Two lanternfish frmo the Museum collection, showing their large eyes of light-producing organs along the sides of their bodies.

Lanternfish form one of the biggest migrations in the animal kingdom © The Trustees of the Natural History Museum, London

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Documenting one of the most abundant vertebrates in the world before it's too late

Lanternfish are one of the most abundant vertebrates on Earth. Yet despite their huge numbers, almost nothing is known about them.

As the fishing industry starts to take more of an interest in these as yet unexploited fishes, researchers have been trying to figure out exactly what role they play in the ocean, how much carbon they are sequestering and precisely how to make any future fisheries sustainable. 

Typically measuring under 15 centimetres long, the small, blunt-headed lanternfish are rather unassuming. But what they lack for in looks, they make for in numbers.

Every day, in every ocean billions of these fish swim from the deep up to the surface and back down again. As they do so, they play a crucial role in the carbon cycle of the oceans, pulling carbon from the surface and depositing down in the deep.

But the exact role that these species play is not particularly well understood.

Sarah Alewijnse is a PhD student at the Museum who is looking into the metabolic rates of deep-sea fish. As part of this research, she has looked at six species of lanternfish.

'We are interested in how much these fish contribute to the carbon cycle, getting to know a bit more about how they might be impacted by climate change, and also getting better models to predict their biomass,' explains Sarah. 'You can get a handle on all of those things by using their metabolic rate.'

To do this, Sarah and her colleagues at the British Antarctic Survey and the University of Southampton have been looking at the inner ear of the fish, which contains a record of their lives not unlike tree rings.

What she has found is that the traditional way for figuring out the metabolic rate, by working out the rate for a well-known species such as cod and then scaling the results to the new species in question, is actually wrong in this case and could mean that large parts of fisheries science might not be reflecting reality.  

The paper has been published in the journal Marine Ecology Progress Series

Four freshly caught lanternfish in a glass dish.

Lanternfish are so named for the constellations of bioluminescent spots scattered across their bellies © James Maclaine

The largest migration on Earth

When sonar was first developed during the second world war and used on boats to scan the ocean depths, it caused a great deal of confusion. When the boats were passing over water known to be thousands of metres deep, it appeared to show that the seafloor was only a few hundred metres below the surface.

It turned out that this phantom bottom effect was a result of one of the biggest migrations in the animal kingdom. 

What the sailors mistook for the seafloor actually turned out to be an extraordinary amount of hungry fish, squid and siphonophores. Every day, millions of these marine organisms migrate from the deep water where they spend the day hiding, to come to the surface at night to feed.

One of the most numerous of these migrating creatures is the lanternfish, so named for the constellations of bioluminescent spots scattered across their bellies. Made up of around 246 species, they are found in every ocean of the world and are so numerous the fish are thought to be the second most common vertebrate on Earth.

But despite this extraordinary biomass, estimated to be several times the volume of the entire world fisheries catch, the fish have largely avoided being exploited by humans. This is down to a rather unfortunate effect they have on people if eaten.

A lanternfish in water.

The fish are hard to study because they are so delicate, meaning they often die when brought to the surface © Love Lego / Shutterstock

'This is because the fish have a really high wax ester content,' explains Sarah. 'So if you eat them you get something call keriorrhea, which is basically really horrible, oily orange diarrhoea.

'But while we can't eat them, people are now looking at using them for fish meal for aquaculture. As the returns from captured fisheries are kind of levelling out, aquaculture has really taken off and so people are starting to be interested in lanternfish fisheries.'

It is this recent rise in aquaculture, in which fish are farmed in pens either close to shore or on land, that has been worrying marine biologists. Some fish that are grown in captivity need to be fed on high protein diets, which usually comes in the form of smaller fish caught from the wild. And the industry has started eyeing up the plentiful lanternfish.

This has spurred researchers to want to find out as much as possible about the fish before plans to exploit them become too developed.

All in the ears

At its very basic, the metabolic rate of a fish is a measure of how much energy an individual uses. This in turn can give an indication of how an animal is breathing, their blood circulation, body temperature control and, ultimately, the rate at which an animal is burning calories.

To figure this out for fish, researchers have traditionally taken a live fish and placed them in special tanks where they could measure every aspect of their biology. But this technique is only possible for a small number of species. 

Two different otiliths, or fish ear bones. One is flat and disc-like while the other is more oval in shape with a fluted edge.

Fish ear bones, or otiliths, are a bit like tree rings in that they grow outwards and record how the fish were living © Mili77 / Shutterstock 

'Lanternfish are kind of inaccessible as they are very good at avoiding nets,' explains Sarah. 'They are also really delicate, so if you catch them then they usually just die because they're not very hardy.' The other drawback of measuring these rates in a tank is that they are simply not very representative of a fish living out in the wild.

But each fish carried with them a record of their metabolic rate, hidden in their ears.

'We used otoliths, or fish ear bones,' says Sarah. 'The otoliths are made up of calcium carbonate similar to seashell material, and we can use the chemical signatures in those to get an estimate of their field metabolic rate.'

'This work has amazing potential, because lab studies can only get you so far and you really want to know what a fish is doing out in the wild. The neat thing about this method is that you can access fish like lanternfish and other deep-sea fish that you can't access as easily as fish which live near the surface.'

The results from Sarah's work show that the lab-based studies are not applicable to wild living lanternfish. This might seem inconsequential, but this discovery may well have huge knock on effects when considering some of the major questions when it comes of these small but mighty fish.

'With this fishery we really have the chance to do things sustainably, and especially because of the carbon cycling we really need to,' explains Sarah. 'But it really shows us again that we just don't have enough information yet to start harvesting these fish in a sustainable way.

'There is so little known about these fish. It is really important to get on it and get things right from the beginning.'