While you were sleeping…

While we all sleep, the greatest migration on the planet occurs in the sea, affecting food production and Earth’s climate. This nightly movement is made up of 10 billion metric tons of animals—a larger net movement of biomass than the migrations of caribou, wildebeest, or zebras on land or Arctic terns in the air. It includes everything from rice-grain-sized invertebrates like copepods, lanternfish measuring 10 centimeters (four inches) long, shrimp, squid, and gelatinous creatures up to 30 meters (98 feet) long.

During the day, these animals reside in the mesopelagic, or “twilight zone,” a region in the water column with only minimal light. At sunset, they move hundreds of meters upward under the cover of darkness to feast near the base of the food chain in surface waters where sunlight allows plants to grow. They descend again at first light to remain hidden from the eyes of their predators. Their migration is the equivalent to running a 10K twice every day and at twice the speed of an Olympic marathon runner.

These vertical migrations of midwater animals were first documented in the 1940s when the US Navy began testing new sonar technology. The sonar readings revealed dense features located more than 300 meters (about 1,000 feet) below the surface and stretching for hundreds of kilometers off the coast of California, in the central Pacific, and in the Atlantic. These mesopelagic animals play a critical role in ocean ecosystems, serving as a food resource for larger animals, including salmon, billfish, marine mammals, and seabirds. In the process of swimming up and down, mesopelagic migrators also act as a biogeochemical conveyor belt, moving nutrients and carbon through the water column—with global consequences.

Carbon dioxide in the surface ocean is taken up by phytoplankton during photosynthesis. If this organic carbon makes it into the deep sea, it can be sequestered from the atmosphere for decades to centuries. About 25 percent of the carbon dioxide generated by burning fossil fuels each year has been sequestered in the ocean. One key in getting the carbon to remain in the ocean rather than returning to the atmosphere is getting it below the mixed layer, which is nearer to the air-sea interface and can range anywhere from 10 to 500 meters (33 to 1,640 feet) in depth. Vertically migrating animals actively transport carbon by feeding near the surface and releasing their waste hundreds of meters deep. While the role of this migration in transporting carbon has only recently received attention, some estimates suggest that diel (daily) migration may account for as much as half of the flux of carbon to the deep sea. Changes in the behavior and quantity of the millions of tiny creatures that vertically migrate could significantly impact Earth’s atmosphere. Changes in vertical migration could raise or lower the temperature of the atmosphere depending on the nature of these changes.

Despite the importance of mesopelagic animals to ocean ecosystems, relatively little is known about these animals, from how long they live and who they eat to how often they migrate and whether this migration varies over time. Global estimates of the total biomass of mesopelagic species are hotly debated, varying from 1,000 to 200,000 million metric tons. MBARI’s nearly 30-year midwater time series, which uses video to estimate animal distributions by depth, abundances, and behavior, is arguably the richest data set on life in the mesopelagic. These records are revealing much about how life in the midwater has responded to our changing climate. To better understand the role of vertical migration throughout Monterey Bay, while untangling the key processes driving the ecological and biogeochemical impacts on this ecosystem, researchers are addressing several new questions:

• Who is migrating—and who isn’t?
• How horizontally extensive are the layers of migrating animals?
• How are mesopelagic animals spatially organized?
• How does vertical migration vary over time, and why?
• What risks and benefits do animals experience during migration?
• What environmental cues are important in migration?
• What role does migration play in connecting the mesopelagic to the continental shelf?

Answering these questions requires a collaborative approach like that championed by MBARI founder David Packard—multi-disciplinary science working hand in hand with engineers and marine operators. The MBARI research teams led by Chris Scholin, Bruce Robison, Kakani Katija, Steven Haddock, Francisco P. Chavez, and Kelly Benoit-Bird are each working on aspects of these questions. They are developing new tools and approaches in conjunction with existing ones, including MBARI’s remotely operated and autonomous underwater vehicles.

Continuous observation with sonar on our cabled observatory reveals that on some days migration ceases, and the arrival of schools of predatory fish or hunting dolphins can trigger new migration patterns—literally overnight. Autonomous platforms like the i2MAP autonomous underwater vehicle (AUV), the smaller long-range AUV (LRAUV), and the Wave Glider surface platform, allow for high-resolution observations of the behavior of animals during migrations. Sampling of environmental DNA (eDNA) and video imaging from these platforms are helping us to learn which species move, which stay put, and how the environment shapes such behaviors. Novel platforms like the Mesobot, being developed in partnership with Woods Hole Oceanographic Institution, are aimed at tracking individual animals for hours to days, giving scientists a fish-eye view of migrating animals.

In 2019, MBARI brought many of these new tools together with more traditional ones for two experiments focused on vertical migration around the cabled observatory in Monterey Bay. For two weeks in the spring and two weeks in the fall, all eyes (and ears) at MBARI were glued to the data coming in from the cabled observatory. Using data from the cabled echosounder, the team carefully selected depths to sample for eDNA with the LRAUV.

This approach combined the ability to describe the biomass of animals and their population movements with information on the identity of animals in a wide range of sizes. But it presented some logistical challenges. Keeping a ball on a football field (the size of the echosounder's view at its widest point) is easy on land. But at sea, this is a formidable challenge for an AUV. Global positioning systems do not work underwater and currents are constantly pushing the vehicles around. MBARI’s engineering and operations teams devised a clever plan—place an autonomous surface vessel above the echosounder to serve as a bridge between GPS satellites and the underwater vehicles, letting researchers precisely target layers of the animals of interest.

One of the long-standing challenges in sampling in the ocean is how to separate the effects of space and time. For example, were the observed changes due to water masses sweeping past horizontally in space, or to a change in animal behavior over time? The use of multiple autonomous platforms and a partnership with the commercial Saildrone surface vessel allowed sampling in more than one place at a time, quantifying the spatial extent of vertical migration in Monterey Bay. As part of a long-standing collaboration, the National Oceanic and Atmospheric Administration’s (NOAA) National Marine Fisheries Service brought their research vessel, the R/V Reuben Lasker, to sample using acoustics and net tows in parallel with the MBARI research vessels Western Flyer and Rachel Carson. Together, the research groups are exploring how new techniques can complement traditional stock-assessment tools to ensure effective fisheries management.

As the team waits for the DNA samples from these field experiments to be analyzed, they are already looking for ways to improve the process. A key to scaling up these measurements is to move from the fixed reference of the cabled observatory to a mobile one. Echosounder sampling, like that used to observe migration from the cabled observatory, will soon be possible from the LRAUV.

The team is building on the success of using an autonomous surface vessel as a navigational gateway to provide a communications hub for the vehicles while they are working. Moving the decision-making about sampling onto the vehicles themselves will enable rapid, dynamic sampling of biological phenomena. Integrating these approaches with new biogeochemistry tools promises insights into the global consequences of vertical migration from food security to global climate.

Our timing could not be better. For the first time, several countries have issued permits to allow fishing access to the mesopelagic to provide fish feed and fish oil. We are in a race to understand this ecosystem before it is exploited. Humankind has a rare opportunity to start with a clean slate, developing sustainable fisheries that acknowledge a range of ecosystem services—and science has an important role to play.