Characterizing the Penobscot River Estuarine Transition Zone to Determine Environmental Challenges for Atlantic Salmon, their Prey, and Other Sea-run Species

Damian Brady, University of Maine, School of Marine Science

Rachel Lasley-Rasher, University of Maine, School of Marine Science


The abundance of Atlantic salmon Salmo salar has precipitously declined in recent decades despite considerable conservation efforts.  One reason for this decline is high marine mortality. Because monthly estuary mortality appears higher than monthly mortality in open ocean systems, estuaries are of interest.  High mortality rates may be due to a combination of physiological stressors, variable prey concentrations, and elevated predation pressure.  Salmon migration success is influenced by physical conditions in the estuarine transition zone such as salinity, temperature, and turbidity.  Especially important is the nature of the saltwater intrusion and how smolts are exposed to seawater and thermal shifts.  Another important factor is the strength of the turbidity maximum which affects both smolt foraging success and predation risk.  Despite the importance of such variables to smolt migration, a thorough characterization of the estuarine transition zone is lacking in the Penobscot estuary.  The location and intensity of the salt intrusion and turbidity maximum is dually affected by riverine and tidal forcing.  We plan to conduct vertical profiles of temperature, salinity, and turbidity throughout the estuarine transition zone during smolt migration.  We will specifically target dates to sample a broad range of discharge values and various tidal stages.  Additionally, we will collect zooplankton (including mysids and fish larvae) samples to characterize the prey field available to migrating smolts.  These data can be combined with ongoing NOAA efforts to determine what combination of physical variables contributes to smolt migration success in the Penobscot estuary.