Delaware Public Media

Enlighten Me: Scientists track American eel population changes in Sussex County

Mar 9, 2018

They’re slippery, slimy, and you can see right through many of them.

This month, young American eels are popping up in Sussex County after making the long journey from the Sargasso Sea in North Atlantic.


Jordan Zimmerman and Chad Betts are standing shin deep in water under a Sussex County bridge at the base of a dam.

The Department of Natural Resources and Environmental Control fisheries scientists are pulling young American eels out of a fishing net.

“So we get all the eels out, put them in this square box,” Zimmerman said. 

They split up the box into a couple of sections - which helps them estimate the number of eels.

“We’ll work them back down until we have a couple hundred here. Since today is a subsample day we’ll take these back to the office and then extrapolate from that number what our total number is,” Zimmerman said.

Zimmerman says the number of eels they catch and their coloration can help determine the timing of their arrival to Delaware’s coast. The more translucent they are, the more recently they’ve arrived.

At this stage, they’re called glass eels.

“When we have a high proportion of pigment stage one which is almost translucent, then we know there’s a new cohort moving in,” Zimmerman said.

American eels are born in the Sargasso Sea. The larvae drift on ocean currents towards estuaries, where they transform into glass eels.

Zimmerman says they’re looking for freshwater habitats, and Sussex County is a perfect fit.

They change into yellow eels and spend three to 10 years in freshwater before maturing into the silver stage.

Then, they migrate back to the Sargasso Sea. 

"They make one contribution to the next generation and they pass on." -Jordan Zimmerman

“All adults spawn in the Sargasso Sea and they spawn once and they die,” Zimmerman said. “They make one contribution to the next generation and they pass on.”

Though scientists know a lot about the American eel’s life cycle, they still lack data on it. Delaware and other Atlantic states sample American eel as part of the Atlantic States Marine Fisheries Commission’s coastwide management plan for the species. Kristen Anstead is a stock assessment scientist with the commission.

“We do consider eel to be ‘data poor’, so we don’t have a lot of data that would feed a big model that would give us reference points — is the stock overfished or is overfishing occurring?” Anstead said.

Anstead says that’s why Delaware’s eel survey helps. There are a lot of questions the commission still wants answers to in order to better manage American eels.

“What is the status of this species? Where does it compare to historic levels? Where does it seem to be going and How much do you need to leave to maintain that species?” Anstead said.

State scientists like DNREC’s Jordan Zimmerman survey glass eels to predict changes to their population.

“In theory if we saw a crash of young-of-the-year eels it helps us understand what the status of the spawning population was at the time,” Zimmerman said.

At a lab in Little Creek, Zimmerman anesthetizes a small sample of eels so he can get their weight, length and color.

Jordan Zimmerman looks at eels through a microscope. The more translucent, the more recently they've arrived to Delaware.
Credit Katie Peikes / Delaware Public Media

He lines the eels up on a white square plate and weighs them. He measures their length in millimeters, then, he puts them under a microscope to look at their pigment stage.

“Translucent - you can actually see the blood flowing through,” Zimmerman said.

The data helps Delaware and the commission track a species whose population has been considered depleted but stable at low levels. The commission says while past data indicates their numbers are stable, an assessment from 2017 still considers the stock depleted. Anstead said they’ll assess the stock again in 2022.

But this could be a good year for eels in Delaware. Last year, the state saw about 300,000 eels over 27 sample days at the Sussex site. Just half way through this year’s sample, they saw about 220,000, giving them hope the American eel is headed in the right direction locally.