Wild Atlantic salmon swim at a fish hatchery in the U.S. Atlantic salmon in the Bay of Fundy and Gulf of Maine regions remain endangered, while the species is extinct in the Great Lakes region, according to the U.S. Geological Survey. (Michael C. York/Associated Press)

About four out of 10 freshwater fish species in North America are in peril, according to a major study by U.S., Canadian and Mexican scientists.

And the number of subspecies of fish populations in trouble has nearly doubled since 1989, the report says.

One biologist called it "silent extinctions" because few people notice the dramatic dwindling of certain populations deep in American lakes, rivers and streams. And while they are unaware, people are the chief cause of the problem by polluting and damming freshwater habitats, experts said.

In the first massive study of freshwater fish on the continent in 19 years, an international team of dozens of scientists looked not just at species, but also at subspecies — physically distinct populations restricted to certain geographic areas. The decline is even more notable among these smaller groups.

The scientists found that 700 smaller but individual fish populations are vulnerable, threatened, or endangered. That's up from 364 subspecies nearly two decades ago.

And 457 entire species are in trouble or already extinct, the study found. Another 86 species are OK as a whole, but have subspecies in trouble.

The study, led by U.S. Geological Survey researchers, is published in the current issue of the journal Fisheries. Researchers looked at thousands of distinct populations of fish that either live in lakes, streams and rivers or those that live in saltwater but which migrate to freshwater at times, such as salmon that return to spawn.

Some vulnerable fish are staples of recreational fishing and the dinner plate. Striped bass that live in the Gulf of Mexico, Bay of Fundy and southern Gulf of St. Lawrence are new to the imperiled list. So are snail bullhead, flat bullhead and spotted bullhead catfish. Sockeye, Chinook, coho, chum and Atlantic salmon populations are also called threatened or endangered in the study. More than two dozen trout populations are considered in trouble.

About six per cent of fish populations that were in peril in 1989, including the Bonneville cutthroat trout, have made a comeback, said lead author Howard Jelks of the U.S. Geological Survey. But one-third of the fish that were in trouble in 1989 are worse off now, said the Gainesville, Fla., biologist.

The study includes far more species and populations than those that are on the official U.S. government endangered species list.

Jelks said the number of species in trouble was close to double what he expected and that means people should be "considerably worried."

The biggest cause, Jelks said, is degraded freshwater habitat, both in quality and quantity of water for fish to live in. Invasive species crowding out native fish is also to blame, he said.

Fish "live in a freshwater habitat that's pretty much under assault by people," said Duke University marine biologist Larry Crowder, who wasn't part of the study. "Things are tanking all around us. When does it have to be bad enough to get people's attention?"

Many of the species in trouble or already extinct are small minnows and darters whose absence is little noticed, but they play a vital role in the food chain.

Hardest hit is Mexico, where nearly half the fish species are in trouble. One in three species in the United States are in peril — up from about one in five in 1989. About 10 per cent of Canadian species dwindled. In the United States, the most vulnerable populations are in the Southeast, not counting Florida.

In the U.S., 263 fish species are in trouble or are already extinct, and nearly 500 have no problems.

The number of fish species and subspecies in North America that went extinct rose from 40 to 61 since 1989, the study said.

Anthony Ricciardi, a McGill University biologist who was not part of the research, found that about 10 years ago freshwater extinctions were happening at a faster pace than on land or in the sea. And yet few people notice, he said.

"A lot of silent extinctions are happening," Ricciardi said. "What we're doing is widespread, it's pervasive and it's rapid."