American Eel

Anguilla rostrata

A Brief Introduction and Guide for Care

Anquilla rostrata

Freshwater eels are the only catadromous fish in North America. This means they spawn in salt water but live as adults in fresh water. (Langlois, 2017)

In late spring to late fall sexually mature eels will embark on a 5,500 kilometer journey to the Sargasso Sea in order to spawn. Spawning occurs from late winter to early spring, with females laying on average 0.5-4 million eggs. Males will fertilize the eggs after they are laid, and only a small percentage will survive to the larva stage. (Langlois, 2017)

Larva, leptocephalus, will hatch about a week after male fertilization. These larvae are transparent and leaf-like in shape. The larvae will passively drift from the Sargasso Sea to the Gulf Stream. They will spend the next 7-12 months drifting in the current until they mature into glass eels (55-65 mm in length) and reach the shallow waters of the continental shelf. Glass eels resemble adults in appearance, but are still transparent. (Langlois, 2017)

As glass eels move closer to the shore line, they will gradually become darker in color and will transition from the water column to the bottom substrate. This transition marks the change from glass eel to elver. (Langlois, 2017)

For 3 months to a few years, elvers will migrate closer to shore and up river tributaries (see map on page 4). When the elvers reach their destination they become immature eels, or yellow eels. Yellow eels that remain in salt water or brackish water will progress through the life cycle quickly (about 9 years). Those that migrate further will change slowly taking up to 50 years to progress to adulthood. The majority of growth for American Eels occurs during the elver stage. (Langlois, 2017)

Adult eels, also known as silver eels, are those that have changed color once more and are now sexually mature. Females will be larger than their male counterparts and will be lighter-colored. They will then begin their journey back to the Sargasso Sea where they will spawn and die. To date, scientists have found no eels that have successfully spawned and returned to freshwater. (Langlois, 2017)

Common name: American Eel
Other names: Anguille, black eel, bronze eel, glass eel, green eel, river eel, silver eel, yellow eel
Description: American Eels are a type of fish that are characterized by a slender, snake-like body with a long dorsal fin originating at the middle of the back and a similar ventral fin. Small pectoral fins can be located behind the head and gill covers. American eels are covered in a mucous coating that serves as protection and to prevent desiccation. Variation in color exists based on the age of the eel, with adults being olive green and brown to greenish-yellow, with a light gray or white belly. Adults can grow to approximately 4 feet in length and weigh an average of 17 pounds.
Habitat: Typically bottom dwellers, eels can be found in a variety of habitats depending on stage of life including freshwater streams, rivers, and lakes or oceanic waters, coastal bays, and estuaries. Eels will borrow and hide amongst sediment, tubes, plants, and other types of shelters.
Diet: Nocturnal Carnivores. Eels are opportunistic hunters and will eat anything living within the same habitat

Life Cycle

Freshwater eels are the only catadromous fish in North America. This means they spawn in salt water but live as adults in fresh water. (Langlois, 2017)

In late spring to late fall sexually mature eels will embark on a 5,500 kilometer journey to the Sargasso Sea in order to spawn. Spawning occurs from late winter to early spring, with females laying on average 0.5-4 million eggs. Males will fertilize the eggs after they are laid, and only a small percentage will survive to the larva stage. (Langlois, 2017)

Larva, leptocephalus, will hatch about a week after male fertilization. These larvae are transparent and leaf-like in shape. The larvae will passively drift from the Sargasso Sea to the Gulf Stream. They will spend the next 7-12 months drifting in the current until they mature into glass eels (55-65 mm in length) and reach the shallow waters of the continental shelf. Glass eels resemble adults in appearance, but are still transparent. (Langlois, 2017)

As glass eels move closer to the shore line, they will gradually become darker in color and will transition from the water column to the bottom substrate. This transition marks the change from glass eel to elver. (Langlois, 2017)

For 3 months to a few years, elvers will migrate closer to shore and up river tributaries (see map on page 4). When the elvers reach their destination they become immature eels, or yellow eels. Yellow eels that remain in salt water or brackish water will progress through the life cycle quickly (about 9 years). Those that migrate further will change slowly taking up to 50 years to progress to adulthood. The majority of growth for American Eels occurs during the elver stage. (Langlois, 2017)

Adult eels, also known as silver eels, are those that have changed color once more and are now sexually mature. Females will be larger than their male counterparts and will be lighter-colored. They will then begin their journey back to the Sargasso Sea where they will spawn and die. To date, scientists have found no eels that have successfully spawned and returned to freshwater. (Langlois, 2017)

Loading...

Loading...

Map of American Eel Migratory Patterns (Langlois, 2017)

Loading...

Once placing in the top three of the North East’s commercial fish harvest, the American Eel is considered a delicacy. However loss of habitat and over-fishing has led to sharp declines in population numbers (Ontario Ministry of Natural Resources, 2010).

American eel fishing and commercial harvest lacked regulation resulting in dramatic over-fishing and sharp declines in population. In the late 1900s Maine, New Hampshire, and Massachusetts alone harvested approximately 80,000 kg yearly. While our Canadian counterparts on Lake Ontario would harvest approximately 223,000 kg. The results of this overfishing is represented by the decline of juvenile eels found in a population study of Lake Ontario. Numbers decreased from 935,000 eels in 1985 to only 8,000 by 1993, with populations nearing zero by 2001 (Ontario Ministry of Natural Resources, 2010).

Unfortunately, the commercial fishing industry wasn’t the only impact on eel population numbers. Construction of dams, turbines, and other irrigation facilities dramatically decreased habitat availability and impacted the migratory routes of the American Eel. Some areas that were once home to large populations of eels have seen a 98.8% decrease in population following the installation of a dam. In addition, eels are very sensitive to low dissolved oxygen levels which are common below dams (Ontario Ministry of Natural Resources, 2010).

Loading...

History

Loading...

Loading...

Once placing in the top three of the North East’s commercial fish harvest, the American Eel is considered a delicacy. However loss of habitat and over-fishing has led to sharp declines in population numbers (Ontario Ministry of Natural Resources, 2010).

American eel fishing and commercial harvest lacked regulation resulting in dramatic over-fishing and sharp declines in population. In the late 1900s Maine, New Hampshire, and Massachusetts alone harvested approximately 80,000 kg yearly. While our Canadian counterparts on Lake Ontario would harvest approximately 223,000 kg. The results of this overfishing is represented by the decline of juvenile eels found in a population study of Lake Ontario. Numbers decreased from 935,000 eels in 1985 to only 8,000 by 1993, with populations nearing zero by 2001 (Ontario Ministry of Natural Resources, 2010).

Unfortunately, the commercial fishing industry wasn’t the only impact on eel population numbers. Construction of dams, turbines, and other irrigation facilities dramatically decreased habitat availability and impacted the migratory routes of the American Eel. Some areas that were once home to large populations of eels have seen a 98.8% decrease in population following the installation of a dam. In addition, eels are very sensitive to low dissolved oxygen levels which are common below dams (Ontario Ministry of Natural Resources, 2010).

Conservation

The American Eel is considered and indicator species, meaning the health of its population is an indicator of the overall health and integrity of an ecosystem. Thus passageways and lifts have been installed in various waterways to allow migratory fish, including eels, a way around dams and other obstructions. The eels provided for the Eels in the Classroom, come from a similar set up and collection basket on the western shore of the Susquehanna river immediately below the Conowingo Dam in Maryland. In 2021, 537,182 eels were collected and transported upstream (Maryland Department of Natural Resources, 2021).

Prepping for the Arrival of Your Classroom Eels

Items to Purchase and Obtain
1. Tank
2. Filter and cartridges
3. Aerator (remember eels are very sensitive to low dissolve oxygen levels)
4. Habitat materials-substrate, plants, rocks. Try to mimic that of a local tributary.
5. A syphon or hose for water changes
6. Water treatment and dechlorinator
7. Full spectrum light (needed for planted aquariums)
8. Screen or flat glass top to prevent eels from escaping

**Tanks should be set up at least 1 month prior to receiving your eels in order to establish a health biome and denitrifying bacteria.

Feeding Your Eels

They will eat ANYTHING that’s alive in your tank (except plants). I have had success with macro-invertebrates, worms, maggots, shiners, brine shrimp, meal worms, and minnows. Freeze-dried blood worms can also be used if you do not have access to live food. **avoid goldfish and other flashy animals, as this will create lazy hunters that will not be well prepared for release.

You might even find they end up eating each other…unfortunately American Eels can and do display cannibalism

Eels are territorial. Try to provide shelter throughout your tank, plants can be used to provide additional shelter within the water column.