Donna Knutson: Dredging, June 16, 2016

NOAA Teacher at Sea Donna Knutson
Aboard R/V Hugh R. Sharp
June 8 – June 24, 2016


2016 Mission: Atlantic Scallop/Benthic Habitat Survey
Geographical Area of Cruise: Northeastern U.S. Atlantic Coast
Date: June 16, 2016




Mission and Geographical Area: 

The University of Delaware’s ship, R/V Sharp, is on a NOAA mission to assess the abundance and age distribution of the Atlantic Sea Scallop along the Eastern U.S. coast from Mid Atlantic Bight to Georges Bank.  NOAA does this survey in accordance with Magnuson Stevens Act requirements.

Me hat

Science and Technology:

Latitude:  40 32.475 N

Longitude:  67 59.499 W

Clouds: overcast

Visibility: 5-6 nautical miles

Wind: 7.4 knots

Wave Height: 1-4 ft.

Water Temperature:  53 F

Air Temperature:  63 F

Sea Level Pressure:  29.9 in of Hg

Water Depth: 103 m


Science Blog:

Paired with the HabCam, dredging adds more data points to the scallop survey and also to habitat mapping.   Various locations are dredged based on a stratified random sampling design.  This method uses the topography of the ocean bottom as a platform and then overlays a grid system on top. The dredged areas, which are selected randomly by a computer program, allow for a good distribution of samples from the area based on topography and depth.

Vic and Tasha sewing up the net on the dredge.

Vic and Tasha sewing up the net on the dredge.

A typical dredge that used for the survey is similar to those used by commercial fisherman, but it is smaller with a width of 8 ft. and weight of 2000 lbs.  It is towed behind a ship with a 9/16 cable attached to a standard winch.  Dredges are made from a heavy metal such as steel and is covered in a chain mesh that is open in the front and closed on the other three sides making a chain linked net made of circular rings.

A fisherman’s dredge has rings large enough for smaller animals to fall through and become released to the bottom once again.  The dredge in a survey has a mesh lining to trap more creatures in order to do a full survey of the animals occupying a specific habitat.

There are three categories of catch received in a dredge: substrate, animals and shell.  A qualitative assessment on percent abundance of each is done for every dredge.  Not all animals are measured, but all are noted in the database.

Dredge being dumped on sorting table.

Dredge being dumped on sorting table.

A length measurement is taken for every scallop, goosefish (also called monkfish), cod, haddock, as well as many types of flounder and skate. A combined mass is taken for each species in that dredged sample.  Some animals are not measured for length, like the wave whelk (a snail), Jonah crab, and fish such as pipefish, ocean pout, red hake, sand lance; for these and several other types of fish, just a count and weight of each species is recorded.

Sorting the dredged material.

Sorting the dredged material.

Other animals may be present, but not

counted or measured and therefore are called bycatch.  Sand dollars make up the majority of bycatch. Sponges, the polychaete Aphrodite, hermit crabs, shrimp and various shells are also sorted through but not counted or measured.

Ocean pout

Ocean pout

All of the dredge material that is captured is returned to the ocean upon the required sorting, counting and measuring.  Unfortunately, most of the fish and invertebrates do not survive the ordeal.  That is why it is important to have a good sampling method and procedure to get the best results from the fewest dredge stations needed.

Goosefish, often called Monkfish, eat anything.

Goosefish, often called Monkfish, eat anything.

The dredge is placed on the bottom for only fifteen minutes.  There are sensors on the frame of the dredge so computers can monitor when the collection was started and when to stop.  Sensors also make certain each dredge is positioned correctly in the water to get the best representation of animals in that small sample area.

Entering the name of the animals to be measured.

Entering the name of the animals to be measured.

Even with sensors and scientists monitoring computers and taking animal measurements, the dredging can only give a 30-40% efficiency rating of the actual animals present. Dredging with the aid of the HabCam and partnerships with many scientific organizations, along with data from commercial fisherman and observer data, create a picture of abundance and distribution which can be mapped.

Adductor muscle the "meat" of the scallop. This on is unhealthy.

Adductor muscle the “meat” of the scallop. This one is unhealthy.

In the scallop survey the emphasis is on where are the most scallops present and this aids fisherman in selecting the best places to fish.  The survey also suggests where areas should be closed to fishing for a period, allowing scallops to grow and mature before harvesting.

This management practice of opening closed areas on a rotational basis has been accepted as beneficial for science, management, and fishermen. This method of balancing conservation and fishing protects habitats while still supplying the world with a food supply that is highly valued.

Personal Blog:

Being part of a dredging team is exciting.  It is a high energy time from the moment the contents are dropped on the sorting platform to the end when everything is rinsed off to get ready for the next drop.

Katryn "Kat" Delgado

Kateryn “Kat” Delgado

I wanted to take pictures of everything, but with gloves on it was hard to participate and help out or just be the bystander/photographer. Kateryn Delgado from Queens NY, a volunteer/student/scientist/yoga instructor/photographer, was very helpful.  She was involved in other surveys and often took pictures for me.

I did find it sad that the animals we sorting were not going to live long once returned to sea, but that is a part of the dredging that is inevitable.  Raw data needs to be collected.  After measuring, a percentage of the scallops were dissected to get their sex, abductor muscle (meat), and stomach.  Shell size was compared to the meat and gonad mass and is also used to age the scallop.  The stomach was removed to test for microplastics.  Dr. Gallager and his research team are studying microplastics in the ocean.   Scallops filter relatively large particles for a filter feeder, and therefore are a good species to monitor the abundance of plastics at the bottom of the ocean.DSCN7891 (2)sunset

The weather has been nice, not very warm, but the waves are low.  Just the way I like them.  We are making our way back to Woods Hole to refuel and get groceries.  I didn’t realize we would split up the leg into two parts.  We should be in around 10:00 a.m.  I’m going to go for a long walk since there is not a lot of opportunity for exercise on the ship.  Hope it’s sunny!


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s