Wombats and the Internal Structure of Scattering Layers
Hey everyone!
Its Ian reporting from team acoustics here on the RV Point Sur. With my advisor Kevin Boswell sitting out this cruise it has been me and my lab mate Aedan Mell running the show on acoustics.
Those of you familiar with the blog may know that this is my third DEEPEND/DSB cruise and in the past, I’ve told you about our general acoustics operations and of course the curious case of swim bladders in mesopelagic fishes. But today I’m going to dive a bit deeper into our autonomous echosounder operations looking at the internal structure of the deep scattering layers.
The deep scattering layer or DSL (pictured below) was discovered back during World War II by sonar technicians and at the time was referred to as a “phantom bottom” as it was located well above where the technicians expected the sea floor to be. Adding to the mystery was that this “phantom bottom” seemed to move up at sunset and then returned to its original depth in the morning. Now of course we know that this layer is actually caused by a thick layer of fish, crustaceans, squids, jellyfish, and other marine organisms, many of which migrate into surface waters at night to feed and return to depth at dawn. A behavior often referred to as diel vertical migration or DVM.
Picture 1. A two day echogram showing the deep-scattering layer (DSL) in the Gulf along with what diel vertical migration (DVM) looks like on our echosounder.
Our shipboard echosounders do an amazing job of capturing not only the location of the DSL but also the changes in its location over time like those caused by DVM. However, due to factors like beam spreading, where the acoustic beam widens out the further from its source it travels, it is largely impossible to make out the individual organisms that make it up. An analogy my lab mate Haley Glasmann likes to use is that looking at the DSL with the shipboard echosounders is like taking a picture of a group from far away, where you can tell it’s a group of people, but you can’t make out the individual faces. However, using an acoustic probe we can effectively cut the distance and get a much closer and detailed look at the internal workings of the DSL thus revealing the faces amongst the crowd.
In my case I’m using a Wideband Autonomous Transceiver (WBAT) that we affectionately referred to as the “wombat”. By attaching the WBAT to the ship’s CTD cage we can effectively bring our echosounders down into the DSL which gives us an unprecedented look into its inhabitants. In my case the goal is to try to make out individual organisms and figure out their rough identities using frequencies responses. Frequency response just refers to the amount of sound an organism scatters at different frequencies (see below) and by using these responses it is then possible to potentially identify different creatures. The ultimate goal of this is to see if these creatures show any sort of horizontal structuring. So, does “like hang out with like” (fish with fish, shrimp with shrimp, etc.) and do the organisms hang out with those that are a similar size to them. Of course, it could also just be a jumbled mess down there but that’s what we’re looking to find out.
Picture 2. The “Wombat” mounted on the Point Sur’s CTD cage
Picture 3. Example of different organism’s frequency responses from Benoit-Bird & Lawson, 2016
Picture 4. Example of collected WBAT data. The little “squiggles” represent individual organisms (or targets)
Aside from the acoustics, the seas have been on a rougher side today, which has put a lot of our operations on hold for the time being. But it looks as if the seas should lie down in the coming days which means a return to sampling and more “wombat” deployments 😊. Also, I have to shout out the awesome Point Sur crew and especially our amazing marine tech Grace, who kindly shares her tech lab with us acoustics people during the cruise and has taught me the ins and outs of operating and deploying the ship’s CTD. They’re ultimately what allows our science to happen.
See you all on the flip,
Ian
