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

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

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.

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).

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

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.

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).

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).

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.

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.

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.

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!

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!

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.