Workpackages

The objective of this work package is to gain information on spawning season of individual fish. The data will be used firstly for the samples from the commercially exploited mixed aggregations of herring as found on the feeding grounds. Secondly, the method will show whether the mixed aggregations of juveniles comprise fish from different spawning seasons. Also, the hypothesis that fish found spawning in a particular season have been spawned in the same season will be tested with samples of ‘ripe and running’ fish collected from the spawning grounds.

Otoliths for microstructure analysis are extracted at sea or in port and will have been subject to shape (WP04) analysis before they are prepared for WP05. The exception are the otoliths of juvenile fish which are not used in WP04 and will be used directly in WP05.
Otoliths will be ground to the nucleus using the method of Karakiri and Westernhagen (1988). Images of the ground and etched surface will be obtained by the use of scanning electron microscopy (SEM). The digital SEM-images will be analysed with an image analysis software to determine the distance from the nucleus to the first winter ring. The widths of the increments within the first year’s growth will give a proxy for the daily growth rate of the larvae and post larvae.
In this work package the emphasis is put on otoliths from fish of the mixed adult aggregations, analysing 100 fish per sample. Only a relatively small fraction of ‘ripe-and-running’ fish from the reference collection will be analysed to test the hypothesis of identity of spawning seasons in the individual. For the aggregations of mixed juveniles a combined approach is chosen: in the first year a reduced number of fish from all sampling sites will be investigated and in the second year the full number of fish will be analysed for those sites that show an appreciable degree of mixing in the first year.

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