Tag Archives | bioacoustics

Listening for Echoes of Krill

Today we went out to search for krill with Dr. Kim Bernard of Oregon State University and Shenandoah Raycroft, her assistant. Krill are an important stepping stone in Antarctic food chains because they eat tiny phytoplankton and then become food for almost all the large Antarctic animals including penguins, seals, and whales. Dr. Bernard and Raycroft use a machine called an echosounder to detect krill in the waters around Palmer Station.

Dr. Bernard set out to sample two areas based on tips she’d received from the radar team and the penguin team. The penguin team had noticed Adelie penguins were staying close to their colony on Torgersen Island, possibly meaning there were krill in those waters. The radar team had noticed an area of light currents near Cormorant Island that might also hold krill. This ability to quickly see data in the field and respond to what it’s telling them is called adaptive sampling, and it’s a major strength of the CONVERGE project.

Read through the slideshow to find out more about how Dr. Bernard’s team measures krill, and then check the bottom of the post to see some of their results:

photocrati gallery

After the team got back to Palmer Station they analyzed the data they had collected. The result was a plot like the one below that shows the readings that the echosounder recorded. The graph shows time on the horizontal axis and depth on the vertical axis.

echosounder_jan8

To read the graph, you just have to imagine yourself in a boat sailing across the top of the image. The echosounder is sensing what’s beneath it—brighter colors mean the echo was louder, which means the object was more dense. That’s why the water appears white (no color) and the rocky seafloor appears red (the brightest color). Those two blue clouds are more dense than water and less dense than rock—they’re groups of krill.

Dr. Bernard analyzed the group on the right and found it contained about 180 pounds of krill stretched over an area 200 feet long and 20 feet high. At less than 0.04 ounces per krill that means, very roughly, there might have been 80,000 krill or more in that one group. Even so, Dr. Bernard said that today was a relatively slow day for a krill survey.

So what does a big krill day look like? When I asked Dr. Bernard this, she pulled up a graph from Dec. 23, 2011. Here it is:

echosounder_dec

You can read this graph just like the one above—look at the huge swath of greenish-blue covering most of the right half of the image above the seafloor! The greenish color is brighter than the blue, meaning that the krill are even more densely packed in these areas. And what about that circular white patch in the middle of the cloud of krill? Dr. Bernard said whales were swimming around the boat while they did this survey. She thinks that one had come through this spot with its mouth open just before the zodiac got there.

In the end, there weren’t large groups of krill near Torgersen or Cormorant Islands today. Dr. Bernard and Dr. Kohut think this may be because of the tidal currents that are happening during this part of the month. We’ll tell you more about that possibility in a post next week. Tomorrow, we’re going to go explore some penguin colonies.

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