Eighth-graders on Staten Island spent their free time this past spring counting — thousands of eels!

Several nature-loving middle-school students stepped into Richmond Creek with nets to gather and tally the vast number of tiny, nearly invisible “glass eels” traveling from the ocean to the Hudson River’s estuaries during their yearly migration.

The students recorded approximately 11,000 of these American eels, which are mostly in their juvenile phase and are known as “glass eels” at this stage due to their almost transparent appearance, as part of the state’s annual eel counting project.

Some of the young enthusiasts even named their slippery catches after their idols, like LeBron James and Michael Jordan, before letting them return to the creek.

“It was a lot of fun to be able to do it,” said one of the young counters, Mark Geissler, 13.

“I was pretty excited to be able to learn more about the eels and to be able to count them and be able to make sure our environment was healthy and the eels were healthy while they were traveling here,” he said.

The teen estimated that he and his friends tallied and released as many as 400 eels in three trips to the creek in April — some of which they named after James and Jordan and other heroes.

The young environmentalists used special nets that helped them see the tiny and otherwise nearly invisible fish as they traveled through the Staten Island waterway — which is one of the dozen Hudson River estuaries the state Department of Environmental Conservation monitors as part of its eel count.

The count’s “robust” eel number is a good sign that the population is holding steady, according to scientists and preliminary data shared with The Post.

The effort, now in its 18th year, registered a total of roughly 140,000 glass eels, which is on par with the average number collected since the program’s inception.

“We’re trying to get a census of baby American eels as they travel from the ocean to the estuary to the watershed,” said project leader Chris Bowser. “We’re trying to get a rough idea on the timing of eel migration and the strength of the migration. How many eels are there year after year?”

“Eels have had a rough century,” Bowser said — with their populations having suffered from overfishing and the introduction of dams, which have blocked their migration.

American eels aren’t endangered, but they are considered “depleted” by the Atlantic States Marine Fisheries Commission.

The local count relies on the power and availability of volunteers — which is a core part of the eel count’s educational mission.

“One of the beautiful things about eels is they’re a very accessible animal. People can come to the river, they can come to their neighborhood stream, even a very urbanized river, and you are going to find these baby eels,” Bowser said. “So it’s an avenue into science, but it’s also an avenue into getting to know your local waterway a little bit better.”

And it’s a hit with students. As with the eels, the young volunteers return year after year to help gauge the fish population.

“The kids love this count. They purchased their own waders so they can come as much as they want,” said Mary Lee, a science teacher at the St. Clare School and Environmental Leaders Fellowship team leader.

Lee has been bringing her students to the eel count since the Richmond Creek monitoring location was established in 2012 — and says the program is so popular that students return to volunteer even after they’ve graduated.

“What’s really fun is these kids have such a good understanding of nature by doing this,” Lee said.

“Like, they really, truly understand that the tides can bring the water levels higher, carry these little eels upstream,” she said, adding that counting the largest number of eels becomes a competition amongst the students.

“It’s almost like status. It’s a big deal. It’s just so fun and exciting, and they all want to be a part of it.”