These tanks of water at Pier 40 hold a variety of fish that normally live in the Hudson River, from horseshoe crabs to oyster toadfish. Marine life experts here say they use them to show the public the river is thriving.

“So many people that think that the Hudson River is a dead river, that it's green, that it's toxic, so we're here to show people that it's very much alive and that it's green because of all the plant life and living things that live in the water, and support this very important ecosystem,” said Melissa Rex, Director of Education for the River Project.

For several months, beginning every April, workers here catch dozens of species of fish and put them in tanks that are hooked up to a system that pumps in water from the river.

“It keeps them at the same temperature and chemical conditions and water conditions that they would normally be, which is important,” said Rex.

The fish are rotated out every couple of months, depending on the water temperature; some fish want to swim elsewhere if the water is too cold or too warm.

Scientists say they occasionally make surprise discoveries, which over time may help them to shed light on the range of temperatures fish can survive in.

“A few weeks back a butterfly fish, which is a tropical fish that was caught - it was about that large and it was extremely rare for that to have happened,” said Tina Walsh, Director of Education and Outreach at Hudson River Park.

With the temperature starting to dip, workers are now getting ready to wrap up the season. That entails taking all the fish they've collected over the last several months and returning them to the water so they can spend the colder months living in their natural habitat.

“It's really important for these animals to get back to their habitats. The river often freezes in the winter, so we need to make sure that the water source is able to be consistent for these fish,” said Walsh.

The event is called the Release of the Fishes. It's happening Thursday afternoon from 4 to 7. The public is invited to come down and help out.

For more information go to HudsonRiverPark.org.