From Sea to Stream: Tracking the Elusive American Eel as it Slithers from Texas’ Estuaries into its Rivers

The American Eel (scientific name Anguilla rostrata) is a species of eel that is found throughout the eastern part of North America, inhabiting coastal streams and rivers from as far north as Canada, to as far south as northern Mexico. Texas is located squarely in that range, and adult American Eels can be found in many of the freshwater habitats of Texas, including rivers and reservoirs. It isn’t a species that is easily observed; adult eels are bottom dwellers that are most active at night, and for this reason they are unlikely to be encountered by recreational anglers very often. However, thanks to Texas Parks and Wildlife Department’s Inland Fisheries Division, adult eels have been caught using a variety of scientific fishing gears meant to target and detect the species directly (Figure 1).

American Eels have a biology that is unlike any other species of fish. They are “catadromous”, meaning that they spawn in saltwater, and thereafter juveniles migrate on currents into coastal and freshwater habitats. There are several species of catadromous fishes in the world, but American Eels take the practice to its extreme: every individual of the species is born in a single location, the Sargasso Sea, which is a large oceanic gyre located off the Atlantic Coast of North America near Bermuda. So, all of the eels that enter coastal inlets across the North American continent (including Texas’s estuaries) start out in the middle of the Atlantic Ocean, spawned at the same time and in the same place. But beyond that, not much at all is known about the pathways they take to get to Texas, or even when they arrive.

Throughout most of the known range of the species on the Atlantic Coast, large migrations of juvenile eels (also called “glass eels” due to the fact that their bodies are devoid of pigment at the earliest stages), have been documented occurring in late winter through early summer. These glass eel runs on the east coast are so numerous that in a handful of the Atlantic states they support a commercial fishery driven by to high demand from international markets. In contrast, virtually nothing is known about the arrival of juvenile American Eels in Texas, or even in the greater Gulf of Mexico. None have ever been observed in Texas’ estuaries, nor have glass eels ever been reported from Texas’ rivers. That is, until now.

To address this knowledge gap, researchers from the Environmental Institute of Houston (located at the University of Houston-Clear Lake, hereafter UHCL) recently headed up a collaborative study aimed at documenting the arrival of juvenile American Eels in Texas. The collaborative team included biologists from UHCL, the Inland Fisheries and Coastal Fisheries divisions within TPWD, as well as biologists at the Lower Colorado River Authority (LCRA). The team set up Irish eel ramps in lower river reaches of 9 rivers and streams across coastal Texas, from Victoria to Port Arthur. Irish eel ramps are a sampling gear that specifically target migrating juvenile eels, and they are usually set up in sections of the river where there is a dramatic change in elevation, such as near dams, culverts, or natural elevation changes. Juvenile eels have a remarkable ability to climb over most obstacles they find in rivers, so long as water is present, so the ramps provide an alternative and easy migration route. The ramps use the force of a gravity fed siphon flow to push the eels into a partially submerged collection bucket that is located next to the ramp. In addition to the ramps, the team collected water samples at ramp sites to detect “cellular” clues from juvenile eels that might be floating in the water column. These cellular clues refer to the DNA that fish (and all organisms) shed in water naturally as they pass through an area. This technique is referred to as “environmental DNA detection” and has been used successfully to monitor the presence of eels on the east coast without seeing the eels themselves.

Both methods paid off. In May of 2022, a late-stage juvenile American Eel (also called an “elver”) was observed in a ramp in the Colorado River near the Bay City dam. Then in January of 2023, the first two glass eels ever observed in Texas were found in an eel ramp in Lynn Bayou, Port Lavaca, less than ½ a mile from the mouth of the bayou in Lavaca Bay). An additional 17 glass eels and 5 elvers were caught in subsequent weeks of the study. Although eels were not observed in every eel ramp in the study, their DNA was. DNA detection of American Eels was made at every ramp site at least once, with the bulk of detections occurring in winter and early spring, which lines up with when juvenile eels are usually detected on the east coast.

Using these innovative sampling methods, the team was able to fill in an important knowledge gap related to the “when” and “where” American Eels traverse Texas’ estuaries and enter the rivers. These types of data can be used for planning more conservation-minded infrastructure along river pathways that can benefit migration of juvenile eels as they make their way upriver. For instance, the LCRA biologists that partnered on the study are now using what they know about juvenile American Eel migration to brainstorm innovative solutions to improve migration for the species. The knowledge gained through the study will also be used to paint a clearer picture of the ways that American Eels fit into estuarine and riverine fish communities as a whole in Texas and will assist biologists as they try to build future strategies for conserving this elusive, imperiled species.