CROTON-ON-HUDSON — A report that shows long-term declines for 10 species of Hudson River fish has spawned a campaign to attack a variety of possible causes.

The environmental group Riverkeeper announced yesterday that it would take aim at power plant fishkills, invasive species, overfishing, and other threats as part of a Hudson Fisheries Defense Campaign.

The group’s president, Alex Matthiessen, said at a riverside news conference that Riverkeeper, which has worked for decades to improve the Hudson, was surprised by the grim findings of a study it commissioned from Pisces Conservation Ltd., a British consultant.

“We might have been resting on our laurels a bit,” he said.

The report, an analysis of data gathered by power companies, found that despite a significant improvement in the Hudson’s water quality, 10 of the 13 species it studied show population declines since the mid-1970s. One fish, the rainbow smelt, has not shown up on the surveys at all in recent years, it said.

The report suggested a variety of causes including global warming — the river is 3.6 degrees warmer — and the invasion of the zebra mussel. It also blamed the five power plants that take in billions of gallons of river water each day — with countless doomed fish — as a coolant.

“Power plants have been slaughtering billions of fish each year,” Mr. Matthiessen said yesterday.

The report says, “Even if the power companies are not the sole cause of degradation of the Hudson River fish community, the loss of such high proportions of the fish populations must be important.”

Riverkeeper has been trying for years to force power plants to upgrade their cooling systems to a closed-cycle type that would use 97% less river water. Mr. Matthiessen said the Clean Water Act requires that power plants update their technology, and he called on the state Department of Environmental Conservation to enforce the requirement.

The biggest fish intake is at the Indian Point nuclear power plants in Buchanan. A spokesman for plant owner Entergy Nuclear, Jim Steets, disputed the study’s findings, insisting that the river’s fish population is “healthy and abundant.”

Mr. Steets said that by using screens, Indian Point safely returns most adult fish to the river. Mr. Matthiessen claimed most of those fish actually die as a result of the process.

Besides getting power plants to upgrade, the Riverkeeper campaign will seek to ban fishing in certain areas away from the Hudson if it interferes with the river’s ecosystem. He said, for example, that overfishing in the ocean and a Delaware Bay operation that catches shad on their way to the Hudson were probably contributing to the problem.

He also said “sloppy development” along the Hudson was interfering with nearshore spawning areas.

Andy Kahnle of the DEC said there are different ways to interpret the data on fish population, but “certain species are definitely in decline.”

Mr. Kahnle noted that several of the species, while down from their 1970s numbers, are up over the last several years. These include white perch, tomcod, and bay anchovy. But the reasons for such increases aren’t any better known than the reasons for the longer-term decreases, he said.

Besides the smelt and shad, the other species reported in decline are the alewife, hogchoker, white catfish, weakfish, and blueback herring.

The striped bass, bluefish, and spottail shiner have increased their numbers, the study says.

Besides the 13 species studied, Mr. Matthiessen said several other Hudson residents are in trouble, including the American eel, the sturgeon, and the smallmouth and largemouth bass.