Community Research and Development Information Service - CORDIS

Phenotypic and Otolith Characterization

For stock assessment purposes different metrics have been applied for separating different stock components in mixed samples. Earlier population based methods using counts of keeled scales and vertebral series have been replaced by individual differentiation using otolith microstructure formed during the larval phase.

The work constituting the present HERGEN result "Phenotypic and Otolith Characterization - 29283" focused on collation and analysis of a number of phenotypic characters and additional development of new otolith based methods to identify spawning populations in mixed samples. A number of growth and maturation parameters differed between spawning components and e.g. mean vertebral counts were found to be different between some populations with the same spawning period.

The performance of the otolith microstructure method was tested and further developed within the project. High reproducibility of the visual inspection method was demonstrated in blind readings of spring and autumn/winter spawning populations. Experienced readers had average misclassification rates of about 2% and between reader differences were about 1%.

An objective method of daily increment width analysis was developed aiming at identification of strayers not spawning in the same season that they were hatched. Otolith silhouette shape was analysed by Elliptic Fourier Transformation (EFT) for a number of spawning populations from all seasons and major sea areas. Cross-validated discriminant analysis of selected EFT components showed classification success ranging from 64-84%. Similarity by Euclidean distance between average population shapes indicated an environmental impact of overlap in summer feeding area.

In general the otolith microstructure methods come out with very precise estimates of hatching period at the individual level, whereas they have limited capability of identifying different populations with the same spawning time. Shape analysis performs better at the population level but is less precise at the individual level. We may therefore conclude that otolith shape in combination with larval microstructure provides an effective additional tool for identification of herring population structure and that as a base line is readily available for splitting mixed stock catches of mature herring into management units. As such they are excellent supplements to microsatellite analysis of mixed stocks.

The prevailing hypothesis underpinning management decisions for herring in the North Sea, Skagerrak, Kattegat and Western Baltic is that spawning time characteristics are sufficient for separating the two major components, North Sea autumn spawners and Western Baltic spring spawners. The combined results on genetics and phenotype in the present project demonstrate strong population structuring following the salinity clines from the North Sea towards the Western Baltic. This structuring indicates important genetic diversity per se but may also help understanding the complex variation in spatial and temporal abundance created by differential migration pattern of the different population subunits.

It is predicted that the combination of the entire range of phenotypic and genetic methods may provide detailed information of population composition in mixed stocks even outside the range of populations present in the base line. Even when retaining the present management units a finer scale resolution of population structure will improve the stock assessment procedure for the area.

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