Stones in the head of a fish – is that possible? Some anglers are aware of stones, or otoliths, in the head of the Madagascar meagre, but it is not generally known that ear-stones, most of them tiny, are found in the skull of all fish.

Otoliths
Ear-stones from various types of fish caught on our coast. Above from left to right: Sompat grunt (Pomadasys jubelini), West Coast seabream, white seabream, Garrick (Lichia amia); below left: kob; bottom right: White barbel (Galeichthys feliceps).

Ear-stones, according to scientists, probably help fish to keep their balance. Otoliths are white and very hard, as they consist mainly of calcium carbonate (aragonite). They are embedded in a jelly-like substance behind the eye and located in a bone cavity. The otoliths in the left and the right side of the head are shaped in a mirror image of one another.

Based on the shape of ear-stones, scientists and experts are able to determine the species of fish, and conclusions about the size of the fish can be drawn from the size of the otoliths. Researchers cut otoliths into thin slices and calculate the exact age of the fish with the help of the annual rings, in the same way as with trees.

Otoliths
A scientist removes an otolith from the head of a West Coast seabream.

A West Coast seabream with a length of 78 centimetres, which was caught in Meob Bay south of Walvis Bay, was 47 years old according to its otoliths. West Coast seabream of the same length, but caught in the slightly warmer waters off Skeleton Coast Park, were between 20 and 22 years old. Fish in warmer waters grow faster than their counterparts in colder waters. A kob with a weight of 60 kilograms and a length of 1.79 metres, which was reeled in at Terrace Bay, was 34 years old according to the annual rings of its otoliths.

Otoliths
Cross-section of an otolith of a 60 kg kob which was 34 years old.

A large fish does not necessarily have large otoliths. The ear-stones of a Sompat grunt were three times larger than those of a Garrick which was double the size and weight of the Sompat grunt.

Otoliths are used to find out what Cape fur seals eat on our coast. Scientists dry the faeces of seals, then grind and sift it to finally pick out the white otoliths. Each fish species can be identified thanks to the unique shape of the ear-stones, and based on its annual rings the age of the fish eaten by a seal is determined. The main diet of seals on the southern coast in the vicinity of Lüderitz consists of Hector’s lantern fish (Lampanyctodes hectoris) and bearded goby (Sufflogobius bibarbatus), whereas bearded goby and Cape horse mackerel (Trachurus capensis) are the main food of seals at Cape Cross in the central section of the coast. This research started 20 years ago.

Author: Dirk Heinrich

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