Many great migrations take place among the animals of this world every year - the wildebeests' circuit of the whole Serengeti, the birds that fly hundreds of miles south every winter to escape the cold, the trans-continental migrations of the Monarch butterfly. However the biggest of all the mass migrations happens every night, and it involves some of the smallest of creatures.
Around dusk billions of tons of zooplankton, fish, krill and more swim their way up to the shallows of the sea, often covering distances tens of thousands of times their own length - 'like a person walking 25 miles each way to get to and from breakfast', as Dr. Deborah Steinberg puts it1. Every morning, they sink back down again. This pattern is known as diurnal, or diel, vertical migration.
People had noticed in the 1800s that fish were often more abundant at night, but the epic scale of their movements only became clear during the Second World War. The navy was initially confused by sonar readings showing a 'false bottom' to the ocean - a layer which could be mistaken for the sea bed, but which was much lower during the daytime than at night. Slowly it became clear that this deep scattering layer, as it became known, was composed of a vast quantity of marine organisms, each making a vertical round-trip of about a kilometre every day.
Why do they do it? I defer here to my friend, the ecologist and singer-songwriter Hannah Werdmuller2:
Much of the ocean is not yet explored
Though submarines pootle about the sea floor
Discovering new species wherever they go
Such as elephant-like-squid and eels that glow.
The following formula's certainly true
Of predation dynamics down in the blue:
From the tiniest plankton to the biggest species yet,
One thing they eat, by another are ate.
Here is where a dilemma arises
For the noble phytoplankton photosynthesises
And is thus confined to where the sun shines through,
And so by necessity what eats them too.
But in the photic zone it is easier to see -
The surface is a dangerous place to be.
The darkness it offers substantial protection
From becoming some predator's tasty confection.
So to the surface fishies travel at night
When there's less chance of being espied in the light
But during the day to escape predation
They return deeper down, and that's diurnal migration.
- The quote is taken from 'The Great Ocean Migration' by David Malmquist, the single best internet source I could find on this. Other information is from a National Geographic poster about great migrations.
- The lyrics of Diurnal Migration, from the album Pre-Apocalyptic Love Song, are reproduced here with permission. Listen to the song for free on her web site - it's great - or as part of the E2 Podcast Season 6, Episode 2, where I also read my own essay, above.