Grinding of otoliths

Catches of herring in the Kattegat, the Skagerrak and the Eastern part of the North Sea are taken from a mixture of two main spawning stocks. These are 1+ ringers of the Western Baltic Spring Spawners (WBSS) and 0-2-ringers from the North Sea Autumn Spawners (NSAS).

Moreover, an uncertain amount of spring spawners belonging to local spawning populations in the Skagerrak/Kattegat area are likely to contribute to the catches.

The introduction of otolith microstructure analysis in 1996-97 (Mosegaard and Popp-Madsen, 1996) enables an accurate and precise split between three groups, autumn, winter and spring-spawners. Today this method is applied for the stock separation in all Danish and Swedish IIIa samples.

The analysis of individual otolith microstructure is used for determination of spawning type in age-class stratified random sub-samples of herring in Division IIIa. However, different populations with similar spawning periods are not resolved with the present level of analysis.

Spring-spawning herring  are identified by broad, distinct growth zones, which increases rapidly near the center of the otoliths. These usually come from Rügen in north east of Germany. There are also "local" populations in the fjords, where zones are narrow, but with high-contrast near the center and then increased sharply.

Autumn spawning herring are identified by narrow growth zones and low contrast. All growth zones seem to have the same width. Often one sees a "barcode" structure (a small area of dense growth zones) about 200 microns from the center.

Winter spawning herring have higher contrast between the rings than the autumn spawners. Regular growth zones with increasing distance. After about 200 microns increases growth significantly.