In this June 2016 file photo, an American shad slips out of Denis Dauphinee's hands during a shad-fishing trip on the Penobscot River in Old Town. Credit: John Holyoke / BDN

BOSTON — Massachusetts state wildlife biologists are participating in an ongoing study of the American shad — a fish that spends more of its life in the ocean, but returns to fresh water to spawn.

During these migrations, shad must travel long distances, and can face barriers like dams.

While there has been long-term monitoring of the upstream migration of adult shad from the ocean to fresh water — particularly at major dams — less is known about the impact of dams on juvenile shad as they travel from fresh water to the ocean.

This fall, biologists from the Massachusetts Division of Fisheries and Wildlife are completing a fifth year of data collection to learn about the relationship between juvenile and adult shad numbers in the Connecticut River.

The small, herring-like fish live from Newfoundland to Florida and play a key role in the food chain of rivers and oceans. They are fished commercially in several East Coast states, though the volume of the fishery is relatively small.

The project is a collaboration with the US Fish and Wildlife Service, the Vermont Fish and Wildlife Department, and the New Hampshire Fish and Game Department.

Shad populations in the Connecticut River have substantially decreased compared to historic levels due to factors like climate change, overfishing, river alterations, pollution, water withdrawals, and dams, wildlife officials said.

There are currently four dams on the Connecticut River (Holyoke, Turners Falls, Vernon, and Bellows Falls) that have varying impacts on adult shad upstream migration, juvenile production, and eventual outmigration of juvenile shad back to the Atlantic Ocean.

Over the past five years, biologists have used electrofishing boats at night to temporarily immobilize juvenile Shad and estimate their numbers and body condition.

Initial findings suggest juvenile shad numbers are down due to poor upstream passage of migrating adult shad and the downstream passage of adults and juveniles.

American shad also serve as prey for many freshwater predators popular with anglers, like small and largemouth bass, walleye, and channel catfish.

The Atlantic States Marine Fisheries Commission last year completed an assessment of the population of American shad and found it to be “depleted.”