The Northwest Watershed Institute completed the first study done in the United States on springtime eagle congregations, investigating the relationship between American bald eagles and intertidal species at Dabob Bay.

Residents recently began noticing large numbers of eagles on the shore during low tides in late spring.

“It’s very unusual to have these spring congregations of up to 175 eagles on the beach,” said Peter Bahls, executive director of the Northwest Watershed Institute.

Bahls and Heather Gordon, a biologist with the NWI, authored the study, which is titled “Bald Eagles, Oyster Beds, and the Plainfin Midshipman: Ecological Relationships in Dabob Bay.”

“The study was about uncovering the mystery of those springtime congregations and having a greater understanding of the ecology of eagles and the shoreline,” Bahls said.

Using a spotting scope and a camera, Bahls and Gordon conducted 22 days of eagle spotting this year between May 15 and July 12 during which they observed foraging behavior.

The study found the eagles mainly fed on plainfin midshipman, a species of bottom-dwelling fish that lives at ocean depths down to nearly 1,000 feet along the Pacific coast from Alaska to Mexico. In the spring, they migrate to the rocky intertidal zone, where they spawn on beaches from May through July.

At Dabob Bay, Bahls and Gordon found plainfin midshipman nests among the oyster beds, where the eagles were found most often at low tide, foraging on foot for nesting midshipmen.

They also were found foraging for stranded midshipmen in exposed eelgrass beds during low tides.

The study showed that, during the more than 70 hours of observation, the plainfin midshipman was the most common prey the researchers observed eagles consume. Of the 617 recorded prey captures, 75 percent were positively identified as midshipmen.

“It shows the relationship between the eagles and what they’re feeding on and the type of beach they have to have,” Bahls said.

The study also sought to show the relationship between humans and the eagle congregations.

The researchers found that foot traffic was both the most common and most disturbing activity to the eagles, but boat traffic and shellfish harvesting also could scare eagles off their feeding grounds.

Bahls said the study was one of the first of its kind in the United States. The only other study of eagles congregating along a shoreline in the spring was done in British Columbia.

The study was funded by landowners in the area and the Port Gamble and Jamestown S’Klallam tribes. After learning results, the two tribes plan to adapt their plan for a commercial aquaculture operation proposed for the Dabob Bay oyster beds.

“The eagle study by NWI is great information for us to have,” Ralph Riccio, shellfish biologist for the Jamestown S’Klallam tribe, stated in a news release from NWI. “The tribes want to make sure our proposed shellfish operation does not harm the eagle convocation at Dabob Bay. We reviewed the findings of the study to help inform our adaptive management and monitoring plan.”

To read the published study, go to the Northwest Watershed Institute website.