Expedition log by MBARI Postdoctoral Fellow Olívia Soares Pereira and Senior Research Specialist John Ryan

The team has been kept busy, with moorings, equipment, and cameras being deployed every morning, followed by dives with the remotely operated vehicle (ROV) Doc Ricketts. For the past three days, we have been exploring the area around Davidson Seamount in support of the Davidson Seamount Hotspot Project. We are currently investigating mysterious furrows in the seafloor.

The team suspects that the furrows may form when beaked whales dive to great depths in pursuit of their prey. As they chase cephalopods or fishes near the seafloor, the whales may collide with the sediment, leaving behind the depressions we see on the seafloor. Before this chase ensues, beaked whales first find their prey in the deep, dark ocean by using echolocation, much like bats do on land. Different beaked whale species use different frequencies (pitches) of sounds, and their distinct echolocation clicks serve as sonic fingerprints that help us identify species. If whales are creating the depressions on the seafloor, our odds of seeing a whale at the moment it swipes with the bottom are extraordinarily low. If we can hear more beaked whale clicks close to areas with abundant furrows, this will be evidence that these formations may be tied to whale activity.

We are deploying hydrophones, or underwater microphones, at four sites around Davidson Seamount to listen for whales. Two are located near areas where furrows have previously been observed, while two are at the same depth but farther away from the seamount, where furrows have not been reported (control sites for comparison). The hydrophone moorings will remain in the ocean for a year, continuously recording the underwater soundscape, to help us determine where beaked whales are more prevalent, which beaked whale species inhabit each area, and how their presence changes seasonally.

With the hydrophones deployed, we began exploring the first control sites with ROV Doc Ricketts. At the first site, we found no furrows, as expected, but the seafloor was covered with munnopsid isopods among large anemones. At the second control site, however, we encountered furrows among a sea of sea cucumbers. We then made our way to the sites along Davidson Seamount where furrows had been observed during previous expeditions, and indeed we found several of them, including some fresh-looking ones. Across all sites, we collected water and sediment for environmental DNA (eDNA) analyses to look for evidence (or lack thereof) of beaked whales and their fish or cephalopod prey. eDNA is genetic material that organisms shed into their surroundings through scales, skin, mucus, and waste, allowing scientists to detect organisms without seeing or collecting them directly. Additional samples were collected to examine the animals living in the sediment.

The furrow investigation is just one part of a larger effort to understand the ecology of Davidson Seamount, and, throughout the cruise, multibeam surveys from the ship are helping us understand prey diversity and abundance across all sites, while nightly CTD (conductivity, temperature, and depth) casts provide environmental data and additional water samples for eDNA analyses. Baited time-lapse cameras were also deployed in hopes of documenting potential prey species.

Together, seafloor observations, acoustic recordings, and biological and environmental samples will help us piece together the story of these mysterious furrows in the seabed.