Shape analysis of otolith annuli in Atlantic herring (Clupea harengus); a new method for tracking fish populations

The mixing of Atlantic herring (Clupea harengus) stocks during the early life phases has important implications for management of the associated fisheries. There are two components within the winter-spawning Celtic Sea stock with characteristic growth and recruitment patterns; Celtic Sea fish that move into the Irish Sea (migrant component) grow more slowly and hence recruit to the adult population later than those that are retained close to the spawning grounds (resident component). The rate of return of the dispersed component to the Celtic Sea, the relative contribution of each component to the adult stock, and its inter annual variation are unknown. A method to discriminate between the migrant and resident components of the winter-spawning Celtic stock, based on shape analysis of the juvenile portion of the otolith, is presented here. Juvenile herring were collected in 2004 and 2005 from nursery grounds in the Irish and Celtic Seas. Autumn- and winter-spawned juveniles were distinguished using larval otolith microstructure measurements. A classification function based on linear otolith measurements and Fourier descriptors was used to differentiate between age-0 winter spawned herring from the two Seas. This classification function was applied to age-1 juveniles from the same areas using otolith descriptors of the region bounded by the first annulus. The dispersed and resident components could be distinguished with a high degree of accuracy (>95%). The potential use of otolith characteristics for tracing juvenile origin in adult fish, assessing levels of natal homing and measuring recruitment levels from each nursery area are discussed.