As the sun sets over Monterey Bay, vast numbers of animals start to rise from the depths of the sea toward the surface waters. During daylight hours, these vertical commuters seek refuge from visual predators like birds, salmon, and tuna in deeper, darker water. But at night, they swim hundreds of meters closer to the surface to feed. The phenomenon, called diel vertical migration (DVM), is the largest mass migration on the planet.

The migrators include both predators (like deep-sea squids) and their prey (such as lanternfish). These animals are engaged in a never-ending evolutionary race, each adapting to the behavior of the other in a bid to survive.

Deep-sea predators have evolved various strategies for hunting migrators. Some active hunters may chase the migrators up toward the surface. Others remain in the depths and wait for their prey to return at dawn. For the prey, it’s a stressful life that teeters between being able to eat or being eaten.

MBARI Senior Scientist Bruce Robison and colleagues Kim Reisenbichler and Rob Sherlock have been working to understand the risks of vertical migration for these prey animals. In a recent article in Frontiers in Marine Science, the researchers describe a new method for estimating the chances that two types of prey—krill and lanternfish—will encounter predators while migrating. The team hopes their new tool will help scientists track changes in predator-prey relationships within this mobile food web.

One of the two prey animals the team studied was krill (Euphausia pacifica and Thysanoessa spinifera)—tiny shrimp-like crustaceans that are eaten by a wide range of animals, from small fish to massive whales. Krill are vertical migrators, usually staying between 100 to 300 meters (300 to 1,000 feet) deep during the day, but moving closer to the surface at night. Many predatory animals rely on these crustaceans as a significant source of food because krill can form large, dense swarms.

One of the major predatory threats to krill are gelatinous colonial animals called siphonophores, distant relatives of jellyfish. One siphonophore—and a principal predator of krill in Monterey Bay—is Nanomia bijuga. At less than 30 centimeters (one foot) long, Nanomia is relatively small but abundant, and it migrates vertically, following prey. Many fish and shrimp that feed on krill also migrate vertically to follow their prey.

According to the researchers’ calculations, the species that presents the greatest threat to krill in Monterey Bay is Praya dubia, a siphonophore that can grow longer than a blue whale and has stinging tentacles up to three meters (10 feet) long. This siphonophore is not known to migrate but presents a formidable threat to vertically migrating animals because of its sheer size.

Additional predators on vertical migrators include comb jellies (ctenophores). Comb jellies do not migrate themselves but can engulf migrating krill and other small animals with sticky lobes, or trap prey with their tentacles.

Despite being filter feeders and not predators, larvaceans in the genus Bathochordaeus also pose a threat to small vertical migrators such as krill. These small tadpole-like animals build sizable mucous filters (“houses”) up to a meter across. Relatively small migrating animals, such as krill, sometimes collide with these houses and become entangled in them.

Lanternfish (myctophids)—the second prey species the research team assessed—are actively hunted by several species of aggressive predatory squids. The squids Gonatus onyx and Gonatus berryi do not migrate vertically but chase and attack migrating lanternfish.

Other squids stay at depth and try to trick their prey by luring them to their death. For example, Chiroteuthis calyx has two extra-long tentacles, which the squid extends while light organs on the tentacle flash, presumably attracting prey. Another squid, Grimalditeuthis bonplandi, has a feeding tentacle with a bulb on the end that can move like a small swimming animal, to entice prey species to come closer.

To quantitatively assess the threats to krill and lanternfish from these predators, the researchers reviewed thousands of hours of video footage from Monterey Bay taken between 1997 and 2015. The team noted the number, size, and cross-sectional area of each predator and prey animal observed in these videos.

MBARI engineer Paul McGill used these data to create a computer model estimating how many times a prey animal would encounter a predator during a daily migration. The researchers used this model to calculate “threat potentials”—percentages indicating the likelihood that a krill or lantern fish would encounter a particular type of predator during each vertical migration.

“Nobody has ever looked at migration from that perspective,” said Robison. “People have always made predictions of predation on vertical migrators based on data from net tows or acoustic surveys. But we spent so much time in the water videotaping the animals that we realized we could look at the risks of migration directly, based on what they actually encounter.”

The researchers note that prey animals have many strategies to avoid predation, including mimicry, schooling, bioluminescence, and avoidance. Because these strategies are not included in the computer model, not all encounters mean certain death. By comparing the calculated threat potentials, however, scientists can contrast the effects of one predator to another. For example, the threat potentials of siphonophores Nanomia bijuga and Praya dubia were very similar for krill, despite N. bijuga being far more abundant, because P. dubia is so much bigger.

Although the researchers focused on just two prey species, Robison says the method can be applied to many other vertical migrators. The team plans to use this method for calculating threat potentials in future studies. “Because ours is the only data set like it in the world, we’ve got the data in hand,” said Robison, “we just need to plug in the numbers for any set of predators and prey.”

Robison believes that this approach could also help scientists better understand the effects of climate change on animals in the ocean. “One of the ways we can use this tool is to calculate and predict impacts of climate change based on what’s going to happen to the krill and their predators,” said Robison. “We know that krill can tolerate environmental changes better than many of their predators, and thus that the ratios of certain predators to their prey will change. This model could help us evaluate and predict future patterns in threat potential as the ocean gets warmer.”

Article by Lara Streiff

Original journal article:

Robison, B. H., R. E. Sherlock, K. R. Reisenbichler, and P. R. McGill. (2020). Running the gauntlet: Assessing the threats to vertical migrators. Frontiers in Marine Science, 7. https://doi.org/10.3389/fmars.2020.00064.

For additional information or images relating to this article, please send an email to pressroom@mbari.org.