Each evening, throughout the World Ocean, multitudes of animals leave the deep, dark waters they inhabit during the day and swim up to the food-rich, nighttime waters near the surface. Each morning, they descend back down to the depths. These movements comprise the largest mass migrations on Earth in a tidal cycle of shifting biomass, a tide driven not by gravity but by light. The animals migrate in order to remain constantly in darkness, to avoid being eaten by visually cued predators. Despite this successful strategy, the migrators still face many other kinds of predators during their vertical transits. Distributed along the migratory path are passive predators, lying in wait to ensnare, entangle, or engulf the vertical commuters. Also closely attendant are layers of active predators, who lure, track, or chase their migrating prey.

In the past, these threats were typically studied by indirect methods such as net tows and acoustics, which could tell us very little about predator prey interactions. Our data obtained directly by MBARI’s ROVs have revealed an abundant and very diverse assemblage of predators confronting the migrators; a group far more complex and dynamic than was previously believed to exist. Based on thousands of hours of in situ observation, we have described predatory threats and assessed the threat potential of various predator types. We also introduce a new way to calculate the threat potential of specific predators.

This research is unprecedented because we examined threat potential from the perspective of the migrator, using high-resolution spatial data to plot the numbers and types of predators in the vertical path of an individual migrator. And for the first time, calculations of predation levels could be adjusted to account for observed behaviors that include: prey capture strategies, predator avoidance tactics, and escape responses. Prior to the MBARI data set, all such insights were purely speculative. The value and scope of in situ midwater research has increased dramatically with the development of new vehicles and advanced technologies that enable investigative practices which were previously possible only in terrestrial and shallow-water research.

Vertically migrating animals comprise a large and active component of the ocean’s biological pump, and when compared with passively sinking detritus, the vertical migrators contribute significantly to the overall flux of particulate organic carbon. Understanding predator/prey patterns and dynamics are critical for quantifying the vast mesopelagic component of the ocean’s carbon cycle, for understanding oceanic community structure and function, and for assessing food web structure.

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