Diel vertical migration

Lore & Background

The deep scattering layer was first identified in the early-to-mid twentieth century when naval and research vessels began using echosounders to map the seafloor. Operators noticed a persistent, horizontal 'false bottom' hovering well above the true seafloor, shifting upward at night and downward by day. What had been a sonar nuisance became one of the most revealing signatures of open-ocean life, demonstrating that the midwater column—far from the benthos and the surface—is teeming with coordinated animal activity.

The behavioural logic of DVM rests on a trade-off. The photic zone offers the highest concentrations of phytoplankton, detritus, and other food, but it also exposes animals to visual predators. By descending into dim or dark water during the day, migrants reduce their visibility while still remaining within reach of sinking particles and prey that drift downward. At night, the light cue relaxes the predation constraint, and the animals rise to exploit the surface food web. The cycle is entrained by the solar day, though internal circadian oscillators allow the rhythm to persist under constant light in laboratory conditions.

DVM is not a single, uniform movement. Different taxa migrate at different depths, at different times, and with different amplitudes. Krill, copepods, myctophids, and squid each contribute distinct layers to the scattering profile, and the composite DSL can span hundreds of metres of vertical space. Seasonal shifts in stratification, prey availability, and predator pressure modulate the depth and timing of the migration, making the DSL a dynamic, multi-species phenomenon rather than a fixed band.

Reader's Guide

Observation: On a midwater trawl station in the open Pacific, a dawn net-haul recovered a dense assemblage of euphausiids, copepods, and juvenile myctophids, all concentrated in the upper 50 m. By 1000 h, a repeat cast at the same coordinate returned a fraction of that biomass, with the bulk of the catch now recorded at 200–400 m on the deck-mounted echosounder. The vertical displacement of several hundred metres within a single morning is the signature of DVM in action.

Inference: The species composition of the surface catch (filter-feeding and small-predator taxa) suggests the animals are exploiting the residual phytoplankton bloom and micro-predator assemblage left by the previous night's grazing. The daytime descent likely reduces encounter rates with visually hunting predators such as larger pelagic fish and seabirds.

Community context: The migrating cohort sits within a broader benthic–pelagic coupling. Particulate organic matter that sinks from the surface is consumed by the descending animals, and their faecal pellets and excreted ammonia are released at depth, subsidising mesopelagic and upper-bathyal communities. In this way, DVM acts as a biological pump, linking surface primary production to deep respiration and remineralisation.

Uncertainty acknowledged: The precise depth and timing of the migration shift with season, local stratification, and prey availability, and the relative contribution of each taxon to the composite scattering layer is still being resolved. Acoustic backscatter alone cannot reliably separate species, so many depth–time profiles remain taxon-ambiguous. Fieldwork combining multi-frequency acoustics, in-situ imaging, and targeted net sampling is ongoing, and several regional patterns—particularly in polar and upwelling systems—remain poorly characterised.

Did You Know?

• The deep scattering layer was first noticed as an apparent 'false seafloor' on echosounder displays, long before its biological origin was understood.
• DVM is considered one of the largest recurring mass migrations of animals on Earth, involving trillions of individuals across all ocean basins.
• The migration is entrained by the light–dark cycle, but internal circadian clocks allow the rhythm to continue under constant illumination in laboratory experiments.
• The daily export of carbon and nutrients by descending migrants is a significant component of the biological pump that sequesters atmospheric CO₂ into the deep ocean.