acoustics – Page 9 – NOAA Teacher at Sea Blog

August 8, 2012August 11, 2021

Johanna Mendillo: Hello pollock…. can you hear me now? August 7, 2012

NOAA Teacher at Sea
Johanna Mendillo
Aboard NOAA ship Oscar Dyson
July 23 – August 10

Mission: Pollock research cruise
Geographical area of the cruise: Bering Sea
Date: Tuesday, August 7, 2012

Location Data from the Bridge:
Latitude: 59^○ 52 ’ N
Longitude: 177^○ 17’ W
Ship speed: 8.0 knots ( 9.2 mph)

Weather Data from the Bridge:
Air temperature: 7.3^○C (45.1ºF)
Surface water temperature: 8.4^○C (47.1ºF)
Wind speed: 4 knots ( 4.6 mph)
Wind direction: 75^○T
Barometric pressure: 1018 millibar (1 atm)

Science and Technology Log:

We are wrapping up our final few sampling transects. Now that you are practically fisheries biologists yourselves from reading this blog, students, we must return to the fundamental question— how do we FIND the pollock out here in the vast Bering Sea? The answer, in one word, is through ACOUSTICS!

Look at all of these birds off the stern! Why do you think they are following us? Are we about to haul up a catch, perhaps?

Hydroacoustics is the study of and application of sound in water. Scientists on the Oscar Dyson use hydroacoustics to detect, assess, and monitor pollock populations in the Bering Sea.

Now, you may have heard of SONAR before and wonder how it connects to the field of hydroacoustics. Well, SONAR (SOund Navigation and Ranging) is an acoustic technique in which scientists send out sound waves and measure the “echo characteristics” of targets in the water when the sound waves bounce back— in this case, the targets are, of course, the pollock! It was originally developed in WWI to help locate enemy submarines! It has been used for scientific research for over 60 years.

(PLEASE NOTE: The words sonar, fishfinders, and echosounders can all be used interchangeably.)

The transducer sends out a signal and waits for the return echo... — The transducer sends out a signal and waits for the return echo once it bounces off the fish’s swim bladder… (Source: http://www.dosits.org)

On the Dyson, there is, not one, but a collection of five transducers on our echosounder, and they are set at five different frequencies. It is lowered beneath the ship’s hull on a retractable centerboard. The transducers are the actual part of the echosounder that act like antennae, both transmitting and receiving return signals.

The transducers transmit (send out) a “pulse” down through the water, at five different speeds ranging from 18-200kHz, which equals 18,000-200,000 sound waves a second!

When the pulse strikes the swim bladders inside the pollock, it gets reflected (bounced back) to the transducer and translated into an image.

First of all, what is a swim bladder? It is simply an organ in fish that helps them stay buoyant, and, in some cases, is important for their hearing.

Swim Bladder (Source: www.education.com) — Swim Bladder (Source: http://www.education.com)

Now, why do the pulses bounce off the swim bladders, you ask? Well, they are filled mostly with air and thus act as a great medium for the sound waves to register and bounce back.

Think of it this way: water and air are two very different types of materials, and they have very different densities. The speed of sound always depends on the material through which the sound waves are traveling through. Because water and air have very different densities, there is a significant difference in the speed of sound through each material, and that difference in speed is what is easy for the sonar to pick up as a signal!

It is the same idea when sound waves are used to hit the bottom of the ocean to measure its depth- it is easy to read that signal because the change in material, from water to solid ground, produces a large change in the speed of the sound waves!

Here is a sonar system measuring the depth of the ocean... — Here is a sonar system measuring the depth of the ocean… (Source: http://www.dosits.org)

Interestingly, different types of fish have different shaped and sized swim bladders, and scientists have learned that they give off different return echos from sonar signals! These show up as slightly different shapes on the computer screen, and are called a fish’s “echo signature”. We know, however, that we will not encounter many fish other than pollock in this area of the Bering Sea, so we do not spend significant time studying the echo signatures on this cruise.

So, what happens when these signals return to the Dyson? They are then processed and transmitted onto the computer screens in the hydroacoutsics lab on board. This place is affectionately known as “the cave” because it has no windows, and it is, in fact, the place where I spend the majority of my time when I am not processing fish! Here it is:

Here is Anatoli observing potential fish for us to go catch!

We spend a lot of time monitoring those computer screens, and when we see lots of “specks” on the screen, we know we have encountered large numbers of pollock!

Here we are approaching a LARGE group of pollock!

When the scientists have discussed and confirmed the presence of pollock, they then call up to the Bridge and announce we are “ready to go fishing” at a certain location and a certain depth range! Then, the scientists will head upstairs to the Bridge to work with the officers and deck crew to supervise the release, trawling, and retrieval of the net.

Now, in addition to the SONAR under the ship, there are sensors attached to the top of the net itself, transmitting back data. All of the return echos get transmitted to different screens on the bridge, so not only can you watch the fish in the water before they are caught, you can also “see” them on a different screen when they are in the net! As I told you in the last post, we will trawl for anywhere from 5-60 minutes, depending on how many fish are in the area!

Left: Echosounder at work/ Right: The "return signature" is visible on the computer! — Left: Echosounder at work/ Right: The “return signature” is visible on the computer! (Source: http://www.dosits.org)

A full catch- success! Without acoustics, it would be much harder for NOAA to monitor and study fish populations.

Personal Log:

In these last few days, we have crossed back and forth from the Russian Exclusive Economic Zone (EEZ) and the U.S. several times. There were some nice views of Eastern Russia before the clouds and fog rolled in!

I can see Russia from my ship! (Photo Credit: Allan Phipps)

In addition, we crossed over the International Date Line! It turns out that everyone on board gets a special certificate called the “Domain of the Golden Dragon” to mark this event. This is just one of a set of unofficial certificates that began with the U.S. Navy! If you spend enough time at sea, you can amass quite a collection- there are also certificates for crossing the Equator, Antarctic Circle, Arctic Circle, transiting the Panama Canal, going around the world, and more…

I will award a prize to the first person who writes back to tell me what does it mean when one goes from a “pollywog” to a “shellback”, in Navy-speak!

Here is a picture of me with the largest pollock I have seen so far- 70cm!

If I am 5' 4", how many 70cm pollock would it take to equal my height? — If I am 5′ 4″, how many 70cm pollock would it take to equal my height?

Lastly, on to some, perhaps, cuter and more cuddly creatures than pollock- pets! Here in the hydroacoustics lab, there is a wall dedicated to pictures of pets owned by the officers, crew, and scientists:

Those are some pretty cute pets left ashore... — Those are some pretty cute pets left ashore…

Clearly, this is a dog crowd! I did learn, however, that our Chief Scientist, Taina, has her cat (Luna) up there! Students, do you remember the name of my cat and, what do you think, should I leave a picture of her up here at sea?

August 7, 2012August 11, 2021

Allan Phipps: Re-verify Our Range to Target… One Ping Only, August 6, 2012

NOAA Teacher at Sea
Allan Phipps
Aboard NOAA Ship Oscar Dyson
July 23 – August 11, 2012


Mission: Alaskan Pollock Mid-water Acoustic Survey
Geographical Area: Bering Sea
Date: August 6, 2012

Location Data
Latitude: 60°55’68” N
Longitude: 179°34’49” E
Ship speed: 11 knots (12.7 mph)

Weather Data from the Bridge
Wind Speed: 10 knots (11.5 mph)
Wind Direction: 300°
Wave Height: 2-4 ft with a 4-6 ft swell
Surface Water Temperature: 8.7°C (47.6°F)
Air Temperature: 8°C (46.4°F)
Barometric Pressure: 1013 millibars (1 atm)

Science and Technology Log

Previously, we learned how the biological trawl data onboard the NOAA Research Vessel Oscar Dyson are collected and analyzed to help calculate biomass of the entire Bering Sea Walleye pollock population. Last blog, I mentioned that the scientific method for estimating the total pollock biomass is not complete without acoustics data, more specifically hydroacoustics! In fact, hydroacoustic data are the real key to estimating how many pollock are in the Bering Sea! That is why our mission is called the Alaskan Pollock Midwater ACOUSTIC-trawl Survey.

Screenshot showing our transects on leg 3 of the pollock midwater acoustic survey. Fish icons indicate where we validated acoustic data with biological sampling. Hydroacoustic data were collected continuously along north/south transects.

The Oscar Dyson is using hydroacoustics to collect data on the schools of fish in the water below us, but we do not know the composition of those schools. Hydroacoustics give us a proxy for the quantity of fish, but we need a closer look. The trawl data provide a sample from each aggregation of schools and allow the NOAA scientists that closer look. The trawl data explain the composition of each school by age, gender and species distribution. Basically, the trawl data verifies and validates the hydroacoustic data. The hydroacoustics data collected over the entire Bering Sea in systematic transects combined with the validating biological data from the numerous individual trawls give scientists a very good estimate for the entire Walleye pollock population in the Bering Sea.

So what is hydroacoustics and how does it work???

Hydroacoustics (“hydro” = water, “acoustics” = sound) is the field of study that deals with underwater sound. Remember, sound is a form of energy that travels in pressure waves. Sound travels roughly 4.3 times faster in water than in air (depending on temperature and salinity of the water). Here is a link with an interactive animation comparing the speed of sound in water, air, and steel! This change in speed will become very important later… keep reading!

Lower sound frequencies travel farther. This is how humpback whales can communicate over great distances with their whale songs! Click on whale songs to hear one!

Whales are not the only aquatic organisms to use sound! Much like dolphins use sound to echo-locate, people use technology to “see” under water using sound energy. We call this technology SONAR (Sound Navigation And Ranging).

dlphfish1 — An animation of dolphin echo-location (courtesy of Discovery of Sound in the Sea).

On a typical recreational watercraft, this technology can be found in the form of a “fish-finder.”

sonar2 — Recreational “fish-finders” can be found on many personal watercraft (courtesy of Discovery of Sound in the Sea).

In commercial fishing, this technology is used in much the same way, just on a larger scale. Here is an animation showing a commercial trawler using SONAR to locate fish.

colrmako — Commercial fishing boat using hydroacoustics to locate fish. This animation illustrates how a fish shows up as an arch on the onboard display (courtesy of Discovery of Sound in the Sea).

The Oscar Dyson has a much more powerful, extremely sensitive, carefully calibrated, scientific version of what many people have on their bass boats. These are mounted on the pod, which is on the bottom of the centerboard, the lowest part of the ship. The Oscar Dyson has an entire suite of SONAR instrumentation including the five SIMRAD EK60 transducers located on the bottom of the centerboard that operate at different Khertz, the SIMRAD ME70 multibeam transducer located on the hull, and a pair of SIMRAD ITI transducers on the trailing edge of the centerboard (one pointed toward the starboard side, the other toward port).

od-acoustics1 — Illustration of the Oscar Dyson showing the hydroacoustic transducers located on the centerboard and the hull of the ship.

This “fish-finder” technology works by emitting a sound wave at a particular frequency and waiting for the sound wave to bounce back (the echo) at the same frequency. The time between sending and receiving the sound wave determines how far away an object is, whether it be the bottom or fish. When the sound waves return from a school of fish, the strength of the returning echo helps determine the fish density (how many fish are there).

An echogram taken from the Oscar Dyson. Shades of yellow and red show extremely large, dense schools of fish. The solid red at the bottom of the picture is the bottom of the sea which is at 94.12 meters at this location.

Another piece of the puzzle… how reflective an individual fish is to sound waves. This is called target strength. Each fish reflects sound energy sent from the transducers, but why? For fish, we rely on the swim bladder, the organ that fish use to stay buoyant in the water column. Since it is filled with air, it reflects sound very well. When the sound energy goes from one medium to another, there is a stronger reflection of that sound energy. The bigger the fish, the bigger the swim bladder; the bigger the swim bladder, the more sound is reflected and received by the transducer. We call this backscatter, or target strength, and use it to estimate the size of the fish we are detecting. This is why fish that have air-filled swim bladders show up nicely on hydroacoustic data while fish that lack swim bladders (like sharks), or that have oil or wax filled swim bladders (like Orange Roughy) have weak signals.

pumkinseed-sunfish-670 — X-ray of fish showing the presence of a swim bladder (courtesy of DeAnza College).

Target strength is how we determine how dense the fish are in a particular school. Scientists take the backscatter that we measure from the transducers and divide that by the target strength for an individual and that gives you the number of individuals that must be there to produce that amount of backscatter. 100 fish produce 100x more echo than a single fish. We extrapolate this information to all the area of the Bering Sea to estimate the pollock population.

Transect_27_awt133 — A close look at part of Transect 27. In this echogram, the area backscatter numerical values are included. At the top of the water column, you can see what are probably jellyfish which have little backscatter since they have no swim bladders. Along the bottom are groundfish. In the center of the water column are several large schools of Walleye pollock with strong backscatter. The square that has a value of 2403.54 shows several large schools!

So the goal is to measure the hydroacoustic density along each transect and extrapolate that data to represent the entire survey area between transects (the area not sampled because the Oscar Dyson can’t cover every square meter of the Bering Sea). When you combine the hydroacoustic data for all of the 30 transects (a total of ~5,000 nautical miles in an area of 100,000 square nautical miles) and the lengths collected in the biological trawl data, you can convert the length data into target strength data to create a distribution of target strengths and find the average target strength for the population. In doing so, you get a complete picture of the Walleye pollock population in the Bering Sea.

Screen shot 2012-08-06 at 4.57.45 PM — The BIG picture. This is the combination of hydroacoustic data and biological trawl data analyzed to show what the entire walleye pollock population looked like for 2009 (courtesy of the Alaska Fisheries Science Center www.afsc.noaa.gov/Publications/ProcRpt/PR2010-03.pdf). Analysis is still being done on the current survey. This year’s results will be out in a report this fall. Expect some changes!

But there’s more!!! Scientists ALSO use hydroacoustic data when trawling to determine if they have caught a large enough sample size to collect fish length data to validate their target strength data. If you recall reading my first blog from sea that taught about the parts of the net, I wrote about and had a drawing of the “kite” on which the “turtle” was attached. The “turtle” is a SIMRAD FS70 trawl SONAR. It has a downward facing transponder that shows a digital “picture” of the size of the net opening. You can also see individual fish and/or schools of fish enter the net by watching this display. Since the scientists only need about 300 fish for a statistically significant sample, they watch this screen carefully so that they do not take more fish than they need. When the lead scientist thinks there are enough fish in the net, she gives the request to the Officer on Deck to “haul back.” Unlike commercial trawlers, a typical trawl on the Oscar Dyson only lasts 25 minutes. Sometimes, we are only officially fishing for 5 minutes if we pull through a large school.

What are the data telling us?

The Walleye pollock data suggest that the population is currently stable; however, there is some evidence of pollock in waters that have traditionally been north of their uppermost documented population range. Are warmer waters due to climate change to blame for this possible shift? Here is an interesting article that addresses this issue and raises several other trends regarding pollock population response to changes in food source and predation due to climate change. Click on the picture to open the article!

fishsticks_title1 — How might climate change affect fish sticks? Click on the picture to read more!

The economic and ecological implications of a shifting pollock population range are a bit unsettling. Fish do not know political boundaries. As the pollock population range possibly shifts north, more of that range will lie within Russian waters than in previous years. This may hurt the U.S. commercial fishing industry as they settle for less of a resource that was once abundant. Since quotas are set based on last year’s numbers, there is a time lag which may result in overfishing in U.S. waters that might lead to a collapse in the Alaskan Walleye pollock fishing industry. The U.S. has invested a tremendous amount of research into maintaining a sustainable pollock fishery. Other countries may be responding to a variety of factors in which sustainability is just one when they are managing pollock stocks and setting catch quotas. Since pollock is a trans-boundary stock, this could lead to greater uncertainty in management of the entire population if pollock increasingly colonize more northern Bering Sea waters as influenced by climate change.

Food for thought…

Next blog, we will learn about cutting edge technology that may eventually make hauling back fish and collecting biological fish data on board the acoustic survey missions obsolete.

Personal Log

It’s tomorrow, TODAY! This morning at 6am Alaska Time, we crossed the International Date Line (IDL). The IDL is at 180° longitude. General Vessel Assistant Brian Kibler and I went out to the bow of the ship so we would be the first onboard to cross the line!

BeringSea — Map of the Bering Sea showing both the International Date Line and the 180th longitude. Our closest point to Russia was 12 nautical miles from Cape Navarin which is very close to 180 longitude.

Over the next two days, our transects take us back and forth over the IDL 3 more times. Fortunately, onboard our Oscar Dyson time warp machine we simply observe the Alaska Time Zone (the time zone from our port of call). With everyone onboard operating different shifts, and with 24/7 operations, it would be quite confusing if we kept changing our clocks to observe the local time zone.

Mariners who cross the IDL when at sea are inducted into the “Order of the Golden Dragon” and receive a certificate with the details of this momentous crossing. There are several other notorious crossing that receive special recognition. They are:

▪     The Order of the Blue Nose for sailors who have crossed the Arctic Circle.
▪     The Order of the Red Nose for sailors who have crossed the Antarctic Circle.
▪     The Order of the Ditch for sailors who have passed through the Panama Canal.
▪     The Order of the Rock for sailors who have transited the Strait of Gibraltar.
▪     The Safari to Suez for sailors who have passed through the Suez Canal.
▪     The Order of the Shellback for sailors who have crossed the Equator.
▪     The Golden Shellback for sailors who have crossed the point where the Equator crosses the International Date Line.
▪     The Emerald Shellback or Royal Diamond Shellback for sailors who cross at 0 degrees off the coast of West Africa (where the Equator crosses the Prime Meridian)
▪     The Realm of the Czars for sailors who crossed into the Black Sea.
▪     The Order of Magellan for sailors who circumnavigated the earth.
▪     The Order of the Lakes for sailors who have sailed on all five Great Lakes.

July 29, 2012August 11, 2021

Allan Phipps: Let the Fishing Begin! July 28, 2012

NOAA Teacher at Sea
Allan Phipps
Aboard NOAA Ship Oscar Dyson
July 23 – August 11, 2012

Mission: Alaskan Pollock Survey
Geographical Area: Bering Sea
Date: July 28, 2012

Location Data
Latitude: 61°24’39″N
Longitude: 177°07’68″W
Ship speed: 3.8 knots (4.4 mph) currently fishing

Weather Data from the Bridge
Wind Speed: 6.9 knots (7.9 mph)
Wind Direction: 30°T
Wave Height: 2ft with 2-4ft swells
Surface Water Temperature: 8.7°C ( 47.7°F)
Air Temperature: 7.9°C ( 46.2°F)
Barometric pressure: 1005.8 millibar (0.99 atm)

OLYMPUS DIGITAL CAMERA — The NOAA Research Vessel Oscar Dyson at port in Dutch Harbor, Alaska.

Science and Technology Log:

Since the main goal of this voyage is the acoustic-trawl survey of the mid-water portion of the Alaskan pollock population, I thought I would start by telling you how we go fishing to catch pollock! This isn’t the type of fishing I’m used to… Alaskan pollock is a semi-demersal species, which means it inhabits from the middle of the water column (mid-water) downward to the seafloor. This mid-water survey is typically carried out once every two years. Another NOAA Fisheries survey, the bottom trawl survey, surveys the bottom-dwelling or demersal portion of the pollock population every year. I will begin by describing how we are fishing for pollock on this acoustic-trawl survey.

The Oscar Dyson carries two different types of trawling nets for capturing fish as part of the mid-water survey, the AWT (Aleutian Wing Trawl which is a mid-water trawl net) and the 83-112 (a bottom-trawl net that is named for the length of its 83 foot long head rope that is at the top of the mouth of the net and the 112 foot long weighted foot rope at the bottom of the mouth of the net). One of the research projects on board the Oscar Dyson is a feasibility study that involves a comparison of the AWT and using the 83-112 bottom-trawl net as if it were a mid-water net. The 83-112 is much smaller than the AWT, so there is concern with the fish avoiding this net and thus causing a reduction in catch. While the bottom trawl survey acquires good information on the bottom-dwelling pollock using the 83-112 bottom trawl, if they also used this net to sample in mid-water they could help “fill in” estimates of mid-water dwelling pollock in years when the acoustic mid-water trawl survey does not occur.

AWT model commented — Scale model of the Aleutian Wing Trawl (AWT) net courtesy of NOAA Scientist Kresimir Williams

When the net is deployed from the ship, the first part of the net in the water is called the cod end. This is where the caught fish end up. The mesh size of the net gets smaller and smaller until the mesh size at the cod end is only ½ inch (The mesh size at the mouth of the net is over 3 meters!).

The AWT is also outfitted with a Cam-Trawl, which is the next major part that hits the water. This is a pair of cameras that help scientists identify and measure the fish that are caught in the net. Eventually, this technology might be used to allow scientists to gather data on fish biomass without having to actually collect any fish (more on this technology later). This piece of equipment has to be “sewn” into the side of the net each time the crew is instructed to deploy the AWT. The crew uses a special type of knot called a “zipper” knot, which allows them to untie the entire length of knots with one pull on the end much like yarn from a sweater comes unraveled.

Along the head rope, there is a piece of net called the “kite” where a series of sensors are attached to help the scientists gather data about the depth of the net, the shape of the net underwater, how large the net opening is, determine if the net is tangled, how far the net is off the bottom, and see an acoustic signal if fish are actually going into the net (more on these sensors later, although the major acoustic sensor is affectionately called the “turtle”).

Once the kite is deployed, a pair of tom weights (each weighing 250 lbs), are attached to the bridal cables to help separate the head rope from the foot rope and ensure the mouth of the net will open. Then, after a good length of cable is let out, the crew transfers the net from the net reel to the two tuna towers and attach the doors. The doors act as hydrofoils and create drag to ensure the net mouth opens wide. Our AWT net usually has a 25 meter opening from head rope to foot rope and a 35 meter opening from side to side.

IMAG0775 — This picture shows the A-frame with the two tuna towers on either side. The AWT is being deployed down the trawl ramp on the stern of the ship.

The scientists use acoustic data to determine at what depth they should fish, then the OOD (Officer on Deck) uses a scope table to determine how much cable to let out in order to reach our target depth. Adjustments to the depth of the head rope can be made by adjusting speed and/or adjusting the length of cable released.

The scientists use more acoustic data sent from the “turtle” to determine when enough fish are caught to have a scientifically viable sample size, then the entire net is hauled in. Once on board, the crew uses a crane to lift the cod end over to the lift-table. The lift-table then dumps the catch into the fish lab where the fish get sorted on a conveyor belt. More on acoustics and what happens in the fish lab in my next blog!

Personal Log:

WOW! What an adventure!!! So I must get you caught up on some of the happenings thus far. After a mix-up where my reservation was cancelled on the Saturday afternoon flight from Anchorage to Dutch Harbor and the threat of being stranded in Anchorage for another day, I finally made it to Dutch. The weather cooperated (which is not the case more often than not), and we landed on Dutch Harbor after a quick refueling stop in King Salmon. Since we landed after 8pm, we went straight to one of the few restaurants in Dutch Harbor and had a late dinner before heading to the Oscar Dyson for the night.

IMAG0751 — My flight after landing in Dutch Harbor, Alaska!

Sunday morning, we went with several of the scientists out to Alaska Ship Supply to get some gear. I picked up my obligatory “Deadliest Catch” shirt and hat as all tourists do here in Dutch Harbor. We made three trips to the airport throughout the day to see if some of the science gear and luggage came, but came back disappointed. On one of our trips to the airport, we had lunch at the airport restaurant. I had Vietnamese Pho, which is a beef noodle soup, but it wasn’t nearly as good as the Pho my wife makes. 🙂 We also drove up the “Tsunami Evacuation Route” to an overlook where we could see all of Dutch Harbor and the town of Unalaska. Later, we drove around Unalaska and stopped to check out some tidal pools on our way back to the Oscar Dyson. In the afternoon, we checked out the World War II museum that was absolutely fascinating! I did not know Dutch Harbor was bombed by the Japanese and that so many American soldiers were stationed in the bunkers surrounding the harbor. For dinner, I had black cod (sablefish) at the Grand Aleutian Hotel. Yummy!

Monday we embarked on our adventure shortly after noon. We had to leave the dock because another ship was scheduled to offload there in the afternoon. The scientists’ equipment arrived on a late Monday morning cargo flight, but they didn’t make it to the ship on time!!! We couldn’t go to sea without them, so we deployed the “Peggy D” to go pick them up and bring them aboard!

IMAG0759 — The Peggy D brings our scientists Rick and Kresimir with their long-awaited research equipment to the Oscar Dyson so we may head out to the Bering Sea!

Once we had our missing scientists, we left the safety of Dutch Harbor and ventured into open water. On our way, we saw dozens of humpback whales! None of the whales breached (jumped out of the water), but several of them fluked (dove and put their tail out of the water).

We started our day and a half journey to get to the starting point of our survey transects (the end point of last month’s survey). On our trip out, we experienced 6 to 10 ft seas and a 25 knot wind. It was a “gentle” welcome to the Bering Sea, but I struggled to get my sea legs underneath me. Meclizine is great motion sickness medication, but it sure knocked me out. I feel better now that I am not taking anything and am used to the rocking deck. While we made our way to our first transect, we had a couple of emergency drills. Here I am with fellow Teacher at Sea, Johanna, in our immersion suits as we completed our abandon ship drill.

On Wednesday morning, we began our first transect and did our first trawl along the transect (more on that later). I learned how to work in the fish lab collecting biological data on the catch we brought on board. I have been struggling to adjust to both my shift, which is 4am to 4pm, and the fact that the sun sets around 1am and rises at about 7am.

Thursday morning I woke on time and observed the survey scientists and crew deploying the CTD (Conductivity, Temperature, Depth) rosette from the hero deck (on the starboard side).

IMAG0828 — Skilled Fisherman Jim is assisting with deploying the CTD.

We also had beautiful clear skies and I was able to see Venus and Jupiter. At sunrise, I saw the GREEN FLASH!!! It was a beautiful start to the day.

We processed one mid-water AWT (Aleutian Wing Trawl) trawl that was all pollock, then switched to the 83-112 bottom trawl net (83 foot long head-rope and 112 foot long foot-rope) and pulled up a lot of jellyfish with our pollock.

Last night, I finally got a really good night sleep! This morning (Friday), I watched the CTD deployment again and learned more about the data being collected (more on this later). No spectacular sunrise this morning as it was the typical gray, foggy weather. I went up and spent some time on the bridge and Chelsea, our navigator/medic, taught me a lot about the instrumentation used for navigating the ship. There sure is a lot of technology on board!!!

From the bridge, we saw a pod of Dall’s Porpoise feeding, splashing around, and moving fast! We processed another AWT trawl of pollock that had quite a few herring mixed in. We traveled further into Russian waters than originally anticipated as we tried to identify the northern boundaries of the pollock population to get the best picture of the entire pollock range. We spotted a huge Russian trawler from the bridge!

We then headed south again towards American waters, but needed to do a quick water column profile test. Since we did not want to stop to drop the CTD again, I got to deploy a XBT (Expendable Bathythermograph)! After all the talk about safety briefings, the use of ballistics, and outfitting me with every piece of safety gear we could muster, I got ready to fire the XBT!!! Turns out, when you pull the firing pin, the XBT just slides out of the tube… no fireworks, no big bang… just a small kurplunk as the XBT enters the water. We all had a good laugh at my expense. See, scientists know how to have fun!

WOW! So I have just scratched the surface of our voyage thus far! Next time, I will give you a snapshot of what life was like aboard the ship.

July 13, 2012August 11, 2021

Amanda Peretich: Awesome Acoustics, July 13, 2012

NOAA Teacher at Sea
Amanda Peretich
Aboard Oscar Dyson
June 30, 2012 – July 18 2012

Mission: Pollock Survey
Geographical area of cruise: Bering Sea
Date: July 13, 2012

Location Data
Latitude: 59ºN
Longitude: 174ºW
Ship speed: 11.7 knots (13.5 mph)

Weather Data from the Bridge
Air temperature: 7.3ºC (45.1ºF)
Surface water temperature: 7.6ºC (45.7ºF)
Wind speed: 4.3 knots (4.9 mph)
Wind direction: 12ºT
Barometric pressure: 1010 millibar (1.0 atm, 757.5 mmHg)

Science and Technology Log

How sonar works: energy (sound) waves are pulsed through the water. When it strikes an object, it bounces back to the receiver. (from http://www.dosits.org/)

Before stepping onto the Oscar Dyson, I wasn’t quite sure about much of the science going on. Did they just put the nets in the water every so often and hope to catch some fish? Carefully lean over the side of the ship saying “here fishy fishy” with the hope that the pollock would find their way into the net? Neither of these scenarios is correct (good thing I’m not actually a fisherman!). So today’s lesson is going to be all about what the chief scientist actually uses to find fish: hydroacoustics (hydro meaning water and acoustics meaning sound). This also involves SONAR, which is short for SOund Navigation And Ranging.

Fishfinding Basics — Fishfinding basics.

If you’ve ever been on a smaller boat, yacht, fishing vessel, or the like, you may have seen something called a fishfinder. The basic concepts are the same as what is happening on the Oscar Dyson. An echosounder sends a pulse of energy waves (sound) through the water. When the pulse strikes an object (such as the swim bladder in fish), it is reflected (bounced) back to the transducer. This signal is then processed and sent to some sort of visual display.

Swim Bladder — Swim bladder in a fish.
(from https://www.meted.ucar.edu/)

The Oscar Dyson uses acoustic quieting technology where the scientists can monitor fish populations without altering their behavior. The Scientific Sonar System and various oceanographic hydrophones (underwater microphones) are raised and lowered through the water column beneath the ship on a retractable centerboard. This is important so that the transducers can be lowered away from the flow noise generated by the hull, which in turn will improve the quality of data collected. In addition, there is a multibeam sonar system located on the forward hull. Ultimately the hydroacoustic data is all used as one piece to the puzzle of measuring the biomass of fish in the survey area.

OD acoustics — The different sonar signal transmitter/receivers (transducers) used on this leg of the pollock survey and their location on the ship.

Neal at work — Chief scientist Neal working away in the Acoustics lab. The second screen from the left on the upper row is showing the information from the ME70 multibeam.

So how does this all work when we are looking for fish? The chief scientist (Neal on the 0400-1600 watch) or another scientist (Denise on the 1600-0400 watch) will spend a lot of time analyzing the various computer screens in the acoustics lab, which has been affectionately termed the “cave” (no windows). They are looking at the information being relayed from both the multibeam and the EK60.

What is a multibeam? The Oscar Dyson has the Simrad ME70 scientific multibeam echosounder. It is located on the hull (underside) of the ship on the front half and sends 31 sonar beams per second down to the bottom of the sea floor.

Aft of the multibeam (on the centerboard) are the five Simrad transducers. It may seem confusing, but hopefully I can walk you through a teensy little bit of how it works when we are looking to trawl for fish.

EK60 Transducer — Information from the EK60 transducer at 18kHz (top) and 38kHz (bottom).

Information from the EK60 echosounder is displayed on the far left screen in the acoustics lab while information for the ME70 multibeam is displayed on the next screen. The darker patches are showing that there are fish in that area. When the scientist first starts to see a good amount of fish, they will “mark” it and keep watching. If the screen fills up with fish (as in the EK60 image), the scientist will call upstairs to the bridge and tell them where to head back to on the transect line to start trawling. Depending on the location of the fish in the water column, it may be a bottom trawl (83-112 net), a midwater trawl (AWT net), or a methot trawl. Side note: the 83-112 midwater comparison trawl that I’ve mentioned before is done almost immediately after an AWT midwater trawl to compare the fish caught in a common area.

Neal on bridge — Chief scientist Neal up on the bridge.

Then the scientist will head upstairs as the deck crew is preparing the net. One of the many sensors attached to the net is called the FS70 fishsounder or “the turtle”, and it is only used during trawls (because it is attached to the headrope). The scientist can “watch” the fish swimming under the ship using the EK60 information combined with the information from the fishsounder. The yellow “turtle” on the right in the image shows how the FS70 is flying in the water. You want minimal pitch and roll and for the front of it to be facing the back of the ship. This way, we can “see” the fish as they are going through the net. The officer of the deck and lead fisherman or head boatswain can adjust various things to keep the turtle in the right orientation. The middle image below is constantly changing on the screen in the bridge as the sonar is sweeping back and forth, so you can almost watch the individual fish enter the net. It was interesting to watch the delay between when you would see the fish from the EK60 (on the left) and when you saw them with the FS70 (middle).

Trawl Fishsounder — Display screens on the bridge used during a trawl.

Once the scientist is satisfied that enough fish have been caught for a sufficient sample size, the net will be hauled back and the acoustics work is done for just a little bit (giving Neal some time to grab some well-deserved coffee and the rest of us time to get our rain gear on to process the fish).

So some of the questions I had asked (that don’t really fit nicely in the information above):

Why do we use different frequencies in the acoustic studies?

Frequency Wavelength — Relationship between frequency and wavelength. (from http://emap-int.com)

This ties right back in to chemistry (and other sciences) with an equation and the relationship between frequency and wavelength (yay!). Basically there is an inverse relationship which means that at a high frequency there is a smaller or shorter wavelength (wavelength is the distance for peak to peak of a wave). At a low frequency, there is a higher or longer wavelength.

At a low frequency, you will see only see things that are larger, like pollock, whereas you will see very small things like krill and zooplankton at higher frequencies. Having information from both types of frequencies is necessary to complete the scientific research on the Oscar Dyson.

Is it possible to see a single fish?
Yes! From sunset to sunrise, the Oscar Dyson doesn’t actually travel the transect lines. This is because the pollock behave differently during darkness than during the day. So instead of traveling between 11 and 12 knots (which is what happens between trawls), it’s almost like the boat is just sitting around for a couple of hours. But during this time, since the boat isn’t moving along quickly, it’s possibly to see individual fish on the sonar as shown in the image.

Hydroacoustics — Hydroacoustic surveys can involve any number of different types and locations of the transducers. (from http://btechgurus.blogspot.com/2012/06/sonar.html)

Personal Log
Today is Friday the 13^th but it was far from unlucky – I finally saw something out in the water other than fog: a boat! Again, all good sightings seem to come from up on the bridge, so I’m thankful for Lieutenant Matt for allowing me to ask a billion questions while I’m up there and teaching me more than I ever thought my brain could hold. He has all of the qualities of a great teacher, which is nice to see.

Ship — The ship we saw up on the bridge this morning from about 5 nautical miles away (left), on the sonar (middle), and through the binoculars (right).

Dancing in the fish lab on the Oscar Dyson — Neal and I dancing while waiting for the fish!

Highlight from the other day? Chief scientist Neal finally dressed out in his Grundens (rain gear) and came to help process a catch in the fish lab! While waiting, he even took a quick second to dance in the doorway (we were “Dougie”-ing) to my music that was playing over the speaker system.

References
NOAA Oscar Dyson flier
NOAA Oscar Dyson Ship Electronics Suite
HTI Sonar
Wikipedia: Sonar
Simrad

June 25, 2012August 11, 2021

Lesley Urasky: Do You See What the Pisces “Hears”?, June 22, 2012

NOAA Teacher at Sea
Lesley Urasky
Aboard the NOAA ship Pisces
June 16 – June 29, 2012

Mission: SEAMAP Caribbean Reef Fish Survey
Geographical area of cruise: St. Croix, U.S. Virgin Islands
Date: June 22, 2012

Location:
Latitude: 18.5472
Longitude: -65.1325

Weather Data from the Bridge:

Air Temperature: 28.6°C (83.5°F)
Wind Speed: 9 knots (10.5 mph), Beaufort scale: 3
Wind Direction: from SE
Relative Humidity: 77%
Barometric Pressure: 1,014.80 mb
Surface Water Temperature: 28.1°C (82.6°F)

Science and Technology Log

Another aspect (much more technical) of the scientific research conducted on this cruise is the collection of acoustic data. This field is continually evolving as the detection resolution improves allowing scientists to more precisely identify fish. This has been used with more success in fisheries farther north because the schools of fish are more likely to be monospecific (a single species). However, the technique still needs improvement in warmer waters where the fish assemblages tend to be multi-specific (having a much greater variety of fish).

General idea behind an acoustic sounder being used to detect fish. (Source: www.biosonicinc.com)

This field of study is called Hydroacoustics (hydro- means water, and acoustics refers to sound). It is the science of how sound moves through water. Leonardo da Vinci noticed how sound travels through water in 1490. He noticed that, “If you cause your ship to stop and place the head of a long tube in the water and place the outer extremity to your ear, you will hear ships at a great distance from you.” (Urick, Robert J. Principles of Underwater Sound, 3rd Edition. New York. McGraw-Hill, 1983.) World War I helped promote innovation in the field, especially with the need for anti-submarine detection devices (Wood, A. B., From the Board of Invention and Research to the Royal Naval Scientific Service, Journal of the Royal Naval Scientific Service Vol 20, No 4, pp 1-100 (185-284)).

Hydroacoustic instruments utilize SOund Navigation and Ranging, more commonly referred to as SONAR. The ship Pisces is equipped with a system located on the center board; this is a flat structure that can be raised/lowered through the water column beneath the center of the ship.

Line drawing of the Pisces showing the location of the center board. — Line drawing of the NOAA ship Pisces showing the location of the center board.

The system used is a sonar beam that is split into quadrants. This instrument is used to assist in determining fish abundance and distribution. The premise is relatively simple: an echo sounder transmits a pulse of energy waves (sound), when the pulse strikes an object, it is reflected (bounced) back to the transducer. The echo sounder is then processed and sent to a video display. This is the same general process behind the recreationally available fishfinder.

Acoustic beam split into quadrants (Source: http://www.htisonar.com

A short burst of energy is focused into a narrow beam. When this beam encounters an object such as a fish, a school of fish, plankton, or other object, some of the energy bounces back up through the water to the transducer. It is the detection of these reflections that allow scientists to determine location, size, and abundance of fish. These reflections show up on our video monitor. These measurements are combined with groundtruthed data (for example, fish collected in the field, camera images).

One of the difficulties in data interpretation is that often, the signals that appear on the computer monitor have false readings. This is a result of the sound wave bouncing multiple times. It travels to the bottom from the transducer, strikes an object, returns to the ship, bounces off the ship back toward the bottom, strikes another object, and is detected yet again.

Real-time annotated echogram at sampling site.

The Pisces is actually home to one of six multi-beam acoustic instruments in the world. Of the six in existence, NOAA has five of them. The benefit of running a multi-beam instrument is that each beam can be set to measure a different frequency (kHz), thus enabling detection of many more features (different species of fish, etc.)

Scientific multibeam echo sounder (Source: www. simrad.com)

Personal Log

Last night the crew of the Pisces carried out a task that they don’t normally perform. The Pisces was created for fisheries research projects – it focuses on collecting fish samples either by bandit reel, longline, or trawling. This particular operation was to deploy the anchor for a buoy that will be attached at a later date. When the buoy is ready to be attached, another vessel will bring it out to the site and divers will go down to the anchor to make the final attachment.

The anchor consists of a huge rebar-reinforced concrete block with a very long chain that has marker floats attached at the end. Logistically, this took some planning; the A-frame had to be raised and the anchor lifted with the Gilson winch with a 1″ spectra line (has an enormous tensile strength). The gate to the ship’s ramp was lowered and the A-frame (or as the deck hands call it, the “Tuna Tower”) repositioned so the anchor was hanging over the water. The rope holding the anchor, chain, and float was cut through, and the anchor plunged to the ocean bottom. Again, the crew made the operation go smoothly and demonstrated their ability to complete unexpectedly assigned tasks.

Today was a slow fishing day – no fish at all. Without any fish to “work up” (collect samples from), the day goes more slowly and we have more down time. With the extra time, I had a chance to interview Kevin Rademacher, the Chief Scientist on the cruise.

LU: What is your official job title and what are your job duties?

KR: I’m a Research Fisheries Biologist. I work for the Reef Fish Unit at the NOAA Fisheries Lab in Pascagoula, MS. I am the Senior Tape Reader/Reviewer, in charge of the readers that analyze the video data we collect from Reef Fish Surveys. I also help plan, organize, and run the surveys. Additionally, I participate in trawl surveys and anything else the lab needs done.

LU: When did you first become interested in the ocean and marine sciences?

KR: I guess that would have been when I was really young. There is a photo from the Panama City, Florida newspaper, two weeks after I was born with my parents pulling me in a homemade wagon along the beach! I knew in junior high school that I wanted to be a cross between Jacques Cousteau and Marlin Perkins of Mutual of Omaha’s Wild Kingdom.

LU: It’s such a broad field; how did you narrow your focus down to what you’re currently doing?

KR: I got lucky and kind of fell into reading underwater videos at the initial stages of the project and fell in love with being the proverbial “fly on the wall”! It has allowed me to see the fish in their natural habitat, different color phases, behavior, etc.

LU: If you were to go into another area of ocean research, what would it be?

KR: Marine Mammal Studies. After college I trained dolphins and sea lions and put on shows with them for a local Oceanarium on the Mississippi Gulf Coast.

LU: What is the biggest challenge in your job?

KR: Communicating with people and writing papers.

Ariane and Kevin — Ariane Frappier and Kevin Rademacher reviewing a dichotomous key in order to determine the species of a fish we caught.

LU: What do you think is the biggest issue of contention in your field?

KR: The impression that commercial fishermen have regarding the work we do to regulate the fisheries they work in.

LU: What are some effects of climate change that you’ve witnessed during your career in fisheries research?

KR: The decline of coral reefs and overfishing of some species.

LU: In what areas of marine science do you foresee a lot of career paths and job opportunities?

KR: Ecosystem management and data modelers. There has also been a decline in taxonomists over the past few decades.

LU: How would you explain your work to a layperson?

KR: I use underwater cameras to help assess populations of reef fish, especially snappers and groupers. The data collected is used to manage those fisheries.

LU: If a high school student wanted to go into your field of study/marine science in general, what kinds of courses would you recommend they take?

KR: Math, Biology, Chemistry, and any other science courses available.

LU: Do you recommend students interested in your field pursue original research as high school students or undergraduates? If so, what kind?

KR: Most definitely! Whatever they are interested in would be beneficial.

Well, only two more days left with the scientists before we pull into San Juan, Puerto Rico. We have 17 more daytime sites to sample and then this survey will be over. The scientific crew will be flying home on the 25th, and once home, their work will really begin. Back in the lab, they will be analyzing the data and reviewing the video. Some of them will be going back out on other cruises. Kevin Rademacher will be going out on another reef fish survey in the eastern Gulf of Mexico. It is currently delayed because of the potential formation of tropical storm Debby. Joey Salisbury has a couple more; he will be going on a longline cruise and then another reef fish survey, both of which will be in the Gulf of Mexico. Arian Frappier will be heading off to begin a masters program in marine systems and coastal studies at Texas A&M Corpus Christi.

After a day’s shore leave in San Juan, I’ll continue on to Mayport on the Pisces. During this time, I’ll focus on the crew members and their jobs. The cruise will definitely take on a different feel at this point, but it will give me an opportunity to explore other ocean related careers.