Eels: An Exploration, from New Zealand to the Sargasso, of the World's Most Mysterious Fish - James Prosek (2010)

Chapter 6. Into the Ocean

The Sargasso Sea

James McCleave has made more expeditions to the Sargasso Sea to look for spawning eels than anyone else alive. Friedrich Tesch, the German eel biologist, and author of Der Aal, now deceased, took McCleave on his first oceanographic cruise in 1974 in the Bay of Biscay. Jim’s first Sargasso trip was in 1981, followed by two in 1983, two in 1984, one in 1985, and one in 1989. I met with him at the Inland Fisheries Office in Bangor, not far from where he taught fisheries oceanography at the University of Maine.

McCleave is soft-spoken and modest about his eel work. He is one of the world’s top eel scientists, along with Katsumi Tsukamoto of Japan, Don Jellyman of New Zealand, Håkan Wickström of Sweden, Willem Dekker of Holland, and John Casselman of Canada. McCleave and I sat in a classroom around a large table.

“I was looking through a list of my publications over the last thirty years,” McCleave laughed, his sea-blue eyes squinting behind his glasses, “and most of them are on eels.”

McCleave theorizes that migrating adult eels identify the spawning grounds not by a geographical feature on the bottom, such as a seamount, but by something more subtle—frontal regions, where two different water masses come together at an area of temperature shift. Out in the middle of the Sargasso, water from the north is being driven south and water from the south is being driven north by trade winds, creating many such fronts, sometimes made visible on the surface by big lines of sargassum weed. It is in these areas where Jim and his colleagues have caught the smallest larvae, those most recently hatched, and therefore closest to the adults.

They cruised back and forth across these frontal regions with acoustic gear, hoping to find echoes that would have been of the right strength to come from an aggregation of spawning eels (sonar detects fish underwater by echoing off the air in their swim bladders).

“We occasionally found echoes that we thought might have been groups of eels,” Jim said, “but by the time we got the ship turned around and dropped the nets, we were off the mark and never caught anything.”

I had a lot of questions for Jim. Where in the water column did the eels reproduce? Were large pelagic fishes out there trying to eat all these eels meeting to lay their eggs? Why did eels go so far to spawn?

Jim smiled politely.

“Dunno,” he laughed, and then gave me his scientific best.

“Eels have been around for about two hundred million years,” he began, “have survived all kinds of climatic changes, including many ice ages, and have been influenced in geographic distribution and speciation by continental drift. The spawning areas have certainly moved and changed over the millennia, so it’s possible that at one time the eels’ spawning area may have been closer to the coast.”^* He reasoned that the American and European eels continued to spawn out in that place, the Sargasso, because it was the only place—with the right temperature and salinity—where the larvae can be reared. “Not all species of Anguilla make such long migrations,” Jim said, “but all the species that we know anything about spawn in warm, salty water out over deep water.”

I asked Jim a series of further questions about eels. After each of them he folded his hands in thought. He twisted his head and then said in his self-effacing way, “We don’t know.” Then he would amplify: “We thought that …” or “We tried to …” After years of studying this fascinating fish, not only in the ocean but in their freshwater habitats, Jim acknowledged that in the end maybe it was not so important to know where they went to spawn.

“Part of it,” he said, “is everyone wanting to solve the mystery. There’s a fellow named Willem Dekker in the Netherlands. He’s the chairman of the International Council for the Exploration of the Sea working group on eels. He’s done all kinds of population dynamics studies on eels and so on. He’s never worked in the ocean, but to him the most burning question is to find the spawning adult European eel in the Sargasso Sea. And I keep saying, ‘Willem, you know, what difference does it make if you actually catch one, ‘cause we know the little larvae are there? ‘ ‘Ah,’ he says, ‘because we haven’t completed the life cycle.’ ”

But, Jim added, “if we knew where the spawning area was, then maybe we could find out why eel populations are in such severe decline. The declines might be due to some feature in the ocean that is affecting larval transport or larval survival, as opposed to things affecting eel populations on land, like dams and overfishing. But without knowing what’s going on in the ocean, it’s hard to say.”

Later that afternoon I drove to the Department of Marine Resources to interview Jim’s colleague Gail Wippelhauser, who had accompanied Jim on several expeditions to the Sargasso. The expeditions usually left in early February from Miami and involved weeks of towing fine-mesh nets for larvae. She described the Sargasso as a place of “huge thermal gradients and mesocurrents.”

Gail spoke of the history of Sargasso expeditions, starting with Johannes Schmidt. After decades of larvae collection, Schmidt concluded in his papers that both American and European eels spawn in a single place, the Sargasso Sea. But, as McCleave points out in his contribution to the book Eel Biology, Schmidt’s case rested on “a modest amount of published data.” The reason the Sargasso proposition has stuck so well is not because it is irrefutable but because Schmidt “presented the case so forcefully that his ideas were largely accepted and still are.”^*

McCleave and others strongly assert that Schmidt’s data were not sufficient to support his grand concluding statement in 1935: “In the Sargasso Sea, the newly hatched larvae of our eels were found, and with the aid of numerous sections of fishing experiments we had been able to determine their distribution and settle conclusively that they are found in an area to the north-east of St. Thomas and south-east of the Bermudas and nowhere else.”

The problem with the phrase “and nowhere else” was that Schmidt had barely taken any samples for eel larvae outside of his purported spawning area (in particular south of 20 degrees north latitude). How could he say for sure that the freshwater eel was not spawning elsewhere when he hadn’t really looked?

Earlier in his life, in 1922, a more modest Schmidt, younger and presumably less focused on securing his legacy, wrote: “I perceived that if the problem were to be solved in anything like a satisfactory manner it would be necessary to ascertain, not only where the youngest larvae were to be found, but also where they were not. Until a comprehensive survey had been obtained as to the distribution and respective density of the various sizes of larvae in all parts of the sea, it would hardly be possible to form definite conclusions as to the origins of the eels of our European continent.”

After Schmidt, the Sargasso Sea was free of eel researchers until 1979, when Friedrich Tesch made his first trip there. This was followed by the various surveys of McCleave from 1981 to 1989, but all these were conducted in the known spawning area, where the anguillid larvae were abundant. And still today, most scientists who are enmeshed in the mystery will tell you that a comprehensive survey of the ocean for eel larvae in other regions outside the known spawning area has not yet been made. The only true answer to the question of whether there is one single or several separate or overlapping spawning areas for American and European eels in the Atlantic is “Dunno.”

Gail said that she and Jim used to get together over a few beers and come up with harebrained ideas about how to find spawning eels. “We thought of attaching balloons to migrating eels that would inflate after a certain amount of time and the eel would pop to the surface.”

Gail described one method they employed in their search for adult eels. “We brought artificially matured female eels and injected them with hormones to make them reproductively ripe. They put these big females out there in cages as decoys on buoys hoping they’d attract males. We lost sight of them. Things disappear very quickly in the ocean.” Gail couldn’t hold back. “I was so mad because I injected all those eels. Jim and I made trips to the Darling marine lab, alternating three times a week, a two-and-a-half-to-three-hour drive, to inject hormones to induce sexual maturation. We had a hundred artificially matured females to take out to the Sargasso Sea and lower in cages attached to buoys to attract males. By the time we got to Miami to catch the ship, most of them had died. And by the time we reached the Sargasso, we had only five. We watched the buoys on the radar. Jim and I took shifts following them. And they disappeared. God, was I angry! All that driving.”

She laughed, then continued talking about a particular trip to the Sargasso with Steve Brandt, a hydroacoustics expert. “We saw one big aggregation on sonar that we thought might have been the eels. We had five or six nets and never saw one.”

Gail spoke of the adult eels returning to the place where they were born, spawning and dying, and then the newly hatched larvae drifting toward the coast from the spawning area. “The currents are so strong, the larvae have no control. They get stuck in the Gulf Stream.” Gail offered no explanation as to how the multitudes of larvae dispense themselves to occupy freshwater habitat throughout the extensive range. How did some know to swim up the Mississippi and others the Delaware, the Hudson, or the St. Lawrence? I asked if it was possible for young eels to return to the same rivers, or at least the same general area, that their parents had migrated from.^* With no uncertainty at all in her voice Gail said, “No. Young eels don’t home to particular rivers. The adults home to where they were born, just like salmon do. Except with eels it happens that their birthplace is an almost featureless place in the open ocean.”^**

Gail said that if she and Jim went back to try to catch adult eels in the Sargasso Sea, they’d bring commercial fishing gear. “We always thought the navy knew where they went,” she added. “It’s really deep near the Puerto Rican trench. They must test their subs down there. Who has all the latest and greatest toys but the navy?”

Gail and Jim agree that it is just a matter of time before the mystery of the eels’ spawning place is unraveled. McCleave writes in Eel Biology: “At some time, smaller telemetering devices will allow direct determination of locations and times of spawning of European and American eels migrating from different parts of their continental ranges.”

If I live to read that headline in the news, it will be a bittersweet day.

^* For instance, when the North American and European continents were closer (and the ocean between them narrower), the eels would not have had to travel so far. But as the plates moved apart and the distance widened, the eels would have had to travel further and further to get to the same general area where optimal spawning conditions exist.

^* K. Aida, K. Tsukamoko, and K. Yamauchi, Eel Biology (Tokyo: SpringerVerlag, 2003).

^* The panmixia hypothesis-that all eels migrate to the Sargasso Sea and randomly mate with each other-was challenged in a paper published in the journal Nature in February 2001, “Genetic Evidence Against Panmixia in the European Eel.” The authors, Thierry Wirth and Louis Bernatchez, posited that the northern (Baltic Sea) and southern (Mediterranean Sea) populations of European eels are two genetically distinct and therefore reproductively isolated groups. Which meant that when the southern population migrated to the ocean to spawn, they did not mix with the northern population, and that the offspring of each group returned to the same general areas from which their parents migrated. This theory has since been proven wrong. It does in fact appear that the entire European population spawns together and that the offspring are randomly dispersed to freshwater rivers throughout the range.

^** The eels’ reproductive strategy is very different from other migratory fish that move between freshwater and saltwater. Salmon, because they spawn in rivers and migrate home to the rivers where they were born, have been repro- ductively isolated in separate river drainages for thousands of years and have evolved into slightly different varieties. Though not significant enough genetically to be considered separate species, each river has a unique population. (In terms of conservation of the species this must be considered. You can’t, for instance, introduce Atlantic Salmon from Canada to replace the extinct popula- tion from the Connecticut River in southern New England and expect them to thrive.) Eels, on the other hand, mix their entire adult population every year reproducing together in the Atlantic Ocean, so the species as a whole is more homogeneous.