Program measures how far herring make it up Plymouth’s Town Brook
Wicked Local Plymouth
Old Colony Memorial
By Frank Mand
fmand@wickedlocal.com
Posted May 5, 2018 at 1:00 PM
“How’s the water?” That was what a passerby humorously called out to a group from the town’s Department of Marine and Environmental Affairs that was splashing around in Town Brook.
PLYMOUTH – “How’s the water?”
That was what a passerby humorously called out to a group from the town’s Department of Marine and Environmental Affairs that was splashing around in Town Brook, just below the grist mill, last Friday afternoon.
Funny, maybe, but that was also the question that this group of wader-wearing town employees was seriously asking the fish that were hurrying upstream to spawn.
How’s the water, or more specifically, are you able to make it upstream to your spawning grounds or are the fish ladders, silt-filled pools and the one remaining dam at Newfield Street keeping you from making it all the way home?
It’s a question that is being posed scientifically.
Led by Abigail Archer, from Woods Hole Sea Grant and the Cape Cod Cooperative Extension, five DMEA staff including Director David Gould spent several days wading in the water.
First they seined the river (caught fish in a net) and placed close to 100 in baskets set down in the water.
Then, one by one, they recorded their length and sex, calling out the numbers to a colleague on the bank.
Then Archer made a small incision in their bellies and inserted a tiny PIT chip into each fish.
PIT stands for Passive Integrated Transponders, which in this case was a quarter-inch sized electronic device with a unique identification number that, when it passes one of over any of a dozen recorders placed up and down Town Brook, records that ID number and the time that this particular fish passed by.
The PIT tags are not meant to count fish. The tried and true method for that is still one person standing at a specific location for a specified period of time watching as each fish passes by and manually clicking a counter.
A count of spawning river herring returning to Town Brook was actually going on at the same time, just upstream on the other side of the grist mill and its fish ladder.
The PITs were being inserted in to the fish so that they could tell Archer just where the collected fish were going, and how far, and whether the fish ladders that are in place today are a help or a hindrance to them.
The plan is to ask these questions now, and again next year after the Holmes Dam – the last dam left on this historic waterway – has been removed.
Archer, who has designed this experiment, explained the goals in a little more detail.
“First there is the question of fish ladders,” Archer said, noting that the Town Brook has three ladders, of two kinds: the Steeppass and the Denil.
There are at least five kinds of fish ladder, she said.
• A pool-weir fishway usually has a hole at the bottom of each pool level so that fish can jump up from level to level.
• Fish using a vertical slot fishway are guided upstream by a narrow passage that winds around the barrier; the water flow is constant and the fish don’t have to jump as much.
• Denil fishways are like choose-your-own-adventure books because baffles create rapids of different speeds so that many fish species can pass.
• Steeppass fishways work similarly to the Denil style, in that there are many different water velocities. They’re usually narrower, though.
• If you want the fishway to blend in with its surroundings, try a natural bypass, which uses natural materials to recreate a stream.
Moving up from the mouth of Town Brook the first ladder you encounter is a Steeppass, best seen downstream from the mill looking back.
The second, another Steeppass, is located underneath Newfield Street.
The third, a Denil, is above ground, between the Town Brook trail and the basketball court, partially hidden by trees and bushes.
By releasing PIT-tagged fish just beneath each of these three ladders the data should reveal how many fish make it up and over each, giving a rough idea of the difficulty, or effectiveness, of each.
The data will also indicate how long it took the fish to make the various waypoints, and how far upstream they traveled.
The data they are collecting could be also used to change what kind of ladders are used for river herring, or what modifications may be necessary to improve their effectiveness, or prompt a decision to switch to different kinds of ladders altogether.
Archer’s project also takes into consideration the effect of dams, as the entire experiment will be repeated next year, after the last of the Town Brook dams is removed this summer.
The hope is that after the dam is removed fish passage further upstream will increase. If that can be shown to be true it will give added scientific validity to other dam removal projects where fish passage has been cited as a factor.
Late word from the team was that the PIT tagging was working, and important data was being obtained.
“Five of our fish made it up the Newfield steeppass and the Denil ladder,” Archer excitedly informed interested parties later in the week. “All five of them were the detected at Boy Scout bridge (where a PIT tag detector is located). As of this morning only one was detected at Morton Park, but we could have missed the others because the tuning board there malfunctioned. I replaced the tuning board this AM and it’s working well now.”
There are other reasons for removing old dams from rivers and streams, chief among them safety.
When many of these dams were originally constructed the population density was very different. Now if one of them were to collapse, the loss of property and potentially life, could be significant. That has always been one of the considerations for dam removal in Plymouth and Town Brook, where over the last 10 years the town has been remarkably successful at receiving grant funds and removing dams.
But that was not the focus on this sunny afternoon. The focus was on fish, and on finding ways to increase their numbers and their vitality. It does little good if the fish find the mouth of the river and begin to make their way upstream only to encounter insurmountable obstacles.
“Plymouth has been a leader, for a long time, in the effort to remove obstacles to fish passage and restore rivers and streams,” Archer told the Old Colony. “With this project we hope to provide additional quantitative data so that we can definitely say that dam removal is an important part of these restoration efforts.
“We hope as well that this data will prove helpful to other communities with river herring runs that are considering dam removal projects.”
For now, though, it was just a pleasant part of the job, for Archer, Gould, and the other DMEA staff.
How’s the water? It depends on whom you ask.
Follow Frank Mand on Twitter @frankmandOCM.
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