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Two methods of determining the age of herring (Clupea harengus L.) from the Bothnian Sea, in the northern Baltic Sea, were compared. Ages were estimated from whole otoliths, which is a routine method for herring age determination, and from neutral red stained otolith cross sections. There was disagreement between the methods, especially in specimens with more than five annual rings. The agreement between the determinations of different age readers was better with the cross section method and, besides, it is clear that with the whole otolith method there is considerable negative bias in old fish. Thus, more herring were classified as more than 15 years old with the cross section method. The study indicates that conventional methodology may generate considerable errors in age distributions, especially in samples which mainly consist of old fish. However, the differences in age reading produced relatively small changes in growth rate estimates, because of the slow growth of the old fish, that are most prone to age determination errors.

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