Autumn meadowhawks — mid-size, red-bodied dragonflies — patrolled over sedges and grass on a recent mild autumn day, at noon at a freshwater wetland in West Tisbury. In drier areas, a few asters still bloomed. From a patch of open water, startled ducks — a pair of green-winged teal, then four gadwall — exploded into flight.
In short, it was a typical grassy wetland — except that prior to last April, this site had spent more than a half-century under water. The areas now vegetated were exposed only after boards blocking the concrete spillway of a broad earthen dam, the lowest dam on the Tiasquam River in West Tisbury, were removed to allow the passage of herring. A low bank and a ring of alder shrubs still mark the outline of the pond.
When the spillway was opened on April 17, the pond’s level dropped gradually over a period of hours, leaving an expanse of bare mud, recalled West Tisbury resident Prudy Burt, who helped pull the boards. Within days, the first shoots of grass and sedge seedlings had emerged from the fertile muck. With no impoundment blunting its energy, the Tiasquam rediscovered its old channel over the next two weeks, scouring through a few inches of sediment down to clean sand and gravel. By midsummer, the site was more marsh than pond, a mix of dense sedges, lazy backwaters, and a briskly flowing stream.
This experiment in stream management emerged from an unusual situation even farther downstream: A routine cut made in the barrier beach of the Tisbury Great Pond in December 2014 persisted far longer than such openings usually do (ultimately into August 2015), overlapping the period when alewives and blueback herring, after several years of growth and maturation at sea, return to coastal streams to spawn. Numbers of herring unprecedented in recent years entered Town Cove, swimming upstream until blocked by the lowest barriers on the Tiasquam and the Mill Brook, the two main tributaries of the Great Pond.
Once prolific herring runs on Martha’s Vineyard, and indeed in southern New England generally, have withered to a tiny fraction of their original size. Causes for this include overfishing, by-catch in the sea herring fishery, impaired water quality, and especially, the presence of dams that bar herring from spawning habitat. In response to the decline of river herring, which provide a critical link in the oceanic food chain, the state has prohibited their harvest and made their restoration a priority.
Schools of herring in Town Cove struck West Tisbury herring warden Johnny Hoy as an opportunity. Mr. Hoy asked the owner and the lessee of the lowest Tiasquam dam (The Trustees of Reservations and Eric and Molly Glasgow of Grey Barn Farm, respectively) for permission to open the spillway and allow herring to continue upstream. Both parties agreed; the state Division of Marine Fisheries, deeply committed to herring restoration, supported the plan; and the West Tisbury Conservation Commission, after reviewing the proposal, issued a letter allowing the dam to be opened.
The herring approved. The day following removal of the boards, recalled Mr. Hoy, alewives were moving up the stream above the drained pond. Hundreds arrived at the next obstacle upstream, the ancient dam at Look’s Pond, where author Geraldine Brooks was out gardening. “At first I thought they were an unfamiliar kind of river grass, their sinuous brown backs arabesquing under the water,” she recalled. “Then I caught the shimmer of their sides.”
This second dam proved impassable for the fish. “Birds feasted on more than a few,” Ms. Brooks reported. “I helped as many as I could [across the dam], lifting them over by hand, until my arms shimmered with scales like a mermaid.”
Like the relentless upstream push of the herring, the rapid transition of the former Farm Pond into marsh simply reflected a normal natural process. Wetlands of all kinds are dynamic habitats. Ecological niches appear and disappear with every change in water level, and changes in water level are nearly constant. Wetland species have evolved to tolerate or even exploit this volatility.
Many wetland plants produce buoyant seeds that can float far from their point of origin, germinating if they wash up in a suitable site. And the seeds of many wetland species are large and thick-shelled, qualities that let them remain viable for years in a pond bottom and then, when the water drops enough to expose the mud where a seed rests, promptly germinate. Drawing down the pond, then, created the opportunity that a host of seeds had been patiently waiting for.
Dams serve a range of purposes: controlling floods, providing power, creating impoundments for recreation or water supply or aesthetics. The costs of meeting these needs, however, have steadily accrued during the centuries since Europeans introduced water power to the Vineyard.
Dams interrupt the movement of fish, including migratory species such as herring that require access to upper reaches of streams in order to reproduce. Dams also tend to dramatically warm a stream, as the exposed surfaces of ponds heat up in the sun. The warmest, least dense water in the pond, at the top of the water column, is typically what goes over the spillway, heating the downstream reach. Cold-water stream specialists, like brook trout, often can’t tolerate the altered conditions, so dams favor generalist aquatic species (catfish, say, or bluegills) over more specialized, often less common fish.
Like many artificial ponds in our region, the Tiasquam Farm Pond, created in the 1960s, had outlived its usefulness. Recalling it humorously as a “hot cesspool,” leaseholder Molly Glasgow noted that modern concerns over microbial contamination prevented Grey Barn Farm from watering livestock directly from the pond. And the few attempts she and her husband Eric made to fish in the pond were, Ms. Glasgow recalls, “vegetarian — we caught nothing but leaves.” So opening the dam to allow fish passage and create a more diverse wetland made perfect sense to her when Mr. Hoy proposed it.
With a significant reach of the Tiasquam newly available to herring, there are plans afoot to install a fish ladder (“not ideal but better than nothing,” in the view of Mr. Hoy) at the Look’s Pond dam. The Farm Pond dam, meanwhile, seems to all parties like a good candidate for complete removal if funding can be found (and assuming a new approval from the Conservation Commission, which regulates changes to wetlands under the state’s Wetlands Protection Act). And it seems likely that the Tiasquam will be more systematically examined for its potential to produce migratory fish.
Following its unusually long opening to the sea, the Great Pond closed. Stream and groundwater flow has now raised its surface high enough into the cove to slow the flow of water through the breached dam. It appears that if the dam is left breached (or removed entirely), the site will be a variable system, wetter or even partially flooded when the Great Pond is at its highest, drier when an opening in the barrier beach drops the water levels downstream.
A mix of wetland plants, probably dominated by grasses and sedges but always subtly changing, will gradually establish itself, featuring species that either tolerate extremes of wetness or can respond quickly when water levels change. In short, the ex-pond will revert to the type of cove-head wetland it was before the dam was built.
Re-establishing a substantial herring run in the Tiasquam is by no means a done deal. Openings of the Tisbury Great Pond would need to be timed to overlap the spring arrival of spawning fish. Water quality and food availability in the stream would have to prove adequate to support young herring reliably. And access to high-quality spawning habitat (our two species of river herring have somewhat differing preferences) would need to be increased by providing passage at other dams sequentially up the stream. Mr. Hoy observed herring eggs in the lowest reach of the Mill Brook, where hundreds of herring proved unwilling to use an existing fish ladder to pass the Mill Dam, and he presumes that fish dumped their eggs in the Tiasquam, as well. Conditions in those fast-moving reaches may have limited breeding success, or prevented it altogether, in 2015.
But none of these steps is impossible, and in the meantime, a half-mile of the Tiasquam has been opened to migratory fish — not just herring but American eel, white perch, and possibly sea-run brook trout (Ms. Burt reports seeing a single brookie in the stream following removal of the boards). And an obsolete pond has resumed its original role as a piece of, rather than an impediment to, the Tisbury Great Pond ecosystem.
“I think it’s important to give nature a chance,” wrote Geraldine Brooks in an email about the brook. “As Rabbi Nachman of Breslov said, ‘If you believe it is possible to destroy, believe it is possible to repair.’”
Matt Pelikan works as an ecologist restoring coastal habitats, and has studied the Vineyard’s birds, bugs, and plants since moving to Oak Bluffs in 1997. His column Wild Side appears regularly in The Times.