2008 Annual Report
Gulf of Maine Research Institute
Spotlight on Science

Innovative Diet Study Promises Clues to Impact of Penobscot River Dam Removal

Two dams in Maine’s Penobscot River are expected to be removed in the near future and a new fish passage will be constructed at a third site. These conservation efforts will restore sea-run fish to nearly 1,000 miles of river habitat and could aid the recovery of a number of key fish species.

With funding from The Nature Conservancy and the Penobscot River Restoration Trust, GMRI scientist Graham Sherwood has teamed up with Karen Wilson from the University of Southern Maine to capture a snapshot of food web dynamics in the region before this major restoration effort gets underway.

Over the past two summers, Sherwood and a team of technicians and interns have spent countless hours angling for large predators such as cod and striped bass and snorkeling for invertebrates such as mussels, crabs, and lobsters at numerous sites throughout Penobscot Bay. They caught few large fish in the upper reaches of the bay, even with the assistance of a recreational fishing guide, suggesting that this area may no longer be home to many large predator species. It is hoped that the removal of the dams may reverse this trend by encouraging more prey species that must spend part of their lives in fresh water (such as alewives) to return to the bay. GMRI also worked with the Maine/New Hampshire Inshore Trawl Survey (DMR) to collect cod, haddock, herring, pollock, redfish, flounders, among other fish species and invertebrates. In parallel, USM scientists collected fish (e.g., smallmouth bass and chain pickerel) and invertebrates in the river and its tributaries.

Back at the lab, a small sample of muscle tissue from each specimen is dissected and preserved for future analysis. Using a novel scientific technique call stable isotope analysis, the researchers will be able to identify carbon and nitrogen signatures to determine the major components of each fish’s diet over the course of its life. In the future, we will be able to compare these isotope signatures to samples collected several years after the dams are removed. This will tell us if and how the relationships between major predator and prey species are changing.

While the primary job of the project to date has been to create an archive of pre-dam removal samples, Sherwood has been able to move ahead with some initial analyses. He has identified an isotope signature that tells us that most fish caught in the bay eat other marine species. Mackerel are the exception with an isotope signature that reflects a diet of more freshwater prey. We suspect that as this highly mobile species feeds it way along the coastline, it carries a freshwater signature into the bay from areas where there are more juvenile alewives leaving the rivers.

Resident cod show no evidence of freshwater influences in Penobscot Bay, but we wonder… how might this change following dam removals? If alewife return in large numbers will cod isotope signatures start to look more like mackerel? Would this signal the beginning of a long-awaited recovery for cod and other groundfish species in Penobscot Bay?

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