What we’ve learned

All of Maine’s native sea-run fish are present in the Penobscot River system. Prior to the Penobscot River estuary survey, researchers had not documented and did not know the full roster of sea-run fish species transiting through the Penobscot River. When we combined our survey data with other available research conducted in the watershed, we determined that all 12 species of sea-run fish were present in the estuary, though not all species used the estuary at the same time. (Lipsky et al 2019)

We have used our survey and monitoring data to explore multiple lines of scientific inquiry, from species-specific to ecological interactions.  Our team has worked to better understand:

The physical characteristics of the estuary itself

  • The physical characteristics of the Penobscot River estuary are described for the first time since 1967. Oxygen levels and temperature are suitable for all Northeast sea-run fish species and life stages. (Meseck et al 2017)

Each individual species of sea-run fish

  • American shad are present and successfully reproducing, even before the dams were removed. (Lipsky et al 2016)
  • Rainbow smelt are present in the estuary most of the year, adults in the spring and juveniles in the fall. (Wood et al 2012)
  • River Herring (alewife and blueback herring) are always present in the estuary, especially age-1 and age-2. (Stevens et al. 2021a)
  • River Herring are not just migrating through the estuary as adults, but also some juveniles are using it as a nursery. (Stevens 2021a)
  • Juvenile river herring are present in the estuary in spring and summer, with age-1 and age-2 herring using the estuary from April to September and age-0 fish using the estuary in the summer. (Lipsky et al. 2019, Stevens et al 2021a)
  • As juveniles, some river herring repeatedly move from ocean to estuary habitats. The prevailing theory had been that juvenile river herring remained exclusively in the ocean once they left the estuary.  (Labonte 2016; Webb 2021)
  • The estuary is an important feeding ground for juvenile alewives. There, they consume estuarine calanoid copepods, barnacle larvae, and mysid shrimp. (Labonte 2016; Webb 2021)

Ecosystem components and processes

  • The make-up of the fish community in the Penobscot estuary varies over time; smelt are common in the spring and fall, Atlantic herring are present in the summer, and river herring are present spring through fall. (Lipsky et al. 2019, Stevens et al 2021b)
  • Zooplankton biomass (roughly equivalent to “weight”) remains relatively constant even as fish biomass increases. (Ambrose 2020)
  • Fish densities have been higher in later years (2019 versus 2012), and fish size is not changing. As a result, fish biomass has increased 3-fold during this time. (Stevens 2019)
  • Fish biomass varies over the course of the year, with highest volumes in summer and spring rather than fall. (Stevens 2019)
  • Fish biomass has increased in recent years, but so has the number of adult river herring. (Stevens 2019)
  • Seals are more common in spring during sea-run fish migration. In recent years (2019 versus 2012), more gray seals have visited the estuary, but there has not been an increase in total seals observed. (Leach 2020)
  • Seal-induced injuries to salmon are declining during a period when river herring and fish  biomass in the estuary are increasing. (Leach et al 2022)