Mill Brook presentation described the natural possibilities

Unimpeded in its upper reaches, Mill Brook winds its way through the Land Bank's Waskosim's Rock preserve. — Photo by Matt Pelikan

Matt Pelikan has been writing the Wild Side column since 2008. He is a restoration ecologist for The Nature Conservancy, and he lives in Oak Bluffs.

Native brook trout cling to survival in the brook’s upper reaches where cold water temperatures prevail.

Aside from a few brilliant seasons catching frogs when I was about nine years old, I haven’t spent much time exploring freshwater habitats. So I was happy to have the opportunity to learn from an expert last weekend, when I attended a presentation by Steve Hurley, Southeast District Fisheries Manager for the Massachusetts Department of Fish and Game.

The presentation, Sunday afternoon at the West Tisbury Public Safety Building, was attended by about 60 interested Islanders. Mr. Hurley discussed the results of two important studies of the Mill Brook, the Vineyard’s most significant stream system, which winds from Chilmark through West Tisbury to empty into the Tisbury Great Pond at the head of Town Cove. The Mill Brook is currently a subject of much study and debate as the Town of West Tisbury contemplates the future of the Mill Pond, an artificial impoundment on this stream just northeast of the town center and adjacent to the Edgartown-West Tisbury Road.

Streams, like any other ecological system, feature astonishing complexity. But as a practical matter, a few simple principles often suffice to explain a great deal about a stream. Drawing on a recent survey of the Mill Brook’s fish and a season-long effort to track water temperature at multiple points in the stream, Mr. Hurley developed an elegantly simple explanation for the distribution of various fish species.

The Island’s streams have always struck me as a bit incongruous in the Vineyard landscape: cold, gravel-bottomed, and often flowing quickly down fairly steep gradients, they seem like bits of Vermont transplanted to the coast. I learned from Mr. Hurley that this impression isn’t so far off. Fed mainly by groundwater, Island streams tend to be cold, and they support some cold-water fish species that one would also find in a northern New England river. The most notable of these is the brook trout, essentially a northern or high-elevation species that approaches the southern and lower limit of its distribution in cold groundwater streams of coastal Massachusetts.

Unlike cold streams in Vermont, though, our streams flow to the sea rather than into lakes or larger river systems. Some brook trout populations have developed a fascinating strategy to take advantage of this: during spring and summer, they descend their streams and enter saltwater, feeding in the rich waters of estuaries before returning to freshwater streams to spawn. In our coastal streams, in fact, such journeys are not unusual: many so-called “diadromous” fish split their time between freshwater and oceanic life. River herring, shad, smelt, salmon, and the American eel are among the diadromous species native to our region and requiring access from salt to fresh water.

Brook trout, wherever they are, require cold water, with about 70 degrees being the upper limit at which they can survive and temperatures below 60 needed for healthy growth. Historically, Mr. Hurley reports, sea-run or “salter” brook trout were abundant in the Mill Brook system. In the 2013 fish survey led by Mr. Hurley, a solid population of native brook trout – both adults and young fish – was documented in the upper reaches of the Mill Brook. But the species was virtually absent from the lower stretches of the stream, which were dominated by fish species that prefer warmer water, and there was no clear evidence of a sea-run brookie population.

An explanation for this pattern came from water temperature data from a survey supported by the Edey Foundation and the Sea-run Brook Trout Coalition. An array of data loggers placed at various points in the stream recorded water temperatures, and the results as presented by Mr. Hurley were startling. Predictably, upper reaches of the brook featured temperatures congenial to brookies. But at each artificial impoundment surveyed lower down in the stream, sun-warmed water from the pond crossed the spillway to heat the stream below. Above and below dams on the Mill Brook, sustained temperatures last summer reached well into the 80s, a level that produces brook trout chowder.

In addition to altering the temperature regime of the stream, Mr. Hurley pointed out, the half-dozen or so major dams on the Mill Brook form impassable physical barriers to brookies and other diadromous fish that might otherwise use the stream. Good numbers of eels were detected in the stream; but generally, the fish survey suggests, the Mill Brook functions poorly if at all as a resource for diadromous fish.

In considering whether to dredge the Mill Pond, remove the dam and restore the stream, or take some other course of action, West Tisbury will need to balance factors including ecology, recreation, scenery, and tradition. But Mr. Hurley’s presentation made it clear that, however natural one may think the Mill Brook is, in fact it’s a system that is vastly altered from its original state. Town leaders and residents should know that however familiar the stream’s artificial impoundments seem, they are recent developments in the overall history of the Mill Brook. And they are features that dramatically interfere with the life cycles of many fish that once flourished here, and could do so again.

For a complete report on the Sunday presentation, click here.