The otoliths appear a few days after the fish is born, as a nucleus around which new layers of calcareous substance will be deposited as the fish grow

Have you ever noticed the age of the fish you are eating?

JAPAN
Saturday, October 17, 2020, 07:10 (GMT + 9)

"How old is this fish?"

Have you ever thought about it when you saw it at a supermarket?

In everyday life, you rarely know the age of a fish.

Regarding the lifespan of creatures living in the sea, some fish live up to 100 years. How long is the life of a fish? As you can see, some fish have longevity, while others grow fast but are short-lived.

So how do you find out the age of a fish? This time 'UMITO', the Maruha Nichiro new educational website, will  tell you how to check the age of fish.

How to find out the age of shellfish

Scallops have a beautiful striped pattern. This line is fan-shaped as the shell grows and is called the "growth line". In the case of scallops, the number of scallops increases once a year, just like the annual rings of a tree, so you can tell the age by looking at the shells.

(Image courtesy of Hokkaido Gyoren/UMITO)

Shellfish grow under various influences such as water temperature, season, and reproductive activity, but shellfish living on tidal flats are particularly affected by the ebb and flow of the tide. While immersed in seawater, it is active and grows its shell, but when the tide is low, it ceases to be active. At this time, the shellfish on the tidal flat increase the growth line by one. It's like recording your activities in the shell.

Check the age of the fish with scales and otoliths

The action of living organisms to make biominerals (raw minerals) such as bones and shells is called biomineralization. It is possible to check the age with otoliths and scales, which are biomineralizations of fish.

In the inner ear of fish, there are three types of stones, flat stones, gravel stones, and stellate stones, one pair on each side of the back of the head. The largest flat stone is called an otolith.

Otolith is a biomineral whose main component is calcium carbonate, and is a tissue related to the sense of balance and hearing of fish. The shape and size of each fish will vary. By the way, the standard Japanese name "Silver white croaker" is said to have been named "Ishimochi" because it has a very large otolith.

(Otolith). Photo: courtesy UMITO/Maruha Nichiro

The otolith grows little by little as the fish grows, but it is not proportional to the size of the fish. For example, the otolith of Atlantic mackerel (adult fish) is about 5 mm, but the otolith of bluefin tuna (adult fish) is about the same size.

Every day, a very thin ring of lines (called a sun ring) is carved on the otolith of a fish, forming a ring pattern (ring-shaped pattern) similar to the annual ring of a tree. In general, fish grow slower when the water temperature is low, so a more prominent ring pattern is engraved in winter. You can guess the age by counting the ring patterns.

However, not all fish are marked with one distinct line per year. In addition, it is difficult to count ring patterns in fish with small otoliths and elderly fish over 10 years old, even when observed under a microscope. In that case, it is necessary to bake the otoliths in the oven or polish them until they are clear.

(Atlantic mackerel scales). Photo: courtesy UMITO/Maruha Nichiro

Similarly, the scales also form a ring pattern, so you can check the age from the scales.

How old is this fish? Let's find out the age of Kinmedai (alfonsino)

Alfonsino (Kinmedai). Photo: courtesy UMITO/Maruha Nichiro

How old do you think this Splendid alfonsino, which is about 28 cm in length? One of the familiar fish, Splendid alfonsino, is a good fish for removing otoliths. A piece of otolith is about 1 cm larger than the fish body and can be taken out at home. Let's check it out immediately.

Splendid alfonsino otoliths are easier to remove when cooked than when they are raw.

. Photo: courtesy UMITO/Maruha Nichiro

The bone indicated by the arrow in the figure at the place where the head and spine are connected is the otolith. There is one on each side.

It's glossy and beautiful.

(Splendid alfonsino otolith). Photo: courtesy UMITO/Maruha Nichiro

A ring pattern was confirmed at the position of the small red dot.

(Splendid alfonsino scales)

I also observed the scales with a microscope. Also from this image, a ring pattern was confirmed in the small red dot part.

As a result of observing the size of the fish, otoliths, and scales, it is estimated that this alfonsino is 3 years old.

Why is it important to know the age of the fish

(School of fish)

Examining otoliths and scale ring patterns is widely used in laboratories as one of the age assessment methods. Depending on the fish species, it may be difficult to read or it may be necessary to make thin slices, but checking the age of the fish in this way is very important from the viewpoint of protecting fishery resources.

Finding out how old and how many fish live will help predict future stock trends. If you keep the catch in line with the speed at which it breeds naturally and grows, you can eat it for a long time. On the other hand, bycatch of over-breeding and pre-breeding fry will eventually lead to depletion of marine resources.

You must try to continue eating delicious fish while preserving the resources of the sea.

Source: UMITO / Maruha Nichiro (Article has been translated from original in Japanese)

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