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Fishing for Data

July 17
Stage 2 – the sorting

"There is the life of the plankton in almost endless variety; there are the many kinds of fish, both surface and bottom living; there are the hosts of different invertebrate creatures on the sea-floor; and there are those almost grotesque forms of pelagic life in the oceans depths. Then there are the squids and cuttlefish, and the porpoises, dolphins and great whales." – Sir Alister Hardy. 

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The jaws of death? The eye of Mordor? At around a metre long and stickily built, this monkfish was by far the biggest catch of the day. (It was subsequently filleted and frozen)

The North Sea might not be the Hollywood of watery ecosystems. It doesn't boast technicoloured fish and coral reefs or have sharks stalking the open waters (though it's also home to some shark species living closer to the bottom). As far as public perception goes, its name seems more synonymous with saltwater, oil and everyday supermarket fish species than with fizzing life, tropical glitz and the sort of creatures we love to anthropomorphise in cartoon and book form.

None of this applies in reality. OK, so it might not be the world's main melting pot of biodiversity, but it's no less ecologically important. The ecosystem is finely-tuned and there is a surprising variety of fish and shellfish species, particularly in the deeper sea and on the very bottom (the benthos). Lots of these are represented in the IBTS catches.

Sorting through

Once the nets are reeled in, the cod end is emptied into a container from which the fish are transferred from the deck on conveyer belts inside to the wet lab – the room where the scientists take over.​

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The cod end is emptied into a container

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'What have we got here then?'

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The catch boards the conveyor belt...

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​...and arrives in the wet lab

From here the first task is to classify the creatures, sorting them into different baskets by species to ease the subsequent measuring, aging and data collection.

This is no light work. Whilst the experts could probably name the fish species blindfolded, the often large number of urchins, crustaceans, starfish, and sponges and so on mean that categorising everything readily can be a challenge.

"Everything's recorded," explains Rupert Wienerroither, expert in amongst other things species identification from the marine institute in Bergen. "We pick out the big fish out to see what's left behind, it's easier that way. Then have a look at what's in the small fish. Most here are Norway pout,"

"We measure 100 individuals of each species from each station. If we don't get 100, then we take what we get. We cannot do anything. If we get just one we get just one."

Is there much diversity?

"There can be quite a lot of diversity. We have some stations where we get up to 50 or 60 taxa, including the benthic organisms. There's a lot of difference between stations. We can compare the same station over different years."

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Sea urchin

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Picking through and sorting a selection of benthic creatures

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The tough egg case of a skate, known as a 'mermaid's purse'. These are laid on the sea floor 

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Here's Rupert recording the length. "When we get skates we tend to measure and put them back alive. Some species are rare – not this one, though they have low fertility rate (fecundity) – just a few eggs a year. They are quite tough."

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Sea cucumber

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Young haddock from close to the seafloor

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Young long rough dab

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A sizeable hake

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Young common octopus. After recording the data, octopuses, squids and cuttlefish – collectively termed as cephalopods – are put back in the water. On board the Johan Hjort this practice is jokingly referred to as 'appeasing the kraken'! 

These taxonomic efforts do come with some challenges, however.

"We break everything down to species level," continues Rupert. "All fish are known, but sometimes with the benthos it can be difficult. We have some groups like sponges where we don't know what they are. A few species we know but some are just 'sponges' – though we try to be as accurate as possible."

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