trawl net – NOAA Teacher at Sea Blog

August 13, 2025August 20, 2025

Joshua Gonzalez: A Tale of Two Hauls: August 13, 2025

NOAA Teacher at Sea

Joshua Gonzalez

Aboard NOAA Ship Bell M. Shimada

August 11 – August 23, 2025

Mission: Integrated West Coast Pelagics Survey (Leg 4)

Geographic Area of Cruise: Pacific Ocean, California Coast

Today’s Date: August 13, 2025

Weather Data from the Bridge:

Latitude: 42° 06.3’N

Longitude: 124° 35.0′ W

Wind speed: 2.4 kts.

Wave height: 1-2 ft.

Air temp.: 12.3° C (54° F)

Sky: Fog

Science and Technology Log

While I was sleeping, the net that we helped set out was brought back in with a haul of hake. Exactly what the morning crew was looking for! It was almost entirely hake, which made the processing extremely straight forward. Some might be inclined to think that this is exactly what science is supposed to look like.

Fast forward to my shift again and shortly after we woke up, there was a haul that was ready to be brought in. We were all very excited for our first chance to process a catch. Well, this time when the net was brought up, it was awfully full. We were excited about the possibility of going through the catch and finding our target CPS, Coastal Pelagic Species. However, it was almost entirely full of krill, and did not have a single specimen of what we were looking for. Now, some might think that this means our haul was a failure and/or it was bad science.

Those people would be wrong. While the first two hauls of this leg of the survey are diametrically different, they are both good. Science requires good data. Data is good when it is reliably accurate. It doesn’t matter if it is larger numbers or zeros. So, in our case, while we didn’t get the information we were looking for, we now know that what we did had a result of zero for our targeted CPS . We also know that where we fished resulted in a catch of krill. This is knowledge that we can use next time to help us get what we are after.

a crewmember lowers a full trawl net, lined with orange buoys, onto the aft deck of NOAA Ship Bell M. Shimada. it is night time. we can see a few crewmembers wearing life vests and hard hats, but our view is obscured by the large net. — About to empty the first catch.

photo of a white plastic bin filled with thousands, probably tens of thousands, of krill. — About to empty the first catch.

In the second catch we did get a few different species. We caught: North Pacific krill, moon jellyfish, and a handful of eulachon. The eulachon were all weighed and measured for length.

Personal Log

Today is my brother’s birthday. Happy birthday! I am feeling much better. I continue to take the sea sickness prevention medicine, but I think I have found my sea legs. I am starting to feel like I know my way around NOAA Ship Shimada more and more, or at least the places I am supposed to go. 🙂 I was happy to make it from my bunk, down past the wet lab and acoustics lab, through the hall, past the mess, down some steps, through a room I never need to stop at, and to the laundry on my very first try! Our ship even has two places to work out. I am not brave enough to try a treadmill when the boat is rocking, but I did take an opportunity to do some jump rope.

I am amazed by the engineers who think through everything that needs to be done to make a ship like ours work. It is a maze of rooms, cords, and more, but all of it is well thought out and has a purpose.

I was a little disappointed today. We had a little extra time in between work during my shift so I went out to check on the stars, but it was foggy out so I could only see a few feet away. But I am learning a lot and having a ton of fun. It will be interesting to see what it will be like when we are getting more and larger hauls in a single shift.

Also, it’s a big deal back in Milwaukee, but the Brewers have won 11 games in a row. One more tonight means free burgers in the city! When I spoke with my wife and kids, they were definitely hoping for a win tonight. Let’s go Brew Crew!

Josh, wearing an orange rain coat, orange gloves, and gray beanie, stands at a metal table in the wet lab over an electronic fish measuring board. he looks down as he lines a slim fish up against the board to take its measurement. — Measuring a fish

Josh, wearing an orange rain coat, orange gloves, and gray beanie, stands at a metal table in the wet lab at an electronic fish measuring board. he looks up for a photo as he lines a slim fish up against the board to take its measurement. in the foreground is a blue tray with a pile of similar fish. — Measuring a fish

Did You Know?

The eulachon is also known as the candle fish. It got the name from the fact that it is so oily that if you dry the fish out, you can light the tail on fire and it will burn like a candle. In the past, the eulachon was prized for its oil. The oil will even be solid at room temperature, similar to butter.

Can you identify this species?

close-up view of a single eulachon in a green plastic basket

You guessed it: Eulachon!

June 20, 2025June 20, 2025

Jenna Cloninger: Anchovy Expert and Pyrosome Party Time, June 15, 2025

NOAA Teacher at Sea

Jenna Cloninger

Aboard Bell M. Shimada

June 11 – June 26, 2025

Mission: Integrated West Coast Pelagics Survey (Leg 1)

Geographic Area of Cruise: Pacific Ocean, California Coast

Today’s Date: June 15, 2025

Track the Ship: Bell M. Shimada

Weather Data Snapshot: 12:23pm, Pacific Daylight Time

Currently, the air temperature is 65°F (18°C) with a wind speed of 10 knots and a wave height of 5 feet. I was finally able to witness a sunrise this morning during my working hours, thanks to clear skies, and I am staying up a little bit past my “bedtime” to enjoy today’s sunshine.

Science and Technology Log

Trawling operations are in full swing here on the ship! Please enjoy this image of me in front of our two trawling nets, which we pull behind the boat at different depths to target different species of fish.

A woman in bright orange overalls and rubber boots poses for a photo in front of two massive spools mounted horizontally above the aft deck, such that they can be wound or unwound. The spools contain teal and yellow netting. One trawl net is partially unrolled, with buoys attached at different points. — Photo of me with our fishing nets, which we use for surface and midwater trawling.

In these first few days, we are seeing many anchovy! I have quickly become an expert at identifying the differences between anchovy and other fishes that may be brought up with our net. In addition to fish species, we see quite a few small squid and some other invertebrates known as pyrosomes in our net. (See the Did You Know? section below for more information.)

close up view of the corner of a plastic teal basket filled with small narrow fish, each about 3-4 inches long. a hand wearing a black glove holds a single fish out for display above the pile. — Photo of a basket of anchovy, with one being held by someone’s hand for a size reference.

After sorting our catch, we measure and weigh a certain number of the target species (sardine, anchovy, and mackerel) to collect data that helps us characterize their species and size distributions. In addition, some specimens are selected for dissection, where we determine the fish’s sex, reproductive stage, and health; collect tissue samples for genetic analysis; and extract otoliths for estimating age.(For more about otoliths, which are also known as ear stones or ear bones, click here.) This information helps scientists monitor fish health through their life history stages. It’s not possible to catch every fish in the ocean, so scientists study a smaller representative group instead, like we are doing aboard NOAA Ship Bell M. Shimada. This age data, along with other information like length, weight, and sex, is used to create computer-generated models of the fish population. When combined with acoustic data, these models help estimate how many fish are in the wild and predict what might happen if people keep fishing.

A woman wearing heavy-duty orange overalls and black gloves stands at a measuring board on a metal table in the wet lab. With her right hand, she uses a tool to measure a small fish placed along the board. She looks down, absorbed in her work. — Photo of me measuring a very small fish with a digital tool called an Ichythystick.

In the picture above, you can see that I am using a special tool called an Ichthystick to digitally measure the length of each fish in a specific subset from our catch. I have discovered that, although I do not normally consider myself squeamish when it comes to science, I am not a fan of dissecting fish for otoliths. Instead, I do a lot of the measuring and weighing of the fish, as well as additional tasks to support my teammates while they work on extracting otoliths.

In addition to trawling for fish, NOAA Ship Bell M. Shimada has a special piece of technology known as a CTD. A CTD is a scientific instrument used in marine science to study the properties of seawater. CTD stands for Conductivity, Temperature, and Depth. These three measurements help scientists understand what the ocean is like at different levels. The CTD device is usually attached to a metal frame and lowered into the ocean from a research ship. As it goes down, it collects data about the water’s temperature, how salty it is (measured by conductivity), and how deep it is. This information helps scientists learn about ocean currents, climate, and marine life. CTDs can also carry bottles that collect water samples from specific depths. Scientists use these samples to test for oxygen, nutrients, or tiny organisms. CTD data is very important for studying how the ocean changes over time. (I have not yet seen the CTD in action, but I pass by it every day on the side deck and am hoping that it will be deployed sometime soon during my working hours.)

Jenna, wearing a Teacher at Sea beanie and a Teacher at Sea t-shirt under heavy orange overalls, stands next to the CTD rosette - a large metal apparatus that hosts both the CTD probe and a ring of gray water sampling bottles. — Photo of me next to a CTD (Conductivity, Temperature, Depth) device for size reference.

Personal Log

Adjusting to life at sea is an ongoing process. I experienced a bit of seasickness yesterday right after lunch, but I was able to go to my stateroom at noon (which is the end of my night shift) and sleep it off until my next shift began at midnight. As a person who traditionally struggles with sleep, I am so exhausted after each shift that I am sleeping much better on the ship than I do at home, which I did not expect! In addition, I am eating much better on the ship than I do at home, thanks to our amazing Chief Steward who has been cooking fabulous meals for us. I have learned that mealtimes are very important on the ship, because sitting with your colleagues while enjoying good food is a boost for team morale and helps everyone stay energized.

Did You Know?

A lot of different animals can become caught in a trawling net while fishing, but pyrosomes are some of the most common animals we see during night trawls (aside from our target species of anchovy, mackerel, and sardine). What are pyrosomes? NOAA’s website tells us that pyrosomes are pelagic tunicates, which are part of the phylum Chordata. In other words, pyrosomes are tough, bumpy, gelatinous tube-like animals that gather in large clusters at the ocean’s surface. Like many jelly-like animals in the ocean, we still don’t know a lot about pyrosomes and how they live. This makes it hard to understand how they might be affecting ocean ecosystems. For example, pyrosomes can grow quickly and filter large amounts of water, which could have a big effect on phytoplankton blooms. Before this experience, I had never even heard of a pyrosome, and now, I feel like I am part of a pyrosome party every night!

top down view of a green plastic basket filled mostly with pyrosomes (which look like pink gelatinous tubes) with some various fish mixed in. — A basket full of pyrosomes (the pink gelatinous tubes) mixed with fish.

July 12, 2024August 8, 2024

Nick Lee: Fishing, Fishing, Fishing, July 10, 2024

NOAA Teacher at Sea
Nick Lee
Aboard NOAA Ship Oscar Dyson
June 29 – July 20, 2024

Mission: Pollock Acoustic-Trawl Survey

Geographic Area of Cruise: Eastern Bering Sea

Date: July 10, 2024

Weather Data from the Bridge:

Latitude: 50° 40.9 N

Longitude: 178° 29.9 W

Wind Speed: 20 knots

Air Temperature: 6.2° Celsius (43.1° Fahrenheit)

Science and Technology Log:

Last blog post, I talked about acoustic backscatter, which scientists on board use to locate fish. When scientists notice high-intensity backscatter – or backscatter that they’re interested in collecting more biological data about – they’ll call the bridge and ask to go fishing. The bridge then makes the announcement over the radio:

“All stations. This is the bridge. We will be fishing, fishing, fishing.”

This announcement sparks a flurry of action from scientists, NOAA officers, and the deck crew. A few scientists go up to the bridge for a marine mammal watch, where they make sure that there are no marine mammals in the area of the operation. NOAA officers navigate to the science team’s target fishing area, and the deck crew prepares the net to go in the water.

Teacher at Sea Nick Lee on marine mammal watch. Nick stands at a window on the bridge and looks out through binoculars at gray waters under a gray sky. — *Marine mammal watch on the bridge.*

Before my cruise, I thought fishing nets were relatively simple and uniform. However, I’ve since learned that the net has many different components and sensors, which help scientists collect additional information about the fish seen with acoustics.

Codend

During the trawl, the net is dragged behind the boat. Near the opening at the mouth of the net, the net’s mesh is over a meter wide. This helps reduce drag from the water, while still funneling fish toward the back of the net. The net gradually gets smaller until the very end of the net – called the codend – where the fish are collected. At the end of each trawl, the net is hauled out of the water, and the contents of the codend are emptied into a sorting table for further processing in the fish lab, where length, weight, sex, and maturity are recorded for a representative sample.

Pocket Nets

In portions of the net with larger mesh, small fish and other organisms can escape through the holes in the mesh. This creates a problem for scientists – a trawl could show that only adult pollock are present in a certain area when in reality the population is mixed, but all of the juveniles escaped! Since scientists will be using trawl samples to understand the overall population of pollock, they want to avoid bias as much as possible in their data.

To get around this problem, scientists are studying the rates at which different sized pollock (and other organisms) escape from the net. They use pocket nets, or small nets made of the same fine mesh as the codend, to get an idea of what escaped from each trawl. Nine pocket nets are attached to the side, top, and bottom of three different sections of the net with varying mesh sizes. As the trawl net is being hauled back on the boat, one of my jobs is to help empty these pocket nets and collect what’s inside.

We’ve mostly found krill and jellyfish, but occasionally we’ll find a larval fish or squid!

CamTrawl

Near the codend, there is also a camera, referred to as CamTrawl. This camera provides scientists with a visual of what is going into the net, and can be used to help identify species and length of fish that are caught.

Scientists deploying PelagiCam. View through a doorway of two scientists in orange foul weather gear and green and white hardhats standing close to one another at the railing of the ship. They face away from the camera. In the foreground, on the deck, is a spool containing yellow cable. — *CamTrawl (left) and scientists Matthew Phillips and Mike Levine deploying PelagiCam (right).*

On this cruise, scientists are also testing a camera that they lower over the side of the ship (without a net), known as PelagiCam. They are hoping that PelagiCam may be able to collect species and length data, supplementing the data captured when processing fish from the trawl. If PelagiCam can record this data accurately, it could provide an efficient complement to trawling, which requires a lot of time and collaboration between different teams of people.

Pollock seen on PelagiCam — *A pollock seen on PelagiCam (left). Posing with scientist Mike Levine in front of the PlagiCam (right).*

Teacher at Sea and scientist posing in front of PelagiCam. They are wearing foul weather gear and hard hats; Mike is also wearing a mask and gloves. — *A pollock seen on PelagiCam (left). Posing with scientist Mike Levine in front of the PlagiCam (right).*

FS70 Net Sounder

The FS70, nicknamed the Turtle, collects acoustic data and produces a live image of the net’s opening when it is in the water. This data allows scientists and the deck crew to monitor the shape of the net while fishing, ensuring that the net opened correctly. It also monitors when fish enter the net.

Live data from the FS70. a square screen mounted in a console displaying sonar data. — *FS70 (left), nicknamed the turtle, and the live data it shows scientists during a trawl.*

Personal Log:

Going fishing can sometimes be a lot of “hurry up and wait.” After the marine mammal watch, at least one scientist stays on the bridge to monitor the net using the FS70, and the others get ready to process the trawl. Letting the net out and hauling it back in is far from simple, however. It requires constant communication between the bridge and the deck crew, and it can be made more complicated by the weather or equipment malfunctions. Once the net is in the water, trawling can take anywhere from 15 minutes to over an hour.

Opening the codend is always exciting, because we’re never quite sure what we caught. While our target is always pollock, we’ll often find other interesting organisms mixed in as well. Some highlights include rockfish, squid, and a smooth lumpsucker.

close-up of a Rockfish in a white bin — *Organisms caught in the trawl so far include rockfish, squid, and smooth lumpsuckers.*

Squid in a white bin — *Organisms caught in the trawl so far include rockfish, squid, and smooth lumpsuckers.*

Did you know?

The net used on NOAA Ship Oscar Dyson was specifically designed for this survey!

June 13, 2024June 13, 2024

Charlotte Sutton: Learning the Lasker, June 11, 2024

NOAA Teacher at Sea

Charlotte Sutton

Aboard NOAA Ship Reuben Lasker

June 7 – June 18, 2024

Mission: Rockfish Recruitment and Ecosystem Assessment Survey (RREAS)

Geographic Area of Cruise: Pacific Ocean; U.S. West Coast

Date: June 11th, 2024

Weather Data from the Bridge

Date: Tuesday, June 11, 2024
Latitude: 35.42 °N
Longitude: 121.22 °W
Sea Wave Height: 4-5 ft
Wind Speed: 4 knots
Air Temperature: 57 ° F
Sky: Foggy / light rain

Science Log

Arriving on the Lasker

We’re off! After landing in San Francisco and driving down to Santa Cruz, I arrived on the NOAA Ship Reuben Lasker by way of small boat transfer. The Lasker was anchored in Monterey Bay, and sent a small boat to pick up myself and some of the science team and crew to be taken aboard. We boarded the small boat, the “RL-2 Shark,” then traveled to the side of the Lasker where we were hoisted up via a winch. I then got a full tour around the ship, and the opportunity to meet many people who work on the Lasker, including members of the science team, NOAA Corps, and Lasker crew.

Teacher at Sea Charlotte Sutton poses for a photo on board the small boat RL-2 Shark. She's wearing a life vest and a hard hat and holding in her hands a jacket with neon yellow reflective coloring. Behind the small boat, we can see the Santa Cruz harbor. — Charlotte boards the RL-2 Shark in Santa Cruz

a view up the portside of NOAA Ship Reuben Lasker, facing the aft deck, as seen from the small boat the water's surface. Winch cables and a frame have been lowered off the side of the large vessel. We can see a davit arm toward the aft. — Charlotte boards the RL-2 Shark in Santa Cruz

The Night Shift

Running a ship like the Lasker is a 24-hour-a-day operation. At all times there are some groups of people sleeping and others who are working. The majority of the science crew works at night, so my day typically begins with dinner at 5:00 pm and then working with the science team from approximately 9:30 pm until 6:30 am. As a morning person this was very difficult at first! But after two nights working, I’m finally adjusting to our new schedule.

What is the Goal of the Survey?

The main scientific focus of the upcoming mission is the Rockfish Recruitment and Ecosystem Assessment Survey (RREAS). This survey has been conducted since 1983, and collects data on rockfish and other organisms in their ecosystem.

Rockfish are a very important fish commercially and recreationally in California and on the West Coast. One of the primary purposes of the survey is to use the data collected to help provide additional information about the management of commercial and recreational fisheries off the west coast.

CTD Operations

On the ship's deck at night, a man stands facing away from the camera, looking down a large apparatus nearly the height of his shoulder. Inside a round metal frame are gray cannisters arranged in a circle (the "rosette"), surrounding a scientific probe mounted in the center. A cable extends from the top of the appartus out of sight. The man wears a hard hat, a life vest, and sunglasses and grasps a gray rope looped through a rung of the rosette. Another man, also wearing life vest and hard hat, is seen at a distance beyond the apparatus. It's nighttime. — CTD rosette, ready to be deployed into the ocean.

I began my first night shift by observing a CTD deployment. CTDs are instruments that measure Conductivity, Temperature and Depth (CTD). CTD measurements are conducted approximately 5-6 times a day, and twice at night. The CTD descends down into the ocean to a depth of up to 500 m . There are other instruments and sensors attached to the CTD that measure things like chlorophyll levels and oxygen levels. The data taken from the water column serves as a foundation for scientists to understand the ocean environment.

All of the CTD data, and all the data that the Lasker collects, is free and available to the public.

Trawling

a hand-drawn diagram of a trawl net in two positions: net while fishing (on top) and net deployment and retrieval (bottom.) The lines are all labeled: we see the headrope (with buoys) at the top of the net, the footrope (small buoys) at the base of the opening, the bridle lines, door leg and transfer lines, the doors, and lines "to trawl winch" and net "to cod end." — Hand-drawn diagram of trawl net, courtesy of scientist Tanya Rogers.

When do we trawl?

The reason the science team trawls at night because there is net avoidance during the daytime, meaning the fish will see the net coming during the day and swim away from it. Other creatures migrate towards the surface at night. In a pattern called vertical migration, these mesopelagic species migrate to shallow waters to feed during the night, while spending day hours at depth.

Having more diverse species to study is useful for the Rockfish Recruitment and Ecosystem Assessment Survey (RREAS). The more data that is collected on rockfish and other species helps scientists to better understand the heath of different fish species, and make predictions and assessments of ocean trends.

How does trawling work?

Each night, the Lasker crew, NOAA corps officers, and science team work together to trawl for different fish species.

“Trawls, which are nets towed behind a boat to collect organisms, have been used by fishers for centuries. Trawls can be divided into three categories based on where they sample the water column: surface, midwater, and bottom.” (NOAA Ocean Exploration)

In our Rockfish Recruitment and Ecosystem Assessment Survey, the science team conducts midwater trawls, at approximately 30m depth to target the fish and other ocean organisms that are targeted for the study.

The last few days we’ve averaged 5 trawls per night. The process begins by deploying the trawling net behind the ship into the midwater section of the water column, and trawling for fish for either 5 or 15 min. After the net is brought in, the contents of the trawl are sorted, measured, and recorded by the science team. This data will be later analyzed to help better understand the ocean ecosystem.

Charlotte stands at a large white bin, about three feet long, containing a pile of small silver-colored fish. She uses two hands to hold up a plastic pitcher filled with a sample of the fish - two other empty pitches rest in the bin. Charlotte wears a coat, orange grundens (fishing overalls), long orange gloves, and her Teacher at Sea beanie hat. — Teacher at Sea Charlotte with the catch of a trawl.

Six people stand three to a side along a long metal table and face the camera for a photo. They are wearing heavy fishing overalls and long orange gloves, and each grasps a pair of tweezers in one hand. On the metal table, white plastic trays contain subsets of the catch; in the foregroud, two of these plastic trays contain organisms that have already been sorted and neatly arranged. — The science team sort fish and other organisms from the trawl.

Personal Log

NOAA Ship Reuben Lasker: My New Home at Sea

NOAA Ship Reuben Lasker (photo courtesy of NOAA)

My new home for my time at sea is the NOAA Ship Reuben Lasker. The Lasker is a NOAA fisheries vessel, with a home port located in San Diego, CA.

“The ship’s primary objective is to support fish, marine mammal, seabird and turtle surveys off the U.S. West Coast and in the eastern tropical Pacific Ocean” (NOAA Office of Marine and Aviation Operations).

During my time at sea, the Lasker will be sailing off the coast of California, sailing out of Santa Cruz and back into port in San Diego.

Living on the ship reminds me a lot of my college dorm room. On the ship most people have roommates, and we all have shared spaces like the mess (cafeteria), science labs, outside decks and places to relax. Everyone aboard the ship has been extremely welcoming and kind, always answering any questions I might have and teaching me about life aboard a ship. I am happy to call the Lasker home over my trip at sea!

a bulletin board housed in a case with sliding glass doors, titled OUR CREW. The background of the display is a nautical chart of the California coast around the Channel Islands, though it is mostly obscured. Photos of the crew members are cut out and pinned all over the chart. There's also a magazine article about Reuben Lasker, the ship's namesake. — There are three major teams working and living as a cohesive unit aboard the *Lasker*. The *Reuben Lasker* crew, NOAA science team, and NOAA Corps officers each have distinct roles and work together each day to accomplish various science projects.

view of a sunset over a calm sea — Sunset aboard the *Lasker*.

Did you know?

Adjusting to working the night shift (approximately 9:00 pm – 7:00 am) as a typical morning person has meant sleep is often on my mind. Chatting before our second night shift, scientist Ily Iglesias shared with me how dolphins sleep. Both dolphins and whales sleep much differently than most mammals. Known as unihemispheric sleep, dolphins

“only rest half of their brain while the other half stays awake to breathe. Also, most whale and dolphin respiratory and digestive tracts are completely separate, so they don’t get water in their lungs when feeding underwater.” (NOAA Fisheries).

August 15, 2023August 15, 2023

Germaine Thomas: What Does Acoustic Trawl Sampling Really Tell Us? August 13, 2023

NOAA Teacher at Sea

Germaine Thomas (she/her)

Aboard NOAA Ship Oscar Dyson

August 7 – August 21, 2023

Mission: Acoustic Trawl Survey (Leg 3 of 3)
Geographic Area of Cruise: Pacific Ocean/ Gulf of Alaska
Date: Sunday, August 13, 2023

Weather Data
Lat 59.12 N, Lon 150.11 W
Sky condition: Partly Cloudy
Wind Speed: 13 knots
Wind Direction: 330°
Air Temp: 14 °C

Science and Technology blog

The ocean is a really big place. We have really only mapped about 5% of the ocean bottom. How do we manage fisheries if we have to count fish in an area that is overwhelmingly large? This is where the genius of acoustics and trawl sampling complement each other. The scientists aboard NOAA Ship Oscar Dyson use the echo sounders to find fish or other animals lurking in the ocean and then they can extrapolate and upscale that data to a much larger area which is covered by their transects.

Wait! That is a lot of information using language that folks don’t really use at the dinner table. Could you please explain this in more basic terms? You bet, as a matter of fact in the last couple of days I have been swimming in a sea of new vocabulary, talking to really smart people and trying to keep up with the conversation that it almost makes my head explode. Don’t worry, I am safe. But it’s really impressive how scientists have developed ways to accurately know fish and marine organism populations in the ocean with out having to sample all of it.

Acoustics

Acoustics uses the echo-sounders a lot like a fish finder, but the ones on NOAA Ship Oscar Dyson are much more capable than the type you would find on your boat. The echo-sounders are attached to the bottom of a lowered centerboard—essentially a large keel—in the center of the boat, and they measure five different frequencies with different wavelengths.

A photo of a computer screen displaying five echograms (graphs of recorded echoes) in a row. Germaine has added annotation: a black arrow points at the top of the echogram with the label "Top of the ocean," and another points to a solid, dark red bar midway down the echogram with the label "bottom of the ocean." Dashed marks, angled up or down, are scattered across the echograms, concentrated in upper portions. Germaine has drawn a black circle around some of these, with the label "The colored marks in the oval indicate "backscatter," which could indicate fish or other marine organisms." At the top of each echogram, in its title, Germaine has circled the frequency measured, but they are difficult to read. — View of the 5 different frequencies measured by the echosounders, one in each frame. The darker marks on the screen could be fish, jellyfish, krill or other marine organisms, this is referred to as “backscatter.” The red circles show the different frequencies used to measure the backscatter.

So, if we can see the fish using acoustics, why do scientists need to sample using a trawl net? As you can see above, the marks in the backscatter can show the depth and the approximate shape of objects, but there is not enough detail to tell exactly what kind of organism is present. Most of the scientists on board have a pretty good idea what kind of fish or organisms are present, but the most definitive way to know is to take a trawl sample.

Trawl Sampling

The trawl net as seen in the picture below is being set off the aft deck.

A crewmember wearing a hard hat, life vest, and heavy work overalls stands off to the side as the trawl net is lowered off the aft deck from a large yellow A-frame. — The part that is in the air is called the codend. That is the section of the net where the specimens are ultimately collected.

view of two rollers - like large spools - containing rolled up fishing nets. the net on the right is orange. the net on the left is white and partially paid out. — The trawl is a about 172 meters long and it stored on these rollers on the back deck.

When the trawl is deployed to the depth that the scientists want to sample, the net will funnel fish and other organisms into it. This is called flying the net.

The screen above diagrams three different views of the net as it is pulled through the water. You can see that the trawl net was not directly behind the boat and went to a depth of 21.5 m.

photo of a computer screen displaying data about the position of the net, along with a more detailed diagram. Germaine has added arrows to label "The doors help open the net" and "the codend at the end of the net that collects the sample." We can see that the set length measures 457 meters. — In this image you can see the net and how far back it trails behind the *Oscar Dyson*.

I just have to include one more view of the trawl net from the bridge as it is pulled behind the boat.

A photo of a computer screen showing a 3-d rendering of the deployed trawl net and the following measurements: door depth port - 16.5 m. door depth starboard.- 15.7 m. door spread - 59.4 m. door pitch port - 4.7 degrees. door pitch starboard - 6.1 degrees. headrope horizontal range - 204 m. headrope true bearing - 326.0 degrees. depth - 21.0 m. change meters/minute - -0.2 m. — This image was taken when the crew was bringing the net back into the boat, so the depth is shallower.

The next image shows the path that the net was pulled through the water.

photo of a computer screen displaying an echogram (graph of recorded echoes.) This echogram shows the returns from a single frequency. Germaine has annotated it with arrows pointing to: Header rope or top of the trawl path, and Footer rope or bottom of the trawl path. Another arrow points to colored specks and reads: The echosounders show backscatter, which could be fish or other organisms. — The acoustics show the backscatter which the scientists make the trawl target. The next step is to process what is captured in the codend of the trawl and see exactly what is present.

Because the trawl is dragged through the water, it catches different organisms at different times. The scientists want to know when the different organisms were caught so they have cleverly attached a camera to the side of the net. Through the camera they can see which type of fish came into the trawl. Ultimately, this links the kind of acoustic backscatter viewed in the echograms recorded during the trawl to exactly the type of organism caught by the trawl.

view of a trapezoidal metal apparatus, containing underwater cameras and floats, attached to a blue trawl net, spread out on deck — The camtrawl: a camera that records the type of fish entering the net and when they enter.

Below is a picture of some fish as they enter the trawl net and move towards the codend.

a photo of a computer screen displaying a black-and-white underwater camera feed. a few fish (pollock) are visible swimming by the net. — The camera is looking across the net as the fish move past. The fish in the picture are pollock, the type of fish we are looking for on this leg of the cruise.

Transect Lines

So how do scientists take this information and extrapolate the data to a broader area? While the Oscar Dyson is out at sea they run transect lines while recording acoustic data. Transect lines are specific paths in the ocean. The picture below shows the transect lines that we plan to do and have done on this leg of the cruise.

a screenshot of an electronic nautical map of the Gulf of Alaska. straight lines extending toward and away from the coast are superimposed across the map. — The red lines are the transects we have done and the blue lines are the transects scientists plan to do in the remainder of this leg of the cruise. If you look closely there are pictures of fish symbols on the transect lines where the ship has made trawl samples.

Using the acoustic data that the echo-sounders provide and verifying the types of fish and other marine organisms through the trawl sampling allows the scientists to predict, with a high level of certainty, the amount and types of marine organisms that are present along the transect lines that were not trawl-sampled. Thus saving the taxpayers money, and allowing fisheries managers to use good data, keeping the fishery viable, and allowing commercial fishing boats to have reasonable catch limits.

Scientist in the Spotlight

Honestly it takes a team to make all of this happen. But, half of our team is sleeping at the moment, I have the night shift from 4pm to 4am, so I am going to introduce one fabulous expert in acoustics and fisheries:

Abigail, wearing a blue hoodie featuring a drawing of a salmon, sits back from a long computer desk with eight computer montiors mounted above and to the side. She smiles at the camera. — Abigail McCarthy in the Acoustics Lab

Abigail McCarthy has been working for MACE: Midwater Assessment and Conservation Engineering Program since 2007. She received her undergraduate degree in Biology from Wellesley College and then obtained a Masters in Fisheries from Oregon State University.

For fun, she surfs and enjoys long-distance prone paddle board races. She has recently found a new love with fly fishing.

Aboard the Ship Oscar Dyson, she is working as a specialist helping to run the acoustics lab.

I asked Abigail what she thought of about her educational experience? She immediately said, “I love learning! High school and college were both a lot of fun.”

What would be a good suggestion for a young aspiring high school student pursuing a degree related to ocean studies or science in general?

Her response was great: “Being curious and working hard is more important than being brilliant. Persistence and determination will get you where you want to be in the future.” Finally, “Learn to code! Become familiar with programing languages like Python and R.”

Hopefully, I answered your burning questions about the use of acoustic trawl sampling, and surveys. Yet, there is so much more to learn. Why not take a trip yourself? Check NOAA’s website out and just apply.