Radio Tracking to Bolster St. Croix Fish Count

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Kevin Bunch
August 23, 2022
milltown dam fish count tagging

Alewives migrating up the St. Croix River are being tagged as part of a new initiative to help track their spawning activities and maintain an annual fish count into the future.

Since the 1980s, the St. Croix International Waterway Commission (SCIWC) has been counting the number of alewives and other fish species crossing Milltown Dam, located between St. Stephen, New Brunswick, and Calais, Maine.

According to SCIWC Programs Administrator Gloria Rodriguez-Whittingham, the fish are counted by hand—a labor-intensive process—or via video feed during the spring run. This occurs within the Milltown Dam’s fish passageway during a 10-minute period each hour. The number of fish counted is then multiplied by six to estimate the overall number for each hour.

Milltown is the lowest dam on the St. Croix River and therefore the first obstacle all migrating fish must cross, but it likely won’t hold that distinction for much longer. Its operator, New Brunswick Power, is working through the permitting process to remove the dam.

When the Milltown Dam is removed, the current fish-counting approach will no longer be viable, Rodriguez-Whittingham said.

Since 2021, the SCIWC has been catching and embedding passive integrated transponder (PIT) tags in migratory alewives. It has also been sampling a calcium structure within fish ears called an otolith that can change based on the location of where a fish lives.

Rodriguez-Whittingham said 500 fish were tagged in 2021, with another 500 tagged in 2022. The fish tagging effort is funded through the IJC’s International Watersheds Initiative; the IJC also has financially supported the fish count in the past.

Using an array of acoustic antennas placed throughout the basin, the SCIWC and its partners can track the tagged fish in the river. These antennas are situated at Milltown Dam, Grand Falls Dam, Woodland Dam, Vanceboro Dam and near the mouth of the Magurrewock River in Maine. Since the tags are passive, they do not require a battery to operate, and will remain active during the lifespan of the fish. As the fish enter the antennas’ range, the antennas receive an acoustic ping from the tags indicating the fish’s location and movement.

“Any time the fish passes an antenna with the tag we inserted, we can see where they’re traveling to,” Rodriguez-Whittingham said. “It also allows us to know where the fish are going, how far they’re traveling, where they’re stopping. It allows us to further track their movements as they’re traveling through the river and gives us the data we want to have to see how far they’re able to get and where they’re spawning, or where they’re choosing to give up and go back.”

pit tag fish insert st croix tracking example

A passive integrated transponder, or PIT, tag being implanted into a smolt in Idaho. The same technology is being used to track alewives in the St. Croix River. Credit: Idaho Department of Fish and Game.

There are several other dams beyond Milltown on the St. Croix River that fish need to overcome as they return from the sea to spawn, and this information can help inform the effectiveness of the fish passageways at each of them, Rodriguez-Whittingham said.

In addition to the tracking project, the traditional fish count also has taken place this year. The total run for 2022 was 712,808 alewives counted crossing Milltown Dam, along with 17 American shad. This is 100,000 more than last year, continuing a trend of larger fish runs returning to the St. Croix River.

Alewife runs–which take place along with other river-spawning species, such as American eel, American shad, sea lamprey and Atlantic salmon–have been historically important for the area. Indigenous communities and settlers catch alewives to serve as bait or to be smoked and eaten. They are a vital part of the broader food web in the region, and therefore a key to ecosystem health for the watershed.

Picture of Kevin Bunch
Kevin Bunch

Kevin Bunch is a writer-communications specialist at the IJC’s US Section office in Washington, D.C.