Sargasso Swimmers: EELS
Scientists work to discover the mysterious story of American eels in Texas.
The strange and convoluted life story of American eels begins in the deep-blue Sargasso Sea, a vast area in the North Atlantic Ocean, oddly bounded not by land but by four clockwise-moving currents (including the Gulf Stream). The mystery of this ancient sea, chronicled by Christopher Columbus, has inspired literature and pop culture; the Sargasso’s crystal waters and steady food supply provide a nursery for eels, including the ones seen in Texas.
The sea is named for its ever-present sargassum, brown seaweed clumped together in huge mats on the surface. This floating ecosystem plays a critical role in the life cycle of many species, including billfish, tuna, sea turtles, migratory birds and whales.
Adult eels spawn in this oceanic sea, their eggs hatching into translucent larvae called leptocephali. These larvae drift on ocean currents until they approach a coast — in the case of American eels (Anguilla rostrata), anywhere along the Atlantic or Gulf of Mexico coasts from Newfoundland to South America. There they transform into tiny, transparent creatures known as glass eels — basically a clear tube with two eyes, says Stephen Curtis, an aquatic biologist at the Texas Parks and Wildlife Department.
Glass eels swim into estuaries and move up rivers, gaining pigment along the way and growing longer, becoming a stage known as elvers. Those in turn mature into yellow eels, which can live for many years in fresh water or salt water. Finally, yellow eels develop reproductive organs and transform into silver eels, which make the long journey back to the Sargasso Sea to spawn and start the story all over again.
These are accepted scientific facts, but the story of eels in Texas has a few holes in it.
“There is more that we don’t know than what we do know about eels here,” says Dean A. Hendrickson, curator of ichthyology at the Hendrickson Lab, one of seven collections in the University of Texas Biodiversity Center. This lab keeps fish specimens and detailed data on where and when they were collected.
“The big questions are when do they make it to Texas, and how often?” Hendrickson says. “When they get here, how successful are they at becoming adults and going back to the Sargasso Sea to spawn? We know a lot about eel life history in general, but when it comes to Texas, we have no clue.”
LOOKING FOR ANSWERS
When scientists feel clueless, they collect more data. That’s what TPWD and the Hendrickson Lab have been doing.
So far, the only eels seen in Texas are yellows.
“We have three museum records of elvers in Texas and one silver eel that washed up on Padre Island,” says Curtis. “All the rest of our sightings are in the yellow eel life stage.”
When the scientists started looking, they found yellow eels in all Texas rivers.
Glass eels and elvers can climb vertical walls and barriers. Yellow eels are no longer found as far up waterways as older records indicate, so some structures may block them. They are unlikely, for example, to make it through the turbines on hydroelectric dams. In fact, some 1,625 tons of European eels were lost to hydropower plants in 2019.
There have been recent sightings of yellow eels in Austin’s Barton Springs Pool, according to Melissa Casarez, the UT lab’s assistant ichthyology collection manager.
Some live in Austin’s Lady Bird Lake as well. Hendrickson says it’s possible that elvers go under lift gates on Longhorn Dam, which forms the lake, or access the lake through underground karst systems. Another possibility: They were there before the dam was built in 1960. That part of the story is a mystery.
It’s odd that a glass eel has never been caught in Texas, Hendrickson says.
“It simply could be that no one has really been looking.”
By contrast, the U.S. East Coast sees seasonal runs of millions of glass eels moving into estuaries and rivers at predictable times. Curtis says those runs occur in Florida in December and January, in the Carolinas in February and March, and in Maine during May and June.
“We don’t get that — or we don’t think we get that — in the Gulf of Mexico,” he says. “Only two glass eels have been collected in the entire Gulf: one in Tamaulipas, Mexico, and one in the Florida Panhandle. We know they come in, but we don’t know if they are in lower numbers, or only come every other year or three years, or what.”
SHARE YOUR EELS
As part of its TPWD-funded work, the UT lab team recruited members of the public, asking people to report whether they or someone they knew saw an eel — or even just thought they did. People could report sightings via a form on the lab’s website or the “Fishes of Texas” link on the iNaturalist app. Curtis and Casarez also collected reports old-school style, via emails and phone calls. Anyone who caught an eel was asked to submit it to the lab.
Citizen scientists responded with enthusiasm; Curtis thinks close to 20 percent of the specimens processed came from the public.
TPWD also funded sampling by volunteers at specific locations along the coast, using eel mops and fyke nets. The mops are saucers studded with short lines of rope, resembling bad Halloween wigs. Fyke nets are small, winged mesh nets with funnels that are set up in the incoming tide. Both tools have been used successfully to catch eels on the East Coast.
The Texas sampling, which wrapped up in May 2021, did not find any of the elusive glass or silver American eels. The effort did collect speckled worm eels (Myrophis punctatusa), a close relative of the American eel, at almost all sampling sites and across all its different life stages. Curtis finds it encouraging that the gear collected a species with a similar life history.
He isn’t giving up. In February 2022, Curtis started setting up prototype eel ramps. These small boxes have water trickling down a ramp for eels to swim up, where they are funneled into a bucket. These have been used regularly for monitoring around the Chesapeake Bay and other areas on the East Coast.
Perhaps it should be no surprise if American eel larvae only rarely and sporadically make it to Texas, given the distance and complexity of a journey from the Sargasso Sea into and across the Gulf, versus a shorter and relatively straight shot to the East Coast.
Currents from the Sargasso Sea into the Gulf are not as regular as those to the East Coast, says Adam Cohen, ichthyology collection manager at the lab. And while those larvae can swim, for the most part they just drift, letting currents do the work. It can take them anywhere from seven to 24 months to make the trip from the Sargasso Sea to a shore.
European eels (Anguilla anguilla), a separate species, also spawn in the Sargasso Sea, although on its eastern side. Their population has declined by 98 percent since 1980, mainly due to overfishing. Glass eels were a popular snack; some places in England held eel-eating contests, and eels are regularly on menus in parts of Asia. Now, trade in European eels is regulated by the European Union and the Convention on International Trade in Endangered Species, an agreement between governments that seeks to ensure that trade does not threaten the survival of wild animals and plants. However, illegal export of these eels to Asia continues, and scientists say the European eel population is on the verge of collapse.
Much of the global industry has moved to the U.S. as a result. Maine is the only state with a significant eel fishery so far, harvesting elvers mainly for sale to aquaculture companies in Asia that raise them to maturity. (Aquaculture operations have been unable to complete the life cycle in captivity and so rely on glass eels or elvers as seed stock.)
Elvers harvested in Maine have been worth as much as $2,000 per pound. While the East Coast harvest is well-managed, says Hendrickson, a black market thrives. That adds an urgency to figuring out the status of this species in Texas and how the Texas population fits in with the larger one.
“I don’t think Texas contributes a lot to the overall population,” Hendrickson says. “We’re probably a small part of the big picture, but we need to know for sure.”
Data on historic ocean currents may provide some insight. The Sargasso Sea is affected by the subtropical North Atlantic gyre (system of currents), the Gulf Stream, warm and cold rings, enormous eddies and the Subtropical Convergence Zone. Research aimed at predicting the timing and magnitude of the arrival of other Sargassum species, such as sea turtles, to Gulf Coast beaches could help clear things up as well.
“Right now, we consider American eels as one single population,” Curtis says. “Eels from Canada, Texas or Costa Rica can spawn with each other. But we’re collecting very basic information such as size and sex distribution, just to piece things together and get a better understanding.”
THE WHERE AND HOW OF EELS
Researchers also are working to determine the age of individual eels, using ear bones called otoliths. These bones form in layers that function much like tree rings in showing an individual’s age and the conditions it experienced in a particular time span. This work could help answer, for example, the question of whether eels got around dams or were there before those dams were built.
“Eels can live a very long time,” Hendrickson says. “The world record is 100 years.”
The otolith work also helps scientists figure out patterns of eel movement.
“We can process otoliths from a yellow eel, and the chemical composition reveals where they were living — whether in salt, brackish or fresh water — which tells us something about their movement patterns,” Curtis says. “Most start with a strong saltwater signature early on. Some remain in brackish water and others move straight into fresh water. Some go back and forth or live right on the tidal line where sometimes their habitat is fresh water and sometimes brackish. The goal is to overlay those chemical signatures with age. The more we understand about them, the better we can manage them.”
Effective management is important because eels play a role in the marine ecosystem.
“They arguably span every aquatic ecosystem you can encounter, from marine to brackish to fresh water,” Curtis says. “They’re a food source for other marine life, mainly other fish. In bays, estuaries and rivers, they are a food source for birds, and they also are predators themselves.”
Eels are intriguing creatures, the stuff of fairy tales and epic legends that still live among us.
“They’re just really cool, and a bit of a mystery,” Curtis says. “The distance eels travel is one of the most impressive in the marine world. They’re not charismatic megafauna, but once people learn that this thing swam thousands of miles, and then swam back … they get an appreciation for eels.”
Eels also could serve as the proverbial canary in the coal mine when it comes to the health of Texas river systems.
“This is the only species in Texas with this unique life history,” Curtis says. “It’s an important species to conserve.”
One whose story is still being written.
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