Love on Rogers Lake: A Tale of Two Alewives
Posted by Richard Conniff on April 21, 2014
We would see amazing things if we could learn to be travelers in our own neighborhoods, Henry David Thoreau once suggested. Lately, I have come to think that this is more true than I had imagined.
Most mornings in warm weather, when I am home in coastal Connecticut, I head out before dawn to row on a 260-acre dogleg of a lake, backed up behind a rickety old dam. And I see plenty of wonderful things as I do my laps: An osprey cruising with a freshly-caught fish carried underneath, like a seaplane pontoon. A kingfisher looping along the shoreline. A newly emerged damselfly riding on my deck for a lap-and-a-half till its wings harden enough for flight. And once, at a distance of 50 feet, a bald eagle scavenging the carcass of a cormorant. But I did not realize until recently that a grand evolutionary experiment was taking place beneath my hull.
Along with other members of my rowing club, The Blood Street Sculls, I spent an inordinate amount of time last year moaning about a project to rebuild the dam where Rogers Lake in Old Lyme, Conn., spills down to become Mill Brook, on route to Long Island Sound and the sea. Construction required dropping the lake level by more than two feet, and that increased the risk for rowers of tearing off a skeg, or ripping out the bottom of a boat, or just spilling ignominiously while running across an unexpectedly low patch.
Now, though, the dam is finished, and starting this month, alewives, also known as river herring, are climbing the new fish ladder there and returning to Rogers Lake from their feeding grounds at sea. The work is part of a coast-wide effort to remove dams, build fish ladders, and improve habitat in the hope of returning the river herring to their former glory.
Alewives are anadromous fish: Born in freshwater, they spend their lives in the ocean, returning annually to their birthplaces to spawn. Until colonial era dams cut off the migration, hundreds of thousands of alewives would have come pouring into Rogers Lake every spring—and into other lakes like it along much of the Atlantic seaboard. Farmers used to apply them to their fields as fertilizer, at a rate of up to 1400 fish per acre. In towns all along the coast, river herring festivals celebrated their arrival.
What’s particularly intriguing about Rogers Lake, though, is that the first dam built on Mill Brook in 1672 inadvertently drew a pre-Darwinian dividing line across a species. On one side, the anadromous alewives continued their old seagoing life. But on the other side, in Rogers Lake, other members of the same species became landlocked.
Over the past 17 years, local conservationists have added fish ladders to the dams lower down on Mill Brook, and about 10,000 anadromous alewives a year have been recovering at least a part of their old migration. But the new fish ladder means they can finally complete the journey–and come face to face with their landlocked cousins, after a separation of 342 years.
Evolution has wrought strange changes in the intervening 100 or so generations, most notably in size. The landlocked fish are now just a third the length of their foot-long seagoing cousins. Somewhere beneath the hull of my rowing shell, it’s as if mastiffs are returning to discover that their distant kinfolk, and potential mates, have become Chihuahuas.
At his laboratory at Yale University, ecologist David Post clambered over obstacles in what would be a walk-in freezer, if it weren’t so full, and eventually pulled out plastic bags containing frozen landlocked and anadromous alewives. “They look like something you’d get out of a pickled herring jar, because that’s basically what they are,” he said. “They’re not nearly as sexy as Atlantic salmon, or as funky as eels. They’re silvery and nondescript, and that’s both the joy and the curse of working with these fish.” (On the sexiness question, one theory holds that the name “alewife” derives from their big-bellied resemblance to a female tavern keeper.)
The joy, for an ecologist, is that alewives are a keystone species. As predators, said Post, they “drive the ecosystem” of every coastal lake and stream from the Carolinas to Maine—or at least the ones to which they can still gain access. As prey, alewives, together with menhaden, Atlantic herring, and other forage fish, are the basic food stock for the entire Atlantic fishery, as well as for seabirds, whales, dolphins, and other species.
Part of the curse is that even biologists have a hard time telling alewives from bluebacks, another species of river herring. At sea, alewives, bluebacks, and Atlantic herring all school together, and to fishermen trawling huge nets for the Atlantic herring, they look the same. As a result, an estimated 3.8 million river herring disappear every year as bycatch, and that may be an important reason the overall river herring population has plummeted by more than half since the 1960s.
Post has been studying alewives in Connecticut lakes for the past decade and now has a National Science Foundation grant to track the outcome of the reunion at Rogers Lake. Before the dams cut them off, he said, the alewives arrived each spring as “slash and burn” predators on the planktonic life in the lake. They picked off everything they could find—mostly fly larvae and feathery little crustaceans like copepods and water fleas—and then headed back out again, leaving what was left of the plankton to recover for another harvest the following spring. “It’s a pretty good lifestyle, if you’re anadromous,” said Post. “You eat everything in sight and go elsewhere.”
But once the alewives became landlocked, they had to get by on the smaller plankton left behind, leading to an almost complete disappearance of large plankton. To human eyes, this effect is almost invisible: Post held up a small jar of water containing thousands of larger plankton from a lake without landlocked alewives, and it looked like a faintly yellow broth. A jar from Rogers Lake, on the other hand, could have passed for clear water. The change in the prey base forced the alewives not just to become significantly smaller but also to develop more closely-spaced gill rakers, apparently so they could strain food from the water, rather than picking it off one particle at a time.
So what will happen when the two forms of alewife come together? The difference in size may not matter for mating, since alewives don’t practice internal fertilization. Instead, they broadcast sperm and eggs into the water simultaneously. But the anadromous alewives begin breeding in April, several weeks ahead of their landlocked cousins. So the two forms may just pass each other with a glance, curious but puzzled.
Or there may just be enough of an overlap for romance. If so, what happens to the young of the year? Will they turn seaward at the end of the summer, as did their anadromous forebears more or less forever? Or will the landlocked lifestyle keep its hold on them?
It is an evolutionary family drama, and it makes me think that Thoreau was right: With every stroke of my oars, I am skimming across the surface of unsuspected mysteries.