When last we left our intrepid alewife friends they were boldly making their way to Billington Sea to spawn, swimming over 1 1/2 miles, up 81 feet, and past various predators (including a portly raccoon). Now, by late May, they’ve finished spawning and we’ve had the pleasure of watching them return to the sea.
Before we let them go to eagerly await the next step in the process–the migration of the new generation, also known as young of the year, to the ocean in late summer–we at the Miller’s Tale want to share a little about how humans have been helping alewives navigate man-made obstructions.
Years ago, alewives had only the usual impediments of predators and exhaustion to stand in the way of a successful spawn. Once people began to build water works, like dams, to harness the power of moving water, it became more difficult or even impossible for alewives and other migrating species to reach their spawning ponds.
As we mentioned in Herring Love, Part I, at some point Plymouth’s colonists realized that they needed to make “suitable provision” for the herring to get to the spawning grounds. Only one court record, however, mentioned why this was necessary. In 1702, Nathaniel Thomas was given permission to build a trough for the herring to go over his mill dam,
“…that soe the fish mite be preserved if it mite be that they mite not be beaten out of the brook.”
(That “mite” be the most confusing sentence ever written!)
The earliest ways to address the problem didn’t involve construction of fishways. In the 1660′s regulations were passed to establish times when herring could not be taken, to require that the mill “stopp water when the tide is out of the pond during the time of the herrings,” and to set men to watch the brook to keep boys, pigs and dogs from “annoying them.”
In the late 1660′s the town began to require that structures be built to provide a “convenient passage” for the herring. Flood gates, waste water courses, troughs, and “throwfares” were all mentioned as possible solutions. Unfortunately, we don’t know what any of these looked like.
Here at the Plimoth Mill today, a dam of about 10 feet provides water for our wheel. The photo below shows the fishway that we use to help the alewives get to the top of the dam.
Here’s a view from the upper level, looking down. There are actually two fishways connected by a level resting area.
All sorts of devices have been built to help fish surmount dams today, the most famous of which is probably the fish ladder, where fish leap or wriggle up shallow steps. At the mill we have a baffle fishway. This kind provides a channel for the fish to swim up. Baffles in the channel break up the flow of water and provide a range of currents so that a variety of fish can successfully use the fishway. A strong current at the mouth of the channel attracts the fish. Our fishway is a type of baffle fishway known as an Alaskan Steeppass. Its modular design and light weight made it perfect to use in remote areas of Alaska.
Once the herring have ascended to the top of the dam, they swim through a culvert under the road and emerge in the Jenney Mill Pond. There they go through a fish counting device which electronically counts each fish (though some timid fish dart in and out and get counted multiple times). The picture below shows David Gould, Plymouth’s Environmental Manager, working at the fish counter. In addition to the electronic data, volunteers take turns manually counting the number of fish that pass by in 10 minute increments and recording air and water temperature and weather conditions. The white platform at the end of the tube provides a contrast which makes it easy to count the fish, although it’s hard to keep track when, at the height of the run, there may be 200 or 300 in ten minutes!
What’s the Sound of 100,000 Alewives Migrating?
That’s what Glorianna Davenport of Tidmarsh Farms wants to find out! Tidmarsh Farms in Plymouth is a wetlands restoration and conservation project which is restoring 250 acres of former cranberry bog into a more natural wetland system. As part of the stewardship program, they are using technology to create an online Living Observatory to help people connect with the natural world in a variety of ways, including by experiencing micro-sounds–sounds so small (like the sounds of dragonfly wings or alewives slipping through the water) that we usually don’t hear them.
“As part of the Living Observatory initiative, we are developing sensor networks that document ecological processes and allow people to experience the data at different spatial and temporal scales. Small, distributed sensor devices capture climate and other environmental data, while others stream audio from high in the trees and underwater.”
Gloria came to Town Brook this spring, before the start of the herring run, to meet David Gould and to talk about placing an audio recording device in Town Brook. An hour later the device was in the water and recording data.
Here are Glorianna and David checking out the equipment before it was placed.
David placing the microphone in the water.
So what is the sound of alewives migrating? And how many alewives participated in the migration this year? We promise we’ll let you know as soon as the data is in.
Hooray for Herring!