Where have all the eels gone?

By Ethan Nedeau

EVERY AUTUMN, American eels (Anguilla rostrata) descend the Gulf of Maine's rivers on a journey toward the warm waters of the Sargasso Sea to spawn and die, returning to a birthplace they left as many as 30 years earlier. We do not know their path nor do we understand how they navigate vast distances. The moon, stars, magnetism and an exceptional homing ability may guide them through the dark waters.

Eels have not revealed their spawning secrets; scientists do not know exactly where eels spawn, at what depth or how they behave when reunited at the spawning area. All eels from throughout their range, Greenland to Central America, converge on the spawning waters at about the same time. This is remarkable considering the vastly different distances they must travel. Northern fish leave much sooner to arrive “on time.” Despite the enormous geographic range of the species, all American eels comprise a single breeding population.

After spawning is completed and eggs hatch, billions of tiny larvae become entrained in the Gulf Stream, Florida Current and Antilles Current. The larvae, called leptocephali, are transparent and shaped like a willow leaf. They can orient themselves and change positions in the water column, but ocean currents dictate their path toward the continent. Some may get pushed southwest into the Gulf of Mexico and wander into estuaries of the Gulf Coast or Central America. Others will be whisked northward by the strong Gulf Stream, spin off into the Labrador Current and turn south toward Newfoundland and the Gulf of St. Lawrence, or north to southern Greenland. Every eel must be prepared to go where currents take it to warm tropical rivers, icy northern waters or anywhere in between.

Leptocephali metamorphose into a more recognizably eel-like juvenile form called glass eels. They are so named because they are transparent and lack pigmentation. Glass eels are stronger swimmers and as currents take them close to the continental shelf and coastal waters, they will swim toward the coast and seek out estuaries. Once in estuaries, they become pigmented and are known as elvers; by this time they are 60 to 130 millimeters (2.5 to 5 inches) long. They swim up tidal rivers during flood tides and retreat to the bottom as tides ebb. They forge upstream with each tidal cycle and gain strength until they can swim against strong currents. Elvers grow into yellow eels, the fourth stage of life.

Over the next few years they may disperse within a watershed, often traveling hundreds of miles in stops and starts, depending on migration barriers and habitat along the way. Some will remain in estuaries and tidal freshwater habitats; these tend to be male fish and may only stay for a few years before returning to the sea. Others have wanderlust and may spend the next 30 years in a watershed - ascending rivers, crossing lakes and pushing toward headwaters. Eels that swim far inland - especially in the north - are mostly female and may grow to over four feet long, far larger than males. Eels toward the northern end of the range are the oldest and largest of their kind and females may produce two or three times more eggs than females from the mid-Atlantic region. Eels prey on or scavenge aquatic invertebrates, amphibians and fish. In turn, large predators such as bass, lake trout, fish-eating birds and mammals may eat them.

When the Sargasso finally calls them back, yellow eels turn to a blackish-bronze color, their eyes enlarge, they fatten and develop a thicker skin and their digestive tract degenerates. They are then silver eels - the fifth and last stage of their lives. Eels are negatively phototactic - they avoid light - and usually wait for the darkest nights to descend rivers. On nights when clouds shroud the moon and rivers swell from autumn rains, silver eels finally begin to swim toward the sea. Then they swim deep below the ocean's surface where light does not penetrate.

Concern about eels

Eels may be declining throughout their range, but most scientists agree that we need much better data. Since all eels come from a single breeding population and disperse somewhat randomly to coastal watersheds, declines in some areas are thought to reflect a range-wide decline. In a 1994 review paper published in the Canadian Journal of Fisheries and Aquatic Sciences, scientists reported an 81-fold decline in yellow eel recruitment to Lake Ontario over an eight-year span, based on the number of fish passing an eel ladder at the R.H. Saunders Hydroelectric Dam on the St. Lawrence River. Research published in Fisheries magazine in 2000, found that seven of 16 long-term datasets reviewed showed a significant decrease in yellow eels and silver eels, and the other datasets showed no trend. In the United States, commercial landings for eels declined from a high of 1.8 million pounds in 1985 to a low of 649,000 pounds in 2002, according to a study released in 2004 by the Atlantic States Marine Fisheries Commission (ASMFC).

It is difficult to demonstrate a significant decline in the eel population. Scientists can count silver eels leaving a river and estimate the number of elvers that return, but know almost nothing about the steps in between. Eel recruitment (the number of juvenile eels returning to estuaries and rivers) can be cyclic and erratic and scientists do not understand long-term population dynamics well enough to put recent declines into perspective. Part of the problem is that, until recently, recruitment was not being measured at all in some areas. Commercial landings provided some evidence of a decline, but these data depend on market conditions and fishing intensity and may not necessarily reflect a declining population. Eels were just not on the radar screen of most fisheries management agencies until very recently, so agencies are now scrambling to develop more consistent monitoring programs for eels. ASMFC is spearheading this effort.

In 2004, due to continued population declines, the ASMFC recommended that the U.S. Fish and Wildlife Service (USFWS) and National Marine Fisheries Service consider designating the Atlantic coastal stock of American eels a species of concern. This action would prompt a status review to determine whether the species should be considered a candidate for federal listing under the Endangered Species Act.

Eels may indeed be experiencing a drastic decline, as some evidence suggests. The perceived eel decline may be due to natural cycles or influenced by factors far beyond the scope of local or regional management agencies. For example, scientists have demonstrated a strong correlation between European eel (Anguilla anguilla) recruitment and the North Atlantic Oscillation, sea-surface temperature anomalies and position of the Gulf Stream. If these hemisphere-scale factors strongly influence American eel recruitment, then we may need more realistic expectations about what local protection and restoration can accomplish.

However, there are clear indications of how we affect eels and how we could reduce mortality, giving us some sense of control. Juvenile eels congregate at the foot of dams; we have severed their paths between the ocean and watersheds. Killed, maimed and stunned eels swirl in eddies below hydropower dams after unwittingly passing through the only downstream path we have provided, through the whirring blades of turbines. Adult eels from many watersheds have cancers, vertebral malformations and high levels of chemical contaminants accumulated during their long residence in polluted waters. Commercial weirs, nets and traps capture elvers trying to swim upstream, yellow eels while in freshwater, and silver eels trying to return to the ocean. In some cases, most or all of the eels that enter a river may never return to the Sargasso Sea to spawn.

Restoring eels

There are ways to protect and restore eels in a given river, but unfortunately, it might make little difference to the population as a whole. Restoring eels in a particular river does not ensure that elvers will return in ensuing years. This is the challenge of trying to manage a species with an enormous geographic range that randomly breeds in an unknown location in the middle of the ocean. Unlike salmon that return to the rivers where they were hatched, young eels do not return to the same rivers where their parents lived. We may be able to restore eels in the Ipswich River in Massachusetts, but how much does this river contribute to the entire spawning stock of the species?

Eels need access to more habitat, and though rivers like the Merrimack, Penobscot and Saint John may provide much more habitat than smaller rivers, all rivers are important. A 1998 USFWS report estimated that dams reduced potential eel habitat by 91 percent from Connecticut to Maine. The American eel population, wide-ranging and previously thought to be robust against local losses, may not be replacing itself. Every river is important because the collective effort to restore rivers represents a culture of environmental concern and because every river restored does add up to more eel habitat.

Restoration Options

One of the best things we can do for eels is to allow them unrestricted access to coastal watersheds and safe passage back to the sea. Thus, we need to focus on man-made migration barriers, especially dams and culverts. With over 60 rivers, hundreds of small streams and perhaps more than 7,000 dams in the Gulf of Maine watershed, it is difficult to decide where to focus limited resources. Four factors may be important in selecting priorities:

1. Focus on hydropower dams that are up for relicensing, especially near the coast.

Hydropower dams are relicensed for 30 to 50 years; so in our lifetimes, we may get one chance to allow eels to get past a particular dam. The relicensing process is an ideal time to consider eel passage because of the intense scrutiny given to the economic and ecological costs and benefits of dam operations. Sometimes eel passage should be provided even when there are no such passageways further downstream or upstream - it just depends on which dams come up for relicensing first.

2. Focus on dams that restrict access to the largest amounts of high - quality habitat.

High-quality habitat for eels may include rivers, ponds and lakes with relatively undisturbed habitat, good water quality, few environmental threats and a good prey base. Low-gradient rivers that connect a series of lakes and wetlands are often very productive habitats.

3. Assess all migration barriers in a watershed and focus on dams that have few or no barriers further upstream.

It makes sense to focus on the first impassable barrier that juvenile eels encounter after entering rivers, but the distance to the next barrier - and habitat quality in between - is also an important consideration. Some rivers have many dams and may require a comprehensive plan to address eel passage at all dams.

4. Evaluate upstream and downstream passage at each dam.

It would seem counterproductive to install elver ladders to get eels upstream if there is not a safe way for them to get back downstream. However, after eels pass a dam they may spend 10 to 30 plus years in the watershed, and allowing upstream passage now can essentially “bank” eels in the watershed with the hope that safe downstream passage will be provided by the time these fish mature.

The life cycle of an American eel

A. After spawning is completed and eggs hatch, billions of tiny larvae become entrained in the Gulf Stream, Florida Current and Antilles Current. The larvae, called leptocephali, are transparent and shaped like a willow leaf. Leptocephali metamorphose into a more recognizably eel-like juvenile form called glass eels.

B. The glass eels migrate into estuaries along the coast, where they become pigmented. These eels are known as elvers. Some elvers remain in the estuaries, but others migrate varying distances upstream, often for several hundred miles, depending on migration barriers and habitat along their way.

C. Now in their yellow eel phase, the American eels will remain in the brackish and fresh waters of the rivers for a few up to 30 years. Females reach a maximum length of five feet, and males grow as long as two feet.

D. Before beginning its life-ending migration back to the waters of the Sargasso Sea to spawn, yellow eels turn to a blackish-bronze color, their eyes enlarge, they fatten and develop a thicker skin and their digestive tract degenerates. They are then silver eels - the last stage of their lives.

Images © Ethan Nedeau

Ethan Nedeau is a science translator for the Gulf of Maine Council. He can be reached at ethannedeau@comcast.net.