Running the Gauntlet: Assessing the Threats to Vertical Migrators

Abstract

Diel vertical migrations (DVM) by zooplankton and nekton are driven by the selective advantage of avoiding visually cued predators near the surface during the hours of daylight. And just as there is a second set of predators that occupy the migrators' dark daytime depths, there is also a diverse suite of predators that comprise a gauntlet of threats during the migrations. Here we examine these migrations from the perspective of the migrators, to enumerate the kinds of predatory threats they face and to assess the threat potential of various predator types. The study is based on thousands of hours of in situ observations and measurements of the mesopelagic community in Monterey Bay, California, conducted chiefly by remotely operated vehicles (ROVs). We provide accounts of some predator/prey interactions, and we introduce a means to calculate the threat potential of specific predators, based on MBARI's long-term time-series of quantitative video surveys.

Zooplankton-Mediated Fluxes in the Eastern Tropical North Atlantic

“…It therefore needs to be stressed that mortality rates at depth might be different to those estimated here. However, Robison et al (2020) note that many different, sometimes specialized mid-water predators pose a considerable threat to the migrating community. Furthermore, the migration activity itself, the changes in abiotic conditions (e.g., temperature, oxygen) and the lack of food might have so far unknown effects on the mortality of the migratory community.…”

“…Furthermore, the migration activity itself, the changes in abiotic conditions (e.g., temperature, oxygen) and the lack of food might have so far unknown effects on the mortality of the migratory community. Natural and/or consumptive mortality might also vary regionally, depending e.g., on the oxygen level at the migration depth or the predator community composition (Robison et al, 2020). Further work, especially to parameterize zooplankton gut flux to and mortality at mid-water depth is needed and will help to reduce the uncertainties associated with the estimation of DVMmediated fluxes.…”

In situ observations show vertical community structure of pelagic fauna in the eastern tropical North Atlantic off Cape Verde

“…This diel vertical migration (DVM) of zooplankton and nekton contributes to a net downward flux of particulate and dissolved matter 20 , 21 . However, the mesopelagic zone also contains a specific set of resident organisms that do not migrate to the surface, and instead feed on sinking particles and the migrant organisms 22 , 23 . Besides the comparatively well-known migration behavior of crustaceans and fishes, some gelatinous organisms such as pyrosomes and salps also migrate on a daily basis 24 , but more observations are needed to quantify taxon-specific distribution and migration patterns.…”

Three-Dimensional Imaging Lidar for Characterizing Particle Fields and Organisms in the Mesopelagic Zone

“…UMSLI, as an instrument originally designed for imaging large marine fish and mammals, has shown extended capabilities for characterization of relatively small particles including marine snow in this field experiment. UMSLI's unobtrusive red laser creates opportunity for observing larger swimming organisms that otherwise practice avoidance behaviors from tow nets and ROVs (Peña, 2019;Urmy et al, 2019;Robison et al, 2020). With demonstrated capabilities of observing nekton in situ simultaneously with particulate organic matter fields, this technology shows potential for studying interactions of aggregates with organisms and resulting transformations through remote, undisturbed observation.…”