acoustic survey – NOAA Teacher at Sea Blog

August 5, 2026August 5, 2026

Stacey Morris: MVP & Investigating the Acoustic Trawl Method, August 3, 2026

NOAA Teacher at Sea

Stacey Morris

Aboard NOAA Ship Reuben Lasker

July 26-August 10, 2026

Mission: Integrated West Coast Pelagics Survey

Geographic Area of Cruise: West Coast Pacific Ocean

Date: August 3 , 2026

Weather Data from the Bridge

Latitude: 43° 06.4 ‘N

Longitude: 124°52.3 ‘W

Wind Speed: 25 kts

Air Temperature: 14.9 °C

Science and Technology Log

To understand more about the acoustic study component of our fish survey, I sat down with Kevin Stierhoff, Chief Scientist and Primary Investigator, and Brad Erisman, marine biologist, to see how our sonar mapping efforts integrate with the nightly trawling operations.

Interview Discussion with Kevin Stierhoff, Chief Scientist:

The Acoustic Trawl Methodology

Kevin: The acoustic trawl method is a globally recognized technique for surveying coastal pelagic species. By utilizing sonar, the ship can efficiently map expansive stretches of the seafloor and record precise echoes from fish populations. These instruments are highly calibrated and sensitive, providing us with accurate data on the biomass below. However, acoustics alone cannot identify the specific species creating the signal. To solve this, we conduct sonar surveys during the day when fish are schooling at depth, and then perform net trawls at night as they rise toward the surface. This nocturnal sampling allows us to determine the species and size composition without the fish avoiding the net as easily. By combining these datasets, we can apply the ratios found in our catches to the sonar echoes gathered during the day. This robust method is the primary tool for managing sardine and anchovy stocks along the West Coast.

photo of a screenshot of the display of the SX90 sonar readings with arrows labeling a bright red blob as a school of fish and a squiggly line as a fish track; this image may be in a textbook — Sonar image of fish school: Image credit NOAA

Historical Context

While simpler sonar and net surveys were conducted as early as the mid-1970s, the modern integrated approach used by the NOAA Southwest Fisheries Science Center in San Diego began in 2008. Since 2012, we have maintained a consistent schedule, performing these surveys at least once annually.

Observing Ecosystem Shifts

The marine environment has changed significantly since the surveys began. In 2012, sardines were the dominant species, but their numbers plummeted leading to the fishery’s closure in 2015. Conversely, northern anchovy and jack mackerel populations have surged, with anchovies remaining the most prevalent species we encounter today. Sardines have yet to show signs of a rebound following their crash a decade ago.

a metal tray filled with thin, silver fish all arranged to face the same direction — Anchovies brought up in one of our trawls this week

Environmental Drivers

There is much discussion regarding why these shifts occur. While fishing pressure reduced stocks in the past, current low levels are likely driven by environmental and biological factors, rather than active overfishing.

Leadership Roles at Sea

The structure of the scientific team ensures the integrity of the data collected. The Principal Investigators (PIs) are responsible for the overall coordination and quality of the long-term survey. On the ship, the Chief Scientist manages daily operations and leads the scientific party, coordinating with the PIs to ensure the survey’s objectives are met successfully.

Advanced Sonar Systems

photo of an illustration printed in a book or on a brochure of a white ship, its centerboard, and swaths of color emanating from the ship or the centerboard representing sound waves. printed on the image is this paragraph: "Multibeam Sonar System provides information on the biomass within the water column and on the type and topography of the seafloor. Drawing courtesy of Kongsberg-Simrad." — Acoustic sonar system under NOAA Reuben Lasker

The ship’s acoustic “eyes” are located on a retractable centerboard, or keel, beneath the hull. While in port, this keel is flush with the ship, but it is lowered once we reach deep water. The Reuben Lasker is equipped with an impressive array of six different sonar frequencies. Low frequencies, like the 18 kilohertz signal, penetrate deep into the ocean to map the seabed, while higher frequencies are better for detecting smaller organisms like krill in the upper water column. We primarily use the 38 kilohertz frequency to measure fish echoes. Anything in the water column with a different density than the surrounding seawater—whether it be a fish, squid, or the ocean floor—reflects sound waves that our instruments carefully measure.

six vertical panels showing backscatter readings at each depth over time, with each panel using a different frequency — Different sonar frequencies used to find fish

Omnidirectional and Multi-beam Sonars

Beyond the downward-looking sonar, we utilize an omnidirectional sonar mounted forward that scans in a radius around the ship. This helps us see fish near the surface that might be missed by the keel-mounted sensors. We also have multi-beam echo sounders, like the ME70 and MS70, which provide detailed three-dimensional views of fish schools and behavior. While these are invaluable for observing marine life, they are more difficult to calibrate for the precise biomass estimates provided by our primary systems.

photo of a computer screen displaying output from the ME70 - backscatter at different frequencies, and a sonar image — sonar image from ME70

photo of an illustration printed in a book or on a brochure depicting a ship at the surface and soundwaves emanating out from its hull; the waves surround a school of fish. — Image credit: NOAA

Survey Transects and Navigation

The survey follows a series of transects that span the U.S. continental shelf from Mexico to Canada. These lines generally extend at least 35 miles offshore to ensure we capture the full range of the species we are monitoring. Each transect provides a localized estimate of biomass, and by repeating these measurements across the entire coast, we can calculate a mean population estimate with statistical confidence. The spacing between these lines—currently 12.5 nautical miles—is a careful balance between our available time at sea and the need for scientific precision. While navigating perpendicular to the coast can sometimes lead to a rougher ride in the troughs of the waves, it remains the most efficient and scientifically sound way to sample across the varying densities of marine life.

simple political map of the west coast of the continental United States, ranging from the border of California and Mexico to Vancouver. small black lines extend out perpendicular to the coastline. each is labeled with a code. — transect lines for the Integrated West Coast Fisheries Survey:
Image Credit: NOAA

Did you Know?

The sonar system on the Reuben Lasker is so sensitive it can detect individual organisms based on their density relative to the water.

two side by side political maps of the western continental United States shown side by side to graph two different types of data by latitude: on the left, density of biomass fish species along the survey transect lines; on the right, proportions of species at sample locations along the transect lines — Density of biomass of fish species (left) and species proportions (right) –Image credit: NOAA

Interview with Brad Erisman, marine biologist about the use of the MVP

Brad: The precision of our acoustic survey relies heavily on the physical properties of the water we traverse. Factors such as temperature and density significantly influence the strength and travel speed of sonar echoes. While we calibrate our instruments in San Diego, the conditions change as we move north along the coast. To maintain the accuracy of our biomass estimates, we must continuously adjust our parameters to account for variations in sound absorption and velocity within the water column.

The Moving Vessel Profiler (MVP)

This is where the MVP, or Moving Vessel Profiler, becomes indispensable. We deploy this specialized probe to capture a comprehensive temperature profile of the water column. These real-time measurements allow us to calculate essential coefficients for our acoustic data. Along every transect, we perform multiple deployments to ensure we have representative environmental data. This constant fine-tuning allows us to produce the most reliable estimates of fish populations possible.

MVP is deployed via crane

Environmental Sensors

The MVP is a sophisticated tool, far more capable than a simple surface drifter. It is equipped with an array of sensors that measure salinity, chlorophyll levels, oxygen concentration, and sound speed. While a drifter only provides a surface snapshot, the MVP reveals the three-dimensional structure of the sea, highlighting fascinating features like thermoclines where warm, shallow water meets the colder, deeper ocean water.

Correlating Fish Patterns with Ocean Data

These environmental datasets help us explain the spatial and vertical distribution of the species we monitor. By collecting in situ data at the same fine scale as our sonar and trawl operations, we can identify correlations between habitat conditions and fish behavior. While satellite data provides a broad overview, the MVP gives us the high-resolution evidence needed to understand why schools appear in certain areas or why species patterns shift across different oceanographic breaks.

photo of a computer screen displaying output from the moving vessel profiler — MVP screen

Efficiency at Sea

The beauty of the Moving Vessel Profiler is in its name—it allows us to sample while the ship is in motion. A traditional Conductivity, Temperature, and Depth (CTD) cast involves a large cage lowered from a stationary ship, which would force us to halt our acoustic sampling. The MVP allows us to gather the necessary data without sacrificing valuable time. Although it doesn’t collect water samples or reach the extreme depths of a stationary cast, it provides exactly what we need to keep the survey on schedule.

A Three-Dimensional View of the Habitat

Oceanographers use these data points to build complex 3D models of the marine environment. These models are vital for understanding fish preferences; for instance, if sardines migrate further north, we can often trace that movement to a specific temperature preference, such as 15-20 C°-degree water, shifting with the currents. This helps us distinguish between a population decline and a simple change in habitat location.

Climate Signals and Regional Patterns

While large-scale climate signals like El Niño or La Niña are often monitored via satellite, our shipboard data helps define how these patterns manifest regionally. By looking at the data across the entire coast, we see the localized reflections of these massive basin-wide shifts, providing a clear picture of how the changing climate impacts our West Coast ecosystems.

close-up photo of a pile of dice of different numbers of sides — D & D dice

Personal Log

We’ve run into windy conditions after we crossed over the border into Oregon. We only were able to do one trawl last night before we had to call it a night due to rough waters. Tonight, we were on watch until midnight to see if things would calm down but it’s still too rocky. To fill the time, we are discovering the delightful world of Dungeons and Dragons, led by the artful storytelling of the Operations Officer, Mike Fuller.

It’s challenging walking down the hallways, and a large wave can scatter anything that isn’t well secured. Luckily the Dramamine is doing its trick and I haven’t felt queasy at all this week.

View of ocean swells out a porthole window

June 19, 2023June 19, 2023

Laura Guertin: Collecting Data: Acoustic Survey, June 19, 2023

What looks like a long fishing rod attached to a ship's rail on the ocean

NOAA Teacher at Sea

Laura Guertin

Aboard NOAA Ship Oscar Dyson

June 10 – June 22, 2023

Mission: 2023 Summer Acoustic-Trawl Survey of Walleye Pollock in the Gulf of Alaska

Geographic Area of Cruise: Islands of Four Mountains area, to Shumagin Islands area
Location (2PM (Alaska Time), June 18): 55^o 15.3391′ N, 160^o 17.8682′ W

Data from 2PM (Alaska Time), June 18, 2023
Air Temperature: 8.9 ^oC
Water Temperature (mid-hull): 7.7^oC
Wind Speed: 4 knots
Wind Direction: 182 degrees
Course Over Ground (COG): 356 degrees
Speed Over Ground (SOG): 12 knots

Date: June 19, 2023

Acoustic fisheries surveys seek to estimate the abundance and distribution of fish in a particular area of the ocean. In my case, this Summer Survey is looking at walleye pollock in the Gulf of Alaska. How is this accomplished? Well, it’s not through this method:

The Alaska walleye pollock is widely distributed in the North Pacific Ocean with the largest concentrations in the eastern Bering Sea. For this expedition, Oscar Dyson is traveling to specific regions in the Gulf of Alaska and running transects perpendicular to the bathymetry/contours (which are not always perpendicular to the shore) to take measurements using acoustics and targeted trawling to determine the abundance and distribution of walleye pollock which informs stock assessment and management models. For this blog post, let’s focus on how and why we can use acoustics to locate fish.

A map of the distribution of walleye pollock in the waters around Alaska. Alaska is centered in this map, but not disconnected from adjacent portions of Canada, and portions of Russia are visible to the east. Colors representing topography are visible, emphasized on the land of Alaska and depicted faintly on Canada and Russia. The ocean is depicted as a solid blue. We see latitude and longitude lines at ten degree intervals. We can see labels for the Beaufort Sea (north of Alaska), Chukchi Sea (northwest), Bering Sea (west), Bristol Bay (southwest), Gulf of Alaska (south and southeast.) The polygon representing the distribution of pollock is shaded with diagonal red lines. It starts in the Chukchi Sea, extends southwest out to the Bering Sea, and curves around the Aleutian Islands, hugging the coastline around the Gulf of Alaska. — Walleye pollock (*Gadus chalcogrammus*) are distributed broadly in the North Pacific Ocean and eastern and western Bering Sea. In the Gulf of Alaska, pollock are considered as a single stock separate from those in the Bering Sea and Aleutian Islands. Image from Alaska Department of Fish and Game.

A screenshot of an electronic nautical chart of the area around the Alaska Peninsula. Overlain on the chart are straight blue lines connecting blue points in a boxy meandering path in and out from the coastline, west to east. A few segments are red instead of blue. — An snapshot of a nautical chart with transects plotted. The first transect was run during Leg 1 on June 14 at the furthest location to the west, then the ship worked its way back east with approximately 40 nautical miles between transects. Once *Oscar Dyson* reached the Shumagin Islands, survey work shifted into this area..

Our story starts with the fish itself. Alaska walleye pollock have a swim bladder. The swim bladder is an internal organ filled with gas that allows a fish to maintain its buoyancy and stability at depth.

One interesting effect of the swim bladder is that it also functions as a resonating chamber that can produce and receive sound through sonar technology. This connection was first discovered in the 1970s, when low-frequency sound waves in the ocean come in contact with swim bladders and they resonated much like a tuning fork and return a strong echo (see WHOI’s Listening for Telltale Echoes from Fish).

illlustrated diagram of the internal anatomy of a boney fish. The swim bladder is located in the middle of the fish, beneath the long, skinny kidney and behind the stomach. — *Internal anatomy of a boney fish. From Wikipedia (CC BY-SA 3.0).*

Illustration of a survey ship on the ocean surface, with the ocean cutaway so that we can see a cone of sound pulses extending out from the ship's hull to the ocean floor. A school of fish is depicted in the middle of the water column, in the cone of sound. — *The sound pulses travel down into the water column, illustrated by the white cones here, and bounce back when encountering resistance.* *(from NOAA Fisheries)*

NOAA Fisheries uses echo sounding, which works by emitting vertical pulses of sound (often referred to as pings), and measuring the return strength and recording the time for the signal to leave and then return. Anything having a different density from the surrounding water (in our case – fish, plankton, air bubbles, the seafloor) can return a signal, or “echo”.

The strength or loudness of the echo is affected by how strongly different ocean elements reflect sound and how far away the source of the element is. The seafloor usually makes the strongest echo because it is composed of rock which has a density different than the density of water. In fish, the swim bladder provides a contrast from the water. In addition, each fish species has a unique target strength or amount of sound reflected to the receiver. The size and shape of the swim bladder influence the target strength. There is a different target strength to length relationship for each species of fish – the larger the fish, the greater the strength of the returning echo.

It’s important to note that echo sounders cannot identify fish species, directly or indirectly. The only way we know which fish species is causing a signal is based on trawl catch composition. There is nothing within the acoustic data that lets us identify fish species, even with the catch data. This is a subtle, but important, distinction. Acoustic data, particularly calibrated acoustic data, in tandem with the information from the trawl, definitely allows us to count fish.

Where is the echo sounder on Oscar Dyson? Look at the figure in the next section of this post – it’s a sketch of NOAA Ship Rainier, but the placement of the echo sounder is the same for Dyson. You can see a rectangular “board” that is extended down from the center of the ship. This is called – what else – the center board! Attached to the bottom of the center board are the echo sounders. When lowered, the echo sounders sit at 9 meters below the level of the sea (~4 meters below the bottom hull of the ship).

Did you know… Southern Resident killer whales use their own echolocation clicks to recognize the size and orientation of a Chinook’s swim bladder? Researchers report that the echo structure of the swim bladders from similar length but different species of salmon were different and probably recognizable by foraging killer whales. (reported in Au et al., 2010)

It starts with a calibration

*Typical setup of the standard target and weight beneath the echo sounder.* *(from NOAA Fisheries)*

Before we can begin collecting data, we need to calibrate the echo sounder. The calibration involves a standard target (a tungsten carbide sphere) with a known target strength. The calibration needs to be completed in waters that are calm and without significant marine life for the best results.

The sphere is suspended below the ship’s hull using monofilament lines fed through downriggers attached to ship railings. One downrigger is in line with the echo sounder on the starboard side, and the other two on the port side. This creates a triangle that suspends the sphere in the center of the echo sounder’s sound beam. By tightening and loosening the lines, the sphere can be positioned under the center of the sound beam and can also be moved throughout the beam. By doing an equipment calibration at the beginning and end of a survey, we can ensure the accuracy of our data.

One of the port side downriggers
A weight that goes at the bottom of the filament to ensure the calibration sphere remains below the echo sounder
The tungsten carbide sphere attached to the line, being lowered over the side

For further exploration

NOAA Ocean Service – Ocean Facts – How do scientists locate schools of fish?

Discovery of Sound in the Sea – How is sound used to locate fish?

NOAA Fisheries – Acoustic Echosounders–Essential Survey Equipment and Acoustic Hake Survey Methods on the West Coast

NOAA Ocean Service – Ocean Facts – What is sonar?

Science – Sounds like my favorite fish – killer whales differentiate salmon species by their sonar echoes

NOAA Fisheries – Sound Strategy: Hunting with the Southern Residents, Part 2

The Pew Charitable Trusts – Advanced Sonar Technology Helps NOAA Count Anchovy

July 4, 2019July 7, 2019

Erica Marlaine: Onboard the City That Never Sleeps, June 28, 2019

NOAA Teacher at Sea

Erica Marlaine

Aboard NOAA Ship Oscar Dyson

June 22 – July 15, 2019

Mission: Pollock Acoustic-Trawl Survey

Geographic Area of Cruise: Gulf of Alaska

Date: June 28, 2018

Weather Data from the Bridge:

Latitude: 58º 28.54 N
Longitude: 154º 46.05 W
Wind Speed: 16.8 knots
Wind Direction: 190º
Air Temperature: 11º Celsius
Barometric Pressure: 102

Science and Technology Log

Scientists aboard NOAA Ship Oscar Dyson are estimating the numbers and biomass of walleye pollock in the Gulf of Alaska. They use acoustics (sound data) to help them do this.

acoustic readout — Acoustic representation of fish in the area

Acoustic representation of fish in an area

Echo sounders send an acoustic signal (ping) into the water. The sound bounces off objects that have a different density than the surrounding water (such as the swim bladder in a fish) and returns back to the echo sounder. Using the speed of sound, this technology can determine how deep the fish are in the water column.

How much sound each object reflects is known as the target strength. The target strength is dependent upon the type of fish and the size of the fish. A bigger fish will give off more of an echo than a small fish will. A fish’s swim bladder is primarily what reflects the sound. Smelt and krill do not have swim bladders. As a result, they do not reflect as much sound as a pollack would. Even though a big fish gives off more sound energy than a small fish of the same species, it is possible that a return echo could indicate either one big fish or several smaller fish clumped together. A big fish of one species could also give off similar sound energy to a big fish of a different species. For that reason, actual fish are collected several times a day in the nets described in a previous blog.

From a net sample, scientists determine the number of each species in the catch as well as the length and weight of individuals of each species.

Additionally, scientists also determine the sex and age of the pollock. The catch data is used to scale the acoustic data, which in turn allows scientists to estimate how many pollock there are of various size and age groups in a given area. These numbers help scientists determine the sustainability of the pollock population, which in turn allows the North Pacific Fishery Management Council to set catch quotas.

Krill Fun Facts:

Krill (aka euphausiids) are small crustaceans (a couple of millimeters long) of the order Euphausiacea. The word “krill” is a Norwegian word meaning “a small fry of fish.” Krill are found in every ocean and are a major food source. They are eaten by fish, whales, seals, penguins, and squid, to name a few. In Japan, the Phillipines, and Russia, krill are also eaten by humans. In Japan, they are called okiami. In the Phillipines and Russia, they are known as camarones. In the Phillipines, krill are also used to make a salty paste called bagoong. Krill are a major source of protein and omega-3 fatty acids.

krill on spoon — There are many kinds of krill. Thus far, in the Gulf of Alaska, we have been seeing mostly *Thysanoessa enermis*, which measure approximately 1/2 inch in length.

Personal Log

People often refer to New York as the city that never sleeps. The same can be said for the NOAA Ship Oscar Dyson. Life onboard the Oscar Dyson carries on 24 hours a day, 7 days a week. There is never a time that the ship is not bustling with activity. Everyone on the boat works 12-hour shifts, so someone is always working while others are sleeping (or doing laundry, exercising, or watching a movie in the lounge before they go to sleep.) Most people on the boat work either the noon to midnight shift or the midnight to noon shift. However, the science team works 4 a.m. to 4 p.m., or 4 p.m. to 4 a.m. I am in the latter group. It was easier to get accustomed to than I had imagined, although it is sometimes confusing when you look at your clock and wonder whether it is 5 a.m. or 5 p.m. since the sun is shining for most of the day. Kodiak has only 4-5 hours of darkness now, and the sun sets at approximately midnight. Therefore, it does not really feel like nighttime for much of my shift.

View — The view from NOAA Ship *Oscar Dyson*

Sunset — Views (and sunsets) like these make it easy to work the night shift!

August 22, 2009April 6, 2021

Patricia Schromen, August 22, 2009

NOAA Teacher at Sea
Patricia Schromen
Onboard NOAA Ship Miller Freeman
August 19-24, 2009

Mission: Hake Survey
Geographical Area: Northwest Pacific Coast
Date: Thursday, August 22, 2009

Bringing in the nets requires attention, strength and teamwork. — Bringing in the nets requires attention and teamwork.

Weather Data from the Bridge
SW wind 10 knots
Wind waves 1 or 2 feet
17 degrees Celsius

Science and Technology Log

In Science we learn that a system consists of many parts working together. This ship is a small integrated system-many teams working together. Each team is accountable for their part of the hake survey. Like any good science investigation there are independent, dependent and controlled variables. There are so many variables involved just to determine where and when to take a fish sample.

Matt directs the crane to move to the right. Looks like some extra squid ink in this haul.

The acoustic scientists constantly monitor sonar images in the acoustics lab. There are ten screens displaying different information in that one room. The skilled scientists decide when it is time to fish by analyzing the data. Different species have different acoustical signatures. Some screens show echograms of marine organisms detected in the water column by the echo sounders. With these echograms, the scientists have become very accurate in predicting what will likely be caught in the net. The OOD (Officer of the Deck) is responsible for driving the ship and observes different data from the bridge. Some of the variables they monitor are weather related; for example: wind speed and direction or swell height and period. Other variables are observed on radar like the other ships in the area. The topography of the ocean floor is also critical when nets are lowered to collect bottom fish. There are numerous sophisticated instruments on the bridge collecting information twenty four hours a day. Well trained officers analyze this data constantly to keep the ship on a safe course.

When the decision to fish has been made more variables are involved. One person must watch for marine mammals for at least 10 minutes prior to fishing. If marine mammals are present in this area then they cannot be disturbed and the scientists will have to delay fishing until the marine mammals leave or find another location to fish. When the nets are deployed the speed of the boat, the tension on the winch, the amount of weight attached will determine how fast the nets reach their target fishing depth. In the small trawl house facing the stern of the ship where the trawl nets are deployed, a variety of net monitoring instruments and the echo sounder are watched. The ship personnel are communicating with the bridge; the deck crew are controlling the winches and net reels and the acoustic scientist is determining exactly how deep and the duration of the trawl. Data is constantly being recorded. There are many decisions that must be made quickly involving numerous variables.

Working together to sort the squid from the hake.

The Hake Survey began in 1977 collecting every three years and then in 2001 it became a biannual survey. Like all experiments there are protocols that must be followed to ensure data quality. Protocols define survey operations from sunrise to sunset. Survey transect line design is also included in the protocols. The US portion of the Hake survey is from approximately 60 nautical miles south of Monterey, California to the US-Canada Border. The exact location of the fishing samples changes based on fish detected in the echograms although the distance between transects is fished at 10 nautical miles. Covering depths of 50-1500 m throughout the survey. Sampling one species to determine the health of fish populations and ocean trends is very dynamic.

Weighing and measuring the hake is easier with automated scales and length boards. — Weighing and measuring the hake.

Personal Log

Science requires team work and accountability. Every crew member has an integral part in making this survey accurate. A willing positive attitude and ability to perform your best is consistently evident on the Miller Freeman. In the past few days, I’ve had the amazing opportunity to assist in collecting the data of most of the parts of this survey, even launching the CTD at night from the “Hero Platform” an extended grate from the quarter deck.

Stomach samples need to be accurately labeled and handled carefully.

Before fishing, I’ve been on the bridge looking for marine mammals. When the fish nets have been recovered and dumped on the sorting table, I’ve sorted, weighed and measured fish. For my first experience in the wet lab, I was pleased to be asked to scan numbers (a relatively clean task) and put otoliths (ear bones) into vials of alcohol. I used forceps instead of a scalpel. Ten stomachs are dissected, placed in cloth bags and preserved in formaldehyde. A label goes into each cloth bag so that the specimen can be cross referenced with the otoliths, weight, length and sex of that hake. With all the high tech equipment it’s surprising that a lowly pencil is the necessary tool but the paper is high tech since it looks regular but is water proof. It was special to record the 100th catch of the survey.

Removing the otolith (ear bone) with one exact incision. An otolith reminds me of a squash seed or a little silver feather in jewelry.

Each barcoded vial is scanned so the otolith number is linked to the weight, length and sex data of the individual hake.

Questions for the Day

How is a fish ear bone (otolith) similar to a tree trunk? (They both have rings that can be counted as a way to determine the age of the fish or the tree.)

The CTD (conductivity, temperature and depth) unit drops 60 meters per minute and the ocean is 425 meters deep at this location; how many minutes will it take the CTD to reach the 420 meter depth?

Think About This: The survey team directs the crane operator to stop the CTD drop within 5 meters of the bottom of the ocean. Can you think of reasons why the delicate machinery is never dropped exactly to the ocean floor? Some possible reasons are:

The swell in the ocean could make the ship higher at that moment;
An object that is not detected on the sonar could be on the ocean floor;
The rosetta or carousel holding the measurement tools might not be level.

Launching the CTD is a cooperative effort. The boom operator works from the deck above in visual contact. Everyone is in radio contact with the bridge since the ship slows down for this data collection.

July 18, 2009April 5, 2021

Jennifer Fry, July 18, 2009

NOAA Teacher at Sea
Jennifer Fry
Onboard NOAA Ship Miller Freeman (tracker)
July 14 – 29, 2009

Mission: 2009 United States/Canada Pacific Hake Acoustic Survey
Geographical area of cruise: North Pacific Ocean from Monterey, CA to British Columbia, CA.
Date: July 18, 2009

Weather Data from the Bridge
Wind speed: 40 knots
Wind direction: 350°from the north
Visibility: foggy Temperature: 12.9°C (dry bulb); 12.0°C (wet bulb)
Wave height: 8-10 feet

Science and Technology Log

Lisa Bonacci, chief scientist and Melanie Johnson, fishery biologist in the Freeman’s acoustics lab

Acoustics: Lisa Bonacci, chief scientist, and Melanie Johnson, fishery biologist, are in the acoustics lab onboard the Miller Freeman as it travels along a transect line. NOAA scientists can detect a variety of marine life under the sea. They use sonar—sound waves bouncing off an object—to detect the animals. There is an onboard sonar system that puts out four different frequencies of sound waves. Each type of fish will give off a different signal depending on its size, shape, and anatomy. The fish are then identified on the sonar computer readout. The strength of the sonar signal will determine the number of hake and the way that they are swimming. As soon as it appears on the sonar as if hake are present, Ms. Bonacci then calls the bridge to request that we trawl for fish.

This is the sonar readout as it’s seen on the computer screen.

Personal Log

The boat was rocking in all directions with 40 knot winds and 8-10 foot waves. The fishing trawl brought up scores of fish including a lot of hake. The sonar signals worked really well to locate them. We dissected and measured many fish, but not before we sat in a giant vat of hake (see photo.) It was a great learning day.

Animals Seen Today
Hake,spiny dogfish, Humbolt squid, Myctophidae, and Birds.

Discovery from the Briny
As the trawl net was raised from the depths
The sun broke through the clouds revealing a sparkling azure sky.
Scores of seagulls circled the stern
In the hopes of a bountiful offering
Tasty morsels from the deep
Soon to be thrown overboard.

American fishery biologist, Melanie Johnson, and Canadian fishery biologist, Chris Grandin, take biological samples.