Tue 28 Oct 2008
Seeing with Sound
If you were a dolphin (or other toothed whale) and you lived in the deep ocean, your eyes wouldn’t help you catch a fish or krill. Your eyes would be useless to you. So how are you going to find food and hang out with your friends? No problem, you have skills: echolocation takes over where your eyes leave off.
As a dolphin you can emit sounds like high pitched clicks. When these clicks hit an object, some of the sound will echo back. By listening to the echo, interpreting the time it takes to return and it’s strength, you can tell if it is a fish, big or little.
Similarly, we are also looking for fish and krill here on the R/V Melville. Our equipment sends out pings, and then captures the sound wave when it returns, with special technological tools such as “the Pod”. ( This apparatus looks to me like a big foot attached to a big fat metal light pole stuck to the port side of the vessel.) It uses echo sounding to convey information about the fish and krill. A little about sound waves: the lower the frequency of the wave the deeper it can go. The lower frequency has a longer wavelength. We use four different frequencies (measured in kilohertz) 38, 70, 120 and 200 to find krill and fish. At 120- 200 khz you can “see krill”. At 38-70 khz you can “see fish”.
Scientists look at visual displays produced by echo sounding and make interpretations about fish type, size and number of fish. Even more fantastic, we can see right down to the bottom of the ocean and all of the minute distributions as we go. There are some really fascinating facts to be extrapolated from the data. For example, some fish and plankton go on migrations! We usually think of a migration as a horizontal movement, the journey across the sea of an animal like a sea turtle, for example. But some sea creatures go on vertical migrations. To explain, many organisms will stay down below the euphotic zone (where light penetrates) because they don’t want to get eaten. At night, they rise up toward the surface to dine on chlorophyll-rich phytoplankton and the small zooplankton. When the sun comes up they travel back down to hide in the dark. This vertical migration is information “the pod” sends to the computers on board our ship. The scientists can see the biomass and the line of descent on the monitors.
The image to the right is a screen shot taken on October 8, 2008 that shows this vertical migration.
Seeing these echos and interpreting them is not enough. We use the trawl method to catch specimens so that we can identify and classify the various species. The trawl nets catch a fraction of the fish (approximately 15%) that is moving along when we cross their paths. But fish can be sneaky. They avoid the nets. So by using both acoustics and a trawl we can get a pretty good picture of what is out here. Each sampling method is like a little window. With each window we have a different perspective. Working as a team, all of the scientists collaborate and share the data they gather. When you put all of the windows together we have a really big view. Last but not least, the Scripps scientists and R/V Melville crew REALLY know how to work like a team, collaborating and sharing their scientific discoveries about the California Current Ecosystem. What an inspiration you all are to me.
Thank you Tony Koslow, SIO, CalCOFI Director for explaining your scientific contributions on board, for editing this report and for sharing with me all the unique creatures of the deep!
Signing off, Laurie Guest, Your Teacher at Sea