The immense spring migration of fish and eels silently swimming up the Hudson River “is one of the greatest shows on earth and it’s happening right now,” says retired New York City police lieutenant George Jackman who tracks the health of the river for the environmental watchdog organization Riverkeeper.

“The season of the wonders is upon us,” Jackman told a small public gathering at Mount Saint Mary College Wednesday.

“The fish are moving in from the ocean just as they have for centuries to exploit the abundance of food and to renew their populations.”

But with one huge drawback, said Jackman.

“The problem is that today this river ecosystem is under tremendous stress from climate change.”

As a result the Hudson’s ecosystem as Hudson knew it long ago “is now unstable from global warming and pollution and that is very frightening”, says Jackman, “because this estuary is so vital to life in the river.”

“We’ve got a lot to lose if pollution gets worse in the Hudson.”

We know, for instance, he said, that an estimated 100 species species of fish dwell in the river along with otters, nesting bald eagles, and more than 100 species of fish, and other wildlife depend on the Hudson River just as they did before Henry Hudson sailed up the river in the Half Moon.

The Hudson is the second largest estuary on the East Coast.

“The whole river ecosystem is under tremendous stress from global warming and man-made pollution,” Jackman said. “I always wanted to know what was in these waters,” Jackman said.

“And what is behind it is one of the great shows on earth and it’s happening now.”

Because of climate change “this river is four degrees warmer that it has been and that could decrease striped bass numbers that already were reduced by historic human over fishing,” said Jackman about a food fish many river creatures and human anglers fish for and depend on.

“Historically, the Hudson has been so abundant in fish it’s been dubbed a “fish factory,” Jackman said.

“When I was a kid,” said Jackman, “I’d go fishing and you always wondered what’s in these waters because there was so much variety.”

There still are huge numbers of different fish including some shad, bass and even some flatfish from the New York harbor area.

Mostly out of sight, almost transparent baby eels no longer than a human pinky fish flood ants up and down the river, oil spills and man made pathogens have seriously undermined the health of the Hudson River.

But nature’s design for the river is resilience, says Jackman.

“Plants provide us with the oxygen and we give them the CO2 in return, what a brilliant design,” said Jackman.

“This river has been a fish factory as the largest estuary on the East Coast.”

And Jackman has faith in the Hudson’s resilience.

“I used to fish every single day and that gave me such satisfaction.”

Meanwhile, Riverkeeper continues to remove man made barriers to fish spawning gourds along the river and educates the public about the mission to remove barriers to migratory fish whose existence depends on access to spawning grounds such as lakes, and large ponds and back country impoundments.