NOAA Ship: Oscar Dyson
When you walk into the acoustics lab you are greeted with an impressive display of primary colors and fascinating images. Sandy, our Acoustician is also there to greet you and help explain the science behind the images of sound. She explained not only the basics of acoustic science; but also shared some fascinating biological phenomena that can be witnessed with this technology.
So first some basics about the acoustics. (Hoping BTW to make Sandy proud about her skills in teaching a physics phobic.. She only made my head hurt a few times..) When you walk in the Acoustics Lab on the Oscar Dyson you will see there are six different acoustic displays in the lab:
- 18 kHz & 38 kHz on one display (These are more common to fishing vessels to distinguish larger fish from jellies, zooplankton and juvenile fish)
- 70 kHz, 120kHz, 200kHz and a 70 kHz with a sideway view from the ship.
The acoustic sounders positioned on the ship’s centerboard emit a ping that is transmitted directly downward from the boat (except one 70kHz pointed sideways).
These pings each have a set characteristic frequency designated by the unit of a kiloHertz (kHz). A kilohertz simply is the thousands of cycles per second that the wave is transmitted. Frequency is indirectly related to wavelength. So if you think about what will fit in-between the waves in the left image it will make sense that lower frequency acoustics are used to identify larger things, while higher frequency captures images of much smaller species or individuals.
Acoustics have the potential to not only identify schools of fish, but also discriminate between species types as well. A characteristic scattering is observed from different types of fish depending on their internal structure (morphology) and composition. For example, whether or not a fish has a swimbladder can be used for identification. A swimbladder will cause a greater acoustic scattering. In terms of composition, jellyfish are over 99% water. The more like the composition of water, the less the sound bounces off the specimen. Therefore, the scatter from the ping of the acoustics is weak and difficult to see on the monitor. However, the jellyfish signature is shows up quite strong on the acoustic monitors. In this case, the size and shape of the jellyfish causes the sound to scatter regardless of its’ composition. So acoustic analysis is not always as straight-forward as the scientists (and fishermen) would like. So how do the scientist tell a jellyfish from a juvenile salmon? Trawling data.. Part of the acoustic’s mission on our BASIS cruise. The scientists would like to develop patterns to match trawls with acoustic signals. Therefore, acoustics can be used in the future more effectively to track and monitor pelagic populations.
Using the 200kHz acoustics, tracking the movement of the zooplankton is quite easy. In the image to the left, taken and archived during the evening hours you can literally see how the zooplankton migrate toward the ocean surface as the sun sets(Around 10PM in these parts..). Trying to avoid their predators, the zooplankton stay near the ocean bottom during the daylight hours, but migrates upward toward their food source, the phytoplankton, once darkness begins to onset.
Riding the Pycnocline
Another interesting physical oceanographic feature you can observe with acoustics is the pycnocline. While you can’t literally see the density change of the water using the acoustics aboard this ship, you can watch the fish hover immediately above this feature.
Yesterday morning’s sunrise was one for the books.. Tuesday was a glorious, sunny day aboard the Dyson.. (Uh.. the answer is YES.. apparently Alaskans do sunbathe in 50 degree weather. As long as the sun is out… I won’t mention any names.) The daytime turned to an evening with a sky full of stars. We then were treated to a spectacular sunrise the next morning with beautiful calm seas. Thanks to Sandy, who captured the picture to the left that morning, while most of us were busy eating omelets and pancakes in the Mess Hall.
It’s fun sometimes when Sandy’s right….
After days of searching for the juvenile salmon, we finally found their playground. One of trawls yesterday brought us over 2,000 sockeye juveniles along with a mess of jellies… After accosting my colleagues with a few paparazzi moments, it was fun to join in to help sort out the catch. And even sometimes when things don’t work out perfectly, finding what you’re looking for makes everything better. To the right is a snapshot of what happens when the bin doesn’t stop in time and the fish/jelly mess overtakes the belt and scientists. Now this is fishin’….