Oceanography After Dark

July 10, 2017 — Hello from the Night’s Watch !  Unlike Jon Snow and his buddies on Game of Thrones, the night watch  on the ship are the folks who work between sunset and sunrise while the turtle-spotting scientists (and probably you) are asleep. It takes a day or two to get your body/brain to switch over to night-time as your working time, but a lot of very cool stuff happens at night.

I’m a bioacoustician. That means I use sound to study the marine environment.  One of my primary tools is an echosounder, which works much like a fish-finder or depth-finder you may have seen on a boat at some point.  Our equipment works similarly to those, it sends out a short pulse of sound (“a ping”) and then we listen for the echoes that come back. The ship we are on (NOAA’s Henry Bigelow) has a very nice, five-frequency, hull-mounted echosounder and as we drive around the ocean, we can see where different fish and plankton (small animals that can’t swim against a current very well) are located in the water column.

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This is an echogram showing where organisms are located in the water column. The five windows show the five different frequencies (or wavelengths) of sound. Different wavelengths can detect some animals better than others. The blue dots represent a layer of animals that are located at different depths in the upper 100 m of the water column. We try to target a specific layer (or depth) when we deploy our nets). Photo Courtesy Joe Warren/Stony Brook University

We use this information to determine at what depths we send our nets down to collect samples of these organisms.  And there’re A LOT of very cool animals that live in the ocean – most of which you probably have never seen or heard of.  One of my students will be blogging about some of these later on.

People washing out large plankton net on deck

Scientist are washing down the net after we bring it back on board the vessel, so that we make sure that we get all the animals in the net into our sample that we process inside the boat in the wet lab. Photo Courtesy Joe Warren/Stony Brook University

Once we have these animals on the boat, we take a small number of them and measure their density (how much they sink or float relative to seawater) and their soundspeed (how fast sound moves through them), and photograph them with a ruler for scale so we know their shape and size.  We use these data to determine how much sound would reflect off a single animal.  With this information, we can convert the echoes (blue dots in the echogram) to how many animals there were beneath the boat.  This process can get very complicated if there are lots of different types of animals in the water – but in many cases we can use the acoustics to map out where the animals are relative to the different types of water we’re encountering.

In our nets, we find lots of different animals :

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(Photos Courtesy Joe Warren/Stony Brook University)

Our cruise track for the past few days has us (at night) going in and out of a warm core ring, which is a chunk of water from the Gulf Stream (so it’s warm and salty) that has spun away from the Gulf Stream and is now traveling up along Georges Bank (southeast of Cape Cod).  What’s really interesting about sampling this warm core ring is that you can find completely different sets of animals when you do a tow inside or outside of the ring.  So you can find tropical species in one tow, and more typical species for coastal New England in the next tow depending on where the ship is.

Color coded chart of ocean water surface temperatures off the Northeatern U.S.

This is a map of sea surface temperatures that are collected by a satellite in space, and made available by scientists at Rutgers University. The left side of the orange-blob of water at the far right-hand side of the image is where we are working currently. This satellite can’t see through clouds so the maps that we get are a function of the weather conditions as well. (Photo Courtesy Joe Warren/Stony Brook University)

Thanks for reading about our science,

Joe Warren

Stony Brook University

Aboard the NOAA Ship Henry B. Bigelow for the 2017 Cetacean and Turtle Cruise

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