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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.
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.
The life cycle of an American eel
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.
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.
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.
Properly designed fishways can allow eel migration. However, salmon or shad fishways typically are designed for strong adult fish that can swim very fast and ascend small drops. These fishways do not work for small juvenile eels that are weak swimmers and cannot leap. Water velocities in excess of their swimming speed may block migration, but eels are good climbers and may ascend vertical surfaces provided there is a wet, rough substrate for them to climb on.
A common fishway design that is specifically intended for elvers is a ramp with baffles and a climbing material, such as artificial mesh or bottlebrushes. The ramp can be installed on the face of, or adjacent to, a dam. Some eel “ladders” actually trap the elvers in a container that volunteers carry above the dam and release. Very few elver ladders have been installed on Gulf of Maine's rivers, but this could be an effective way of restoring eels and involving citizens in restoration projects.
Culverts are an important impediment to juvenile eels because they concentrate flow, create high water velocities that may exceed the swimming speed of eels and usually have a smooth surface that does not provide a flow refuge. Some culverts are elevated at one or both ends, and drops of only a few centimeters may be enough to block juvenile eels. Problem culverts should be replaced with adequately sized culverts with natural bottom habitat and hydraulic conditions that do not restrict eel movement.
Providing safe passage for ocean-bound eels is a different challenge than getting juveniles upstream. Dams do not necessarily hinder downstream migration, especially low-head dams where water flows over the top or through open gates. The main problem is hydroelectric dams where water passes through turbines. The most effective way of getting eels past a hydroelectric dam is to turn off the turbines or increase spill over a dam. Some hydropower companies have license articles that require them to turn off turbines at night during peak migration. Another effective way to get eels downstream is to remove dams altogether. Dam removal reconnects fragmented river systems, restores habitat for migratory and resident fish, restores natural flow regimes and may improve water quality.
The American eel may lack the charisma of other species whose habitat spans the ocean, tidewaters and headwaters of the Gulf of Maine watershed. We embrace Atlantic salmon, shad and sturgeon, but the American eel - snake-like and active on the darkest nights - receives a more reluctant welcome. Yet eels are one of the most interesting and poorly understood fish species in our region. They will test our ability and resolve to reconnect fragmented ecosystems for the benefit of all species, and to think locally and globally to protect our natural resources.
Ethan Nedeau is a science translator for the Gulf of Maine Council. He can be reached at firstname.lastname@example.org.