Hi Readers! Guest blogger Selene Fregosi here. I’m part of the Cetacean Research Program (CRP) at the Pacific Islands Fisheries Science Center (PIFSC), the counterpart to SWFSC which is located in Honolulu, HI. I’m collaborating with Shannon, Kourtney, and Cory on a cross-center project that aims to increase the use of autonomous, or uncrewed, systems (UxS) to collect valuable data about where cetaceans (whales and dolphins) live to help NOAA manage and conserve these species. Once these instruments are deployed, they will spend 9 weeks at sea, our longest mission to date!
Gliders as underwater listening devices
My role in the project is glider pilot and bioacoustician. I have been working with passive acoustic underwater gliders for over 10 years; the focus of my PhD research was on how these types of recording platforms compare to more traditional stationary recorders and with surface drifting recorders (like those used in the ADRIFT project). We’ve found that gliders with hydrophones work really well to acoustically survey hard-to-reach areas. They can complement existing efforts with ships and stationary recorders to provide a cost-effective way to collect more data on how many whales and dolphins are in an area and where and when they are present (e.g., Fregosi et al. 2020 and Fregosi et al. 2022).
Seagliders were originally developed by the Applied Physics Laboratory at the University of Washington. There are a few ‘brands’ of gliders now available. SWFSC has been flying Slocum gliders for many years in Antarctica (you can read an excellent blog series by SWFSC’s Jen Walsh for more!). The Seaglider ‘flies’ by making small buoyancy changes that cause it to repeatedly sink (or dive) to 1000 m and then float (climb) back to the surface. Because it has wings, the small buoyancy changes create forward movement. It takes 4-6 hours for a Seaglider to complete one dive cycle where it dives down and comes back up to the surface. Once the glider arrives at the surface, it communicates with the pilot via satellite and things start to get interesting.
As a trained Seaglider pilot, my job is to monitor the glider and make any necessary modifications to keep it diving safely and efficiently. The glider sends files with information like its GPS location, how much battery is left, and how strong the ocean currents are. This helps me understand its performance during the last dive cycle and allows me to assess how quickly it is traveling and check if it is staying on its planned trackline. I watch for any critical warning or error messages and make sure it is communicating consistently. If it’s NOT doing any of those things then I’ve got to figure out why, and try to fix them. Just as the glider sends me information, I am able to send it information via satellite. I can send updated command files each time it surfaces to make adjustments to dive depth and direction, various communication settings, and glider orientation (pitch and roll).
This constant monitoring every few hours sometimes means middle of the night wake ups to check on the glider, lots of discussion with other experienced pilots to try to figure out what is going on, and occasionally emergency recoveries (usually in bad weather of course). This is a lot for just one glider, and having three out at once is quite a juggling act! Fortunately, I work with a great team of pilots in Oregon and Hawaiʻi (shout out to Erik Norris and Calla Lloyd-Lim of PIFSC and Dave Mellinger of Oregon State University) so we are able to share the load and help each other through the hiccups over the very busy 9 weeks.
Depending on what type of sensors it carries, how energy efficient those sensors are, and how much data each sensor collects and can store, a glider can be out for a few weeks to many months at a time. Our gliders have a sensor to measure temperature and salinity of the seawater and a hydrophone and acoustic recording system to record underwater sound. Since passive acoustic data takes up a lot of memory space and passive acoustic recorders require a lot of power to run, we’ve traditionally been limited to 4-8 weeks. Thanks to the development of an updated, more energy efficient, acoustic recording system, this mission is 9 weeks and we are hoping to deploy for 10 or more weeks in future missions. The temperature and salinity data is transmitted in real-time when the glider surfaces. Acoustic data is very large and it is impractical to transmit it over satellite so we don’t have access to the recordings until the glider is safely back on land.
As I mentioned, SWFSC’s program uses a different brand of glider. They also have a different overarching research goal and so use a different suite of sensors. They use active acoustics to look for krill - the gliders produce sound and listen for the echoes allowing them to measure krill biomass. Our group employs passive acoustic methods. We are simply listening for any sounds occurring underwater, including anything from whales and dolphins to fish, ships, earthquakes, and any other source of sound.
A complementary glider and ship survey
So, back to this cross-center project I mentioned. I’m currently piloting three gliders that are surveying waters off the Oregon and California coasts. The gliders were deployed in August and September out of Newport, OR. From there, two gliders have stayed relatively closer to shore (although still 10s of miles out) to survey the more nearshore waters (red and yellow lines on the map below). The third glider is traveling up to 150 nmi offshore to survey the deep offshore waters. All three gliders will be retrieved at the end of October near Eureka, CA. We are able to use these gliders through a collaboration with the Cooperative Institute for Marine and Ecosystem Resources Studies at Oregon State University and NOAA’s Pacific Marine Environmental Laboratory.
What is unique about this glider survey is that it is occurring in approximately the same time and place as a ship-based visual and acoustic survey. CalCurCEAS (California Current Cetacean and Ecosystem Assessment Survey) is a large-scale survey along the entire West Coast of the US dedicated to collecting data used to better understand the populations and habitats of whales, dolphins, and seabirds. While visual observations are the main type of data collected to estimate animal abundance, the team is also deploying drifting acoustic recorders to collect passive acoustic data of whales and dolphins throughout the study area.
Gliders move much slower (~½ knot) than ships (~10 knots), so the gliders cannot survey the entire area the ship can cover in the same amount of time. So, we selected a single ‘leg’ of the ship-based survey and are flying the gliders in that same area at approximately the same time. The ship will survey the Oregon region in just 3 weeks (Sep 8 to Sep 27), while the glider will survey that same area over 9 weeks (Aug 23 to Oct 24). While not in the exact place at the exact same time, this coordinated effort still gives us data from two different platforms in the same region and season, which we need to help advance the use of passive acoustic gliders for cetacean surveys.
Gliders have the potential to greatly increase the amount of information about where cetaceans live and the habitat characteristics of those regions to complement costly ship-based surveys. Since gliders can also collect data in rough and stormy conditions that are unsuitable for visual observations, they can also fill in data gaps in certain areas at any time of year. By surveying with multiple platform types in the same region at the same time, we will be able to compare what the gliders and drifting recorders detected acoustically to the visual observations from the ship. However, more work is required to properly combine the visual observations and different passive acoustic data sets to provide more accurate information about cetaceans and their habitat for management and conservation purposes. Our hope is that eventually NOAA will be regularly using passive acoustic gliders in many of their survey regions to help us better understand trends in cetacean populations!
