Mill Brook water study is not good news for wild brook trout

The Mill Pond is part of the Mill Brook system. — File photo by Ralph Stewart

State fisheries manager says high water temperatures are a barrier to survival.

In a presentation Sunday on the fisheries ecology of Mill Brook, Division of Fisheries and Wildlife (DFW) Southeast District fisheries manager Steve Hurley described a complex ecosystem in which wild brook trout and other native species struggle to survive. Mr. Hurley presented the results of a water temperature study that revealed readings that at the height of the summer could prove lethal to trout.

According to the study, which regularly measured water temperatures at nine locations along the Mill Brook system during the summer months and continuously from July 18-20, the temperatures are dangerously high, at temperatures at which brook trout have been eliminated from the lower stretches of Mill Brook.

For example, one graph Mr. Hurley offered showed that temperatures below Roth Pond averaged 78 degrees Fahrenheit in July, and reached as high as 92 degrees at midday on July 19, 2013. Above Priester’s Pond, where there are still wild brook trout, the temperatures also reached the high 80s (see related chart).

Brook trout are a cold water species that require temperatures between 33 and 53 degrees for egg development, spawn optimally at 51 degrees, and grow best at 59 degrees. Ten degrees higher than this is the upper limit for the survival of the brook trout population.

“If these fish are there at all, they’re hanging on to the last bits of habitat,” said Mr. Hurley, a trout biologist, during the presentation. “When the water is revealed to the sun, it’s easy for the temperature to reach lethal limits.”

The Mill Brook begins in Chilmark, passes through several artificial impoundments in West Tisbury, such as Mill Pond, and streams into Tisbury Great Pond. A combination of dams and changes made for agricultural purposes have altered the ecosystem, limiting habitat for brook trout that were once in abundance on the Island. An important factor in the decrease of the population is water temperature, which has increased, in part because of a reduction in the wetland plant life that shields streams and ponds from excessive solar heating.

Mr. Hurley placed significant blame on dams, of which there are more than 2,645 in Massachusetts, 10 along Mill Brook.

“If you have a dam that’s no longer in operation,” he said, “consider breaching it and letting the stream return to its natural form.”

About 60 Islanders attended the two hour presentation at the Public Safety Building in West Tisbury, during which Mr. Hurley also addressed a 2012 fish census and the history of the Mill Brook system.

Mr. Hurley noted that the water temperature study was completed almost 70 years after his predecessors in the Massachusetts Fish and Game Division highlighted similar issues in a letter dated August 8, 1955, in which they recommended a temperature study to Nelson S. Bryant, Sr., a selectman then, in response to a request for help to maintain the brook system “suitable for maximum trout utilization.”

The unnamed aquatic biologist wrote, “One undesirable feature of the system, as I saw it, is that the series of ponds (Priester’s Pond, etc.) in the upper system, must cause considerable warming up of the water … No doubt small spring brooks or spring holes will always carry over some trout, but large quantities of suitable cold water is vital for maximum trout utilization and survival.”

He added, “I would strongly urge, possibly with the help of the Rod and Gun Club, to take water temperatures in Priester’s, Mill Pond and Mill Brook during August. Both surface and bottom temperatures should be taken. I think Howard Andrews, secretary of the Rod and Gun Club, still has a Max-Min. thermometer of ours, which could be used.”

Click here for an accompanying “Wild Side” column on the Mill Brook talk