NOAA Teacher at Sea: Story Miller
NOAA Ship: Oscar Dyson
Time: 1940 ADT
Wind: 8 knots (approx. 9.2 mph or 14.8 km/h)
Direction: 270° (W)
Sea Temperature: 9.2°C (approx. 48.6°F)
Air Temperature: 9.1°C (approx. 48.4°F)
Barometric Pressure (mb): 1007
Swell Height: 1 foot (about 30.5 cm)
Wave Height: 0-1 foot (about 30.5 cm)
There are many different groups of people working aboard the ship, Oscar Dyson – Scientists, NOAA Corps officers, Deck Hands, Engineers, Survey Technicians, and Cooks. Within the science department, there are 12 members aboard and two Teachers at Sea which totals to 14 souls. For this third leg of pollock surveys, the chief scientist is Taina Honkalehto. Her job aboard the ship is to plan the scientific activities and make the decisions on how best to carry out that plan. Of the scientist crew, there are two Russian scientists that are conducting their own research in collaboration with NOAA.
This pollock survey, which focuses on determining abundance and distribution, is an important component of the fishing industry in the United States. According to The Bering Sea Project, “The largest concentrations of pollock occur in the eastern Bering Sea,” and more specifically, “Walleye pollock support the largest single commercial fishery in the U.S., producing the largest catch of any one species inhabiting the 200-mile US Exclusive Economic Zone.” Additionally, the pollock industry is incredibly important to the people living in Dutch Harbor and Unalaska because pollock is one of the main fishes processed there and has helped classify Dutch Harbor as America’s #1 fishing port in the USA for fish landed (NOAA, 2009).
There are two summer surveys being conducted to estimate the Bering Sea pollock population: Acoustic-Trawl Survey and the Bottom-Trawl Survey. Currently on the Oscar Dyson we are conducting the Acoustic -Trawl Survey. After we catch the fish, we combine the acoustics, fish samples, and CTD deployment data, to draw conclusions that help us estimate population size and ecological factors of pollock. Remember, in order for pollock to live where they do, they need food and so when we extract stomach samples, we are looking for what pollock prey upon (mostly krill). Besides, food, other important aspects of their habitat must be in place for their survival. The CTD data – water temperature, salinity, nutrients, oxygen, and chlorophyll – help us understand how the distribution of pollock has changed in past years and may also provide information about how it could change in the future.
However, not all of the scientists on board are collecting data related to pollock. Currently we have two other subgroups with one observing seabirds and the other observing marine mammals. The crew observing seabirds have a goal of observing species seen during the tour to determine seabird species distribution and abundance. The marine mammal observers are working to obtain current data on cetacean species distribution and abundance.
The Teachers At Sea (TAS), which currently include Obed Fulcar (New York, New York) and myself (Dutch Harbor, AK) have an important role of working under the scientists and other crew members to learn about the research being conducted in an attempt to bring real science into the classrooms.
Because acoustics is a major tool used in pollock survey, I feel it would be beneficial to provide a few details on how it works. Remember, referring to Blog #2 “the ship has Transducers that send pings of sound energy down through the ocean and when they hit some object, such as the bottom of the ocean or a fish, in this case they are hitting the swim bladders of the fish, some of the energy in the sound ping is returned to the ship and received by our echo sounding system in the acoustics lab of the ship.” It is important to note that the acoustics under the water are different than in the air because the pressure in each location is different. Inside the acoustics lab there are many different screens that display the pings at different frequencies of sound waves. We know that jellyfish tend to show up the best from the low frequencies. Acoustics is a good tool to use to study pollock because pollock is the primary fish species inhabiting the middle-waters of the Bering Sea shelf. For example, bottom fish are difficult to see because the acoustic signals from the seafloor are too strong and tend to hide the bottom fish signals. Acoustic signals that we see on the computer screen rely on the actual physiological make-up of the fish. Also, the behavior of pollock plays a role in how we can see them acoustically. For example, salmon do not swim in large schools like pollock. When we see large schools of pollock on the acoustic screens, density determines the color – blue usually is reflecting a couple fish whereas red represents a high density of fish – and the shape of the schools tend to be typical of pollock. Through acoustics, we are able to survey pollock over a wide area and gain information regarding their distribution and population.
Prior to fishing, we consistently monitor the screens as the ship travels up and down the rectangular transects you can see when you view the ship’s path on ShipTracker. When we observe schools of fish, we need to decide whether they are large enough to sample the fish with the trawl. Because we also want to target certain ages of fish, it is important to be able to estimate their size.
We can estimate size through a method using additional measurements from the acoustic data. We draw a box around an area that is not densely packed with pollock so it is easier to distinguish an individual acoustic image of a fish. The software we have gives us the average intensity of the acoustic pixels. We call this intensity target strength which translates to the size of the echo. Because the size of the swim bladder is proportional to the size of the fish, we can use the intensity of the echo off the swim bladder to estimate the size of the pollock. In short, target strength depends on the size of the swim bladder and features of the swim bladder can be used to predict fish size.
We can use an equation for calculating decibels to help us estimate the size of the fish in the school we might target. For my friends and students who are math gurus, the equation is TS = 20Log(length cm) + b20. The b20 variable is different for different fish species and so for Walleye Pollock in the Bering Sea, b20 is -66. Therefore, the equation for Walleye Pollock is TSpollock = 20Log(length cm) – 66.
To provide an example of how the equation works, lets say that the average length of a two year-old pollock is 25 cm and that is the size we want to target. We take that 25 centimeters and “plug it” into the section of the equation that stands for length in centimeters. Scientific calculators are wonderful devices for logarithms as they have the Log function already installed, and if you plug in 20Log(25) – 66 into the calculator, the answer -38.4 translates into the target strength that would show up on the screen. So if we find schools of pollock and see that the target strength is close to -38.4, then we know the echosounder is observing two-year old pollock.
Once acoustics have determined that we need to fish, they send the coordinates they want the Officer of the Deck (OOD, a.k.a. the NOAA Corps officer on watch on the bridge) to follow and the officers drive the ship to the location. On the bridge of the ship, the scientists are able to see the acoustic screens and are able to keep an eye on the location of the fish, relative to the transducer underneath. From there the Lead Fisherman or Chief Bosun operates the machinery required to put the trawls in the ocean. After the large mesh net is placed in the ocean, the crew put on a sensor that measures water depth and temperature. They also install a tool, called a headrope unit, that is similar to a mini transducer which makes an image of the mouth of the net and allows the scientists to watch fish entering the net from the bridge.
Once the fish are caught, the deck crew will draw the nets back onto the boat using hydraulics. From the stern (back of the boat), the fish go into the fish lab on a conveyer belt where we sort, sex, measure, and extract stomachs and otoliths. Since being on the ship, during my shift we have been averaging two trawls per day.
My morning started off fantastic as I was able to launch an XBT into the water again. By the time I was beginning to type this blog we passed over a school of pollock and decided that we needed to turn around and go fishing. Approximately two hours of sorting commenced before I was able to return. I learned that acoustics is a very difficult concept to explain as there are many factors in mathematics and physics that are complicated to translate into layman’s terms. I ended up spending a lot of time reading a textbook on the research the theories of using acoustics on wild fish. Please do not hesitate to ask in the comment box below this post if you have questions!!!
Overall, there was a good assortment of fish today and I stayed fairly busy in the fish lab collecting pollock sample data!
Animals Seen Today:
Northern Fur Seal
Something to Ponder:
Life at sea can be an amazing experience but there are many things people may take for granted when living on land. For example, consider the possibility of becoming hurt on the job, or developing a medical condition such as a rash or appendicitis. From the middle of the ocean, it is very difficult to reach a doctor to get a diagnosis. On board the ship, we have some medical supplies but typically there is not a licensed doctor on board the ship. Would you know how to respond to an emergency if it were to happen? If you have taken a First Aid or CPR class, do you remember what you need to do? How would you react? What would you do to reach help? Who could respond to your call?
For the Oscar Dyson we have the following protocols:
1. Contact the medical officer on board for an initial diagnosis.
2. If the condition requires advanced medical care, he or she will contact the medical officer on call at the NOAA Marine Operations Center.
3. In the case of an emergency and when the Marine Center cannot be contacted, he or she will contact the Maritime Medical Assistance (MMA).
4. If needed, we will arrange for a medevac (medical evacuation) which could involve the US Coast Guard and/or head back to port.