Salmon team and predators ready for Spring on the Penobscot


This is the computer readout from our two frequency echosounders.  The water surface and bottom are thick red lines, the specks in between the lines are likely fish. Photo by NOAA/NEFSC/NEST

Another beautiful spring day in Maine means that salmon biologists are busy in the field.  Since our last post, we’ve been waiting for the warmth of spring to arrive, bringing with it the sea-run fish migrations.  Last Friday we conducted one of our biweekly fish surveys of the Penobscot Estuary.  We used single frequency echosounders, also known as “fish finders”, to measure the amount of fish in the water column as we cruised through the estuary.  Generally this technique is called “hydroacoustics,”and uses sound waves to record what types of objects are in the water.  In our case we are using it to identify fish in the water.

For the past five years, we have made the same cruise along the Penobscot Estuary and are accumulating the data from echosounders which translate into the number of fish.  This allows us to describe patterns of fish abundance.  Many of these fish are sea-run species that make a migration inland during the spring.

Why is this important? Because one aspect of our research is understanding the dynamics of Atlantic salmon smolt survival as they migrate from rivers to the ocean.  Previous studies have shown that predators are a big factor. This survey is designed to figure out where predators such as large fish, cormorants, and seals congregate relative to their prey – namely, fish.  While the echosounders are counting fish, we observe the birds or mammals that we see, record their species, number, and behavior (feeding or resting).  Describing the overlapping distributions of predators and prey is the first step in understanding the complex ecological interactions that salmon are part of as they migrate to sea.


A pair of loons looking for a meal in the freshwater portion of the Penobscot River estuary. Photo by NOAA/NEFSC/NEST



A harbor seal sits on a haul-out ledge. The Penobscot Estuary survey records the number and location of any marine mammals encountered to help in determining areas of potentially high predation for Atlantic salmon smolts as they migrate the estuary. Photo by NOAA/NEFSC/NEST


Roosting double crested cormorants. These birds are exceptional fishers that eat migrating Atlantic salmon smolts in Maine Rivers.  Photo by NOAA/NEFSC/NEST

In this survey our nice day turn unsettled when Northwest winds instigated a small craft advisory for the area.  Luckily for us we were already north of the more exposed areas of the survey and could safely continue on our mission.  We saw a number of cormorants and seals but they were mostly at roosting and haul-out areas.


Justin Stevens, contract NOAA Fisheries biologist, navigating the Penobscot Estuary during a fish survey using hydroacoustics. The weather looks nice but mid-survey a small craft advisory was issued for the general area and this is the face of a boat operator scowling at the news.


The number of fish counted with the echosounder was less than we have seen in previous years by this time, but water temperatures are still around 10°C (50°F) so we probably are not close to the peak of the fish migration.  My gut tells me that things will be changing soon given we have started to have some sunny days with temperatures in the 80s.  One thing that is guaranteed, things will be different next time we are out.


The NOAA Fisheries Northeast Salmon Team

Penobscot River, Maine





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