Date of Award


Document Type

Open Access Thesis

Degree Name

Master of Science (MS)



First Advisor

Karen Wilson

Second Advisor

Chris Maher

Third Advisor

Rachel Lasley-Rasher


Oiadromous fish provide ecological subsidies to freshwater and marine food

webs, connecting both ecosystems. A main goal of the Penobscot River Restoration

Project was to increase connectivity between food webs by removing two mainstem

dams, improving fish passage, and reintroducing river herring through stocking.

Oiadromous fish now reach historic spawning habitat that was not accessible for

centuries. As a result, river herring runs in the Penobscot River increased from

2,336 fish in 2009 to over 3 million fish by 2018. To assess food web connectivity in

the Penobscot watershed, I analyzed stable isotopes from samples collected before

(2009-2010) and after (2020-2021) dam removals by sampling species ranging in

trophic level from piscivorous fish to baseline primary consumers from three

mainstem and three major upstream tributary sites. I targeted top fish predators

that can consume adult river herring directly. Pre-restoration, I found little evidence

of marine derived nutrient (MON) assimilation in freshwater food webs, with the

exception of a mainstem site below all dams. Post-restoration, MON assimilation

increased only below what is now the lowest dam on the river, likely due to

migration delays aggregating more fish for a longer period of time than in free-flowing

river sections. Where changes in MON assimilation occurred, I saw evidence

of bottom-up enrichment of the food web. This pattern of enrichment has been

measured in smaller rivers with spawning runs dominated by river herring. These

results may be one of the first in a river of this size (watershed area 22,300 km2)

and restoration of this magnitude, suggesting that even in larger rivers with greater

"dilution effects," effects of river herring on the transfer of nutrients from marine to

freshwaters are detectable. In the Penobscot Watershed, river herring currently

dominates the sea-run fish population but only comprise 20% of conservative

estimates of historic run size based on spawning habitat available before dam

construction. As sea-run species increase in abundance, I expect MDN to be

detectable beyond points of aggregation.


Fourth Advisor: Graham Sherwood



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