrunk by 81 percent on average since 1970, a new report finds.

"The catastrophic decline in migratory fish populations is a deafening wake-up call for the world," said Herman Wanningen, founder of the World Fish Migration Foundation, one of the groups behind the report. "We cannot continue to let them slip silently away."

The analysis, published by a coalition of conservation groups, finds that fish have been in decline for 30 years, and that their collapse is most severe in Latin America and in Europe. Humans are driving the losses by overfishing, polluting waterways, damming rivers, converting wetlands to farmland, and by fueling warming.

The report offered a silver lining, however, finding that nearly one-third of species studied have grown in number. Analysts credited the creation of new fish sanctuaries, greater legal protections for migrating fish, and the removal of dams. Last year, 487 barriers came down in Europe, while in the U.S., the biggest-ever dam removal got underway on the Klamath River in California and Oregon.

The report called for expanding such efforts to better protect migratory fish. Said Michele Thieme, of the World Wildlife Fund, "Prioritizing river protection, restoration, and connectivity is key to safeguarding these species."

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SCCF: Invasive Freshwater Fish Are Back

SANIBEL-CAPTIVA CONSERVATION FOUNDATION

The Sanibel-Captiva Conservation Foundation reported that before Hurricane Ian, the lakes and marshes of Sanibel were filled with fish. The native sunfish and bass were there, but in the last 20 years they had become outnumbered by non-native invasive Mayan cichlids and blue tilapia.

"At nearly any freshwater spot on Sanibel you could observe large nests along the shorelines made by tilapia and the smaller holes cleared by the cichlids," research associate Mark Thompson said. "Typically, if you thought you saw a sunfish, you were actually looking at a cichlid."

Mayan cichlids and tilapia out-compete the native fish because they:

– Can exist in low-oxygen conditions

– Will eat almost anything

SANIBEL-CAPTIVA CONSERVATION FOUNDATION Mayan cichlids gather in groups as juveniles near submerged limbs and detritus.

– Protect their young by holding them in their mouths until they get old enough to fend for themselves

– Begin spawning when they are 3 months old

– Are aggressive (and easy to catch)

"If you came to Sanibel within a few days of Ian's surge event, you noticed the mud covering everything and lots of dead fish," Thompson said. "It was truly stunning the number of dead tilapia and cichlids that paved the island's roads and yards."

Since Sanibel's lakes contain so much food, including algae and phytoplankton, the fish could sustain "bloom" levels. The sudden replacement of fresh water with salt water during the hurricane's surge event killed most of the island's freshwater fish.

SANIBEL-CAPTIVA CONSERVATION FOUNDATION Small shallow ponds such as this one on a city of Sanibel preserve are becoming fresh more quickly than larger deeper ponds. Freshwater fish will repopulate these ponds before the larger ones.

The SCCF reported that since Ian, the waterbodies have contained estuarine fish, mosquitofish and killifish. Lakes and marshes that contain large volumes of water may take years to become fresh water again. However, many shallow ponds and the Sanibel Slough are now barely fresh enough to support the non-native Mayan cichlid, which can also tolerate slightly salty water.

"Fish watchers around the island have been wondering when we would see them return — knowing that Mayan cichlids would be the first," he said. "Now that they are here, the only thing that will limit their range is how salty the lakes are."

Recently, Thompson and his cast-netting son pulled in living proof that Mayan cichlids have returned to the Sanibel Slough. In addition, a blue tilapia was also found in the Slough near Casa Ybel Road.

The SCCF noted that the Sanibel Slough is currently about 7-9 practical salinity units (PSU). Fresh water is 0-2 PSU and Gulf water is 30-36 PSU. Some of the biggest lakes on Sanibel are still 25-36 PSU, but where there are low salinity pools, Mayan cichlids will find a way to get there.

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