Fish have ears. Really. They're quite small and have no opening to the outside world carrying sound through the body. For the past seven years, Simon Thorrold, a university professor, has been examining fish ears, small round ear bones called otoliths. As fish grow, so do their otoliths. Each day, their otoliths gain a ring of calcium carbonate  . By looking through a microscope and counting these rings, Thorrold can determine the exact age of a young fish. As a fish gets older, its otoliths no longer get daily rings. Instead, they get yearly rings, which can also be counted, giving information about the fish's age, just like the growth rings of a tree. Ring counting is nothing new to fish scientists. But Thorrold has turned to a new direction. They're examining the chemical elements   of each otolith ring. The daily ring gives us the time, but chemistry tells us about the environment in which the fish swam on any given day. These elements tell us about the chemistry of the water that the fish was in. It also says something about water temperature, which determines how much of these elements will gather within each otolith ring. Thorrold can tell, for example, if a fish spent time in the open ocean before entering the less salty water of coastal areas. He can basically tell where fish are spending their time at any given stage of history. In the case of the Atlantic croaker, a popular saltwater food fish, Thorrold and his assistant have successfully followed the travelling of young fish from mid-ocean to the coast, a journey of many hundreds of miles. This is important to managers in the fish industry, who know nearly nothing about the track of the young fish for most food fish in the ocean. Eager to learn about his technology, fish scientists are now lending Thorrold their ears. Why is it important to study the chemistry of otolith rings?
The elements of the otoliths can tell the history of the sea. Chemical contents of otoliths can tell how fast fish can swim. We can know more about fish and their living environment. Scientists can know exactly how old a fish is.