In the Ocean’s Twilight Zone, a Fish That Could Feed the World – or Destroy It

In 2010, another Spanish expedition set off from Cádiz, tracing much of the original route and studying what the oceans are like today.

The team measured pollutants, plastics and chemicals that were not there in Malaspina and Bustamante’s time. They collected samples of seawater and plankton. And all the way through the 31,000-mile voyage, the ship’s sonar was switched on, listening for echoes from below. Their chief targets? Small silver fish that look like sardines or anchovies – only with bigger eyes and rows of spots that glow in the dark.

They are lanternfish: there are about 250 species and they are not only the most common fish in the oceans’ twilight zone but the most abundant vertebrates on the planet. Huge numbers were first noticed during the second world war, when naval sonar operators saw echoes from what appeared to be a solid seabed, one that rose to the surface at night and fell back down at daybreak. In fact, the pulses of sound were echoing off the swim bladders – the internal gas-filled bubbles – of billions of lanternfish, as they congregated in dense layers hiding in the deep, then at sunset swam up thousands of metres to feed at the surface. Every night, along with other animals, such as the squid that prey on them, lanternfish undergo the greatest animal migration on the planet.

Before the 2010 Malaspina expedition, studies based on trawl surveys estimated that the twilight zone contains about a gigatonne (1bn tonnes) of fish. But this was most likely an underestimate, it turns out, because lanternfish avoid being caught by swimming away from the open nets. The Malaspina acoustic survey did not rely on nets, and in 2014 its research led to new estimates of the abundance of twilight-zone fish, ranging between 10 and 20 gigatonnes.

The prospect of such a colossal harvest raised an old question: could fish from the twilight zone help to feed a growing human population? (...)

Climate consequences

In contrast to extremely slow-growing deep-sea species such as orange roughy, lanternfish are more likely to withstand substantial hunting pressure; they are much faster growing, and their lives are measured in months, some living for less than two years. Nevertheless, fishing in the twilight zone could trigger a different kind of catastrophe by disrupting the way lanternfish and similar species help regulate the climate.

Their daily routine of swimming up and down forms vital connections between the surface and the deep by boosting the “particle injection pumps”. This is the process of little fish feeding in the shallows, then plunging downwards, where they are eaten by bigger fish that remain in the deep, thereby “pumping” carbon dioxide from the atmosphere into the deep ocean where it can be stored. If particles sink below 1,000 metres their carbon can be stored for up to 1,000 years before returning to the surface. A study of the continental slope off western Ireland estimated that deep-dwelling fish capture and store the equivalent of 1m tonnes of CO2 a year. 

by Helen Scales, The Guardian |  Read more:

Image: Morgan Trimble/Alamy