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Fishing for Data

July 20
Stage four – aging the fish

"​​The sea lives in every one of us …” – Wyland

The single most interesting thing I've learned since being involved with marine science is the fact that you can ascertain the age of a fish by counting the rings on its ear stones, otoliths. 'Really? Like with a tree?' is the typical response, mine included. Yes, like a tree, only you don't take an axe or chainsaw to the fish's midriff; instead you remove the tiny structures from behind the fish's brain with surgical precision.

Adding up the rings in this way provides scientists with a toolkit with which they can reveal data about a given individual's diet, health and, crucially for this survey and consequential stock assessments, age.

Today I was shown exactly how these calcium carbonate structures are extracted from the targeted species and read by the scientists on the Johan Hjort 

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Firstly the otoliths have to be parted from their freshly-dead owners (to avoid any structural loss over time). This is done by slicing through the fish's head behind the eyes and under the brain, though the knife work can differ with species or size. The two otoliths are picked out with tweezers and deposited into paper bags, which also list the fish's age length and sex. 

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The scientists read many fish otoliths for each trawl during the day. They're all filed systematically in packets ready for the information to be stored electronically.

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To read the ear stones, like these saithe ones above, they are split into two so their cross-sections can be studied under the microscope. One part is then mounted on a lump of plasticine under the microscope (below). 

With most demersal fish such as saithe, the otoliths don't need to undergo any extra treatment, although a dab of water on the stone at this stage can help with boosting the contrast and definition. For the same reason, the scientist checking the age will tend to hold a pencil in their left hand to block the beam of light. 

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​Once the equipment is focused and the eye is in, the scientist counts the opaque, whiter rings of the otolith outwards from the central nucleus. As the otolith isn't completely round and more of a jagged oblong shape, care must be taken to count the bands of incomplete rings on the narrower top part.

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Top: the saithe in question, which turned out to be five years old. Bottom: a haddock otolith.​ As well as changes in the formation between species, separate populations of the same species in different parts of the world will develop unique structures.


The method is a generally a little different with the smaller otoliths of pelagic fish like herring (below). In this case they are steeped in a clear resin ​solution known as Entellan that then sets and hardens around the stones.​​ This helps boost the contrast once the microscope lens is on it.

otolith10.jpgHerring otoliths

There are two alternating types of ring on each otolith: transparent and opaque. Whilst Norway counts the opaque, other countries go for the transparent ones. It was explained to me that it doesn't really matter which one you go for as long as you're consistent.​

Actually, the shaded concentric bands imprinted on each stone tell different stories, with the darker transparent rings denoting winter growth and the opaque lighter ones summer growth. The distinction between the two types is clearer cut in geographical regions and habitats where the seasonal change is more dramatic such as the Barents Sea and here, the North Sea (as opposed to more temperate, less variable climes). In this case, it is possible to see the marked increase in feeding during the summer months. ​

But that's not where the magic ends. With many species it is possible to go one step further and grind a part of the stone down in order to read its daily growth when it was younger. In this sense, measuring growth rate is also possible.

​And further still, there are more advanced techniques out there through which nitrogen isotopes can be analysed in order to determine what trophic level​ a fish has been feeding at. More on that here. ​

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