Secrets of the otoliths

A group of researchers from disciplines as diverse as climatology, fishery biology, geochemistry, palaeozoology, anthropology and archaeology are studying cod otoliths in the search for important historical data.

Scientists have long been using otoliths to determine the age of individual fish. The otolith has a crystalline structure of calcium carbonate, which grows very slightly every day in the life of a fish. It lies within a fluid-filled pocket, and acts as a stability organ. The number of layers that the otolith consists of tells us how old the fish is, in the same way as we can find the age of a tree by counting its annual rings. By combining different types of expertise and analytical methods, however, we can obtain a great deal more information from these tiny structures.

Revealing historical climate data

Because the otoliths grow with every new layer of material deposited, its chemical composition can tell us something about where the fish has lived, and for how long. Otoliths from fish from the Baltic, for example, contain more barium and less strontium than those of the ones from the coast of Norway . Findings of this sort tell us something of imports and trading patterns in the Middle Ages, and about how resources were exploited. They also enable us to read off the temperature: the ratio of two isotopes of oxygen in otoliths is temperature-dependent. In collaboration with the University’s Bjerknes Centre, the temperature is measured on a monthly basis, and this provides an overview of summer and winter temperatures far back in time.

“The otoliths can also help to produce important data regarding climatic change at high latitudes. We can supply both raw data for climate models and data on human behaviour in historical time,” says Professor Audrey Geffen of the Department of Biology.

While a good-quality sediment core can provide information on temperatures on a ten-year to fifty-year timescale, the otoliths are capable of providing information about variations in the course of a single year. Among other things, it is possible that some of the changes that had been observed in the production and availability of certain species of fish have been closely related to short-term alternations in climate and temperature.

International expertise gathers in Bergen

“What interested me was that this was a very “Norwegian” project, at the same time as it is of international calibre. It has basically grown by itself. We had no need to look for expertise elsewhere, because everyone was here already. This is where the internationally recognised research groups are, and we also have the high technology we need to perform the analyses,” says Geffen, who herself comes from the USA . The investigators in the project belong to the Departments of Biology and Geosciences, as well as the Natural History Collections, the Bjerknes Climate Research Centre and the Institute of Marine Research.

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