LDWF Biologists Seek To Learn More About The American Eel’s Life Cycle In Louisiana
story by ROBBY MAXWELL and TREY ILES, LDWF
It may look strange to most folks and it certainly won’t win any beauty contests. Some people may find the species unsettling because of its snake-like appearance. But, looks aside, you would be hard pressed to find a fish as elusive, unique and mysterious as the American eel (Anguilla rostrata).
The distance this fish travels to complete its life cycle is hard to imagine. The American eel’s journey begins and ends in the Sargasso Sea, which is a 2 million square mile area of the North Atlantic between the West Indies and the Azores.
Think of the life cycle of the salmon, only in reverse. Salmon grow and mature in the ocean, swim inland up rivers, spawn then die. This cycle is called anadromy and fish that follow this pattern are described as anadromous.
The American eel, however, grows and matures in rivers, swims downriver and back to the Sargasso Sea where it spawns then dies. This cycle is called catadromy and fish that follow this pattern are described as catadromous. The American eel has the most extreme migration of any catadromous fish in North America.
Unlike many species, the American eel is only known to have a single breeding population because of the convergence of the fish in one place to spawn. Therefore, the eels found in Louisiana are from the same breeding population as those found in Central and South America, the east coast of the United States and in Greenland. There are theories of less significant spawning grounds in the Gulf of Mexico and the Caribbean but no proof has been found to support them.
The American eel is thought to spawn in deep water but they’ve never been recorded spawning and their eggs have never been observed in their spawning grounds. It is known, however, that the females release 20-30 million eggs during the spawn. These eggs rise to the surface and hatch into larvae, which drift along the various currents of the Atlantic Ocean. The larvae, which look like small knife blades or willow leaves, are known as leptocephali.
From their spawning grounds, it takes a year or more for the larval eels to make their way to the U.S. coast. Some eel migrations in other parts of the world are reported to be even longer, up to three years. Once they get close to the coast, they develop into what are known as glass eels, which are 2-3 inches long and look like pieces of clear spaghetti. It is at this stage that eels are first found in coastal bays and rivers.
As they continue to grow, they transform into the stage where they are called elvers, when they begin to develop green-brown to gray pigmentation. Then it’s on to the yellow eel stage, the final stage before adult maturation.
By this point they’ve found a temporary home in nearshore marine waters, lakes, ponds, rivers or streams. It takes about a year for them to reach the coastal areas of the United States and Canada.
In the southeastern United States, studies show most yellow eels begin to sexually mature between 5-12 years but maturation can occur anywhere between 3-40 years as they remain in the freshwater environment.
When American eels reach sexual maturity, they become known as silver eels and begin their journey back to the Sargasso Sea to spawn and die. Another impressive attribute during this final migration back to the Sargasso Sea is the transformation from a bottom dwelling freshwater fish to an ocean going fish. Adult females often grow to sizes more than three feet long but can be up to 5 feet in length and weigh four to five pounds.
For as big a fish as they are, and as common as they are in the Bayou State, not much is known about their life history in Louisiana.
That’s why the Louisiana Department of Wildlife and Fisheries (LDWF) has begun to study the species more closely. Thanks to a State Wildlife Grant received last year by LDWF inland fisheries biologists in Lake Charles, and with the help from other fisheries biologists throughout the state and across the country, the department is working to better understand a fish that is currently listed as a species of greatest conservation need as defined in LDWF’s State Wildlife Action Plan.
THE LOUISIANA MIGRATION
Eels can be very difficult and time consuming to catch, as LDWF biologists have found while attempting to trap them in the state. Even though it’s the same species of fish across its range, methods that are used in Delaware or South Carolina may not work in Louisiana or the next state over or even the next river or waterbody over.
LDWF is using a model found to be successful in Florida. Eels are being collected as bycatch that is harvested statewide while conducting routine electrofishing or netting methods. The eels are frozen and delivered to those biologists studying the species.
The objectives in studying the American eel are to determine age and size structure, sex ratio and habitat associations and other population characteristics in state waterways. The secondary objective is determining how to catch them.
LDWF is working to gather baseline and preliminary information on the American eel in Louisiana to determine how best to manage and preserve the species in state waters and to better understand how it spends its days here. The data gathered within the state will also help to inform other American eel researchers across its range as earlier studies have helped to supplement LDWF research.
There is no robust commercial market for the species currently in Louisiana. It is possible that this could be an issue in the future because of the demand in other parts of the world. In fact, there is concern, supported by decades of data, that American eel numbers are declining along the Eastern Atlantic Seaboard.
The American eel is considered a delicacy in Asia and Europe with young eels, known as glass eels, fetching from $1,000-$2,000 per pound. The glass eels are generally shipped to rearing ponds in China where they are grown to marketable size. Fishery managers have determined that overharvesting the glass eels can have a detrimental effect on the eel population and such harvest has been outlawed in most states.
As with many species, habitat quality is a problem. Dams and other obstructions in rivers and streams block the American eels’ migration corridors and patterns. This may be the case in some parts of Louisiana though it’s not as big of an issue as it is in some other states.
Knowing that dams inhibit their movement, eel ramps have been developed and used in the Sabine River to study the feasibility of assisting their upstream movement. Changes in salinity and marsh loss also mean a change in eel habitat.
The American eel can absorb oxygen through its skin and gills and can travel over land in some cases, typically over wet grass or mud.
An Asian parasite that affects the American eels’ swim bladder is taking its toll in parts of its range as it affects their ability to remain buoyant and swim vertically in the water column during their migration to spawning grounds. The good news in Louisiana is that the parasite has not been found here.
It’s true that the American eel doesn’t have the charismatic appeal of the alligator, Louisiana black bear or eastern brown pelican. Nevertheless, it is vital to learn about its time in the state during its life cycle.
It’s a species that other animals enjoy making a meal of, from fish and birds to humans, and is a vital part of the ecosystem.
Eels are an indicator of river connectivity as they depend on good water flow and free passage to move up and down the rivers to complete their various life stages. Other migratory fish and arthropods like shrimp are not able migrate up and down a river or estuary system if there is an impediment to their passage. So the presence of eels is a good sign that these other species can complete their life cycles.
American eels have economic and ecological importance from the very local level all the way up to the global scale. LDWF is working to ensure that it gains an understanding of how to manage a sustainable population of this fascinating and crucial resource for generations to come.
For more information on the American eel, contact Robby Maxwell at firstname.lastname@example.org or 337-491-2587.
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