Otolith shape analysis: its application for discriminating between stocks of Irish Sea and Celtic Sea herring (Clupea harengus) in the Irish Sea

The extensive movement of Celtic Sea juvenile Atlantic herring (Clupea harengus) during the first year of life into the Irish Sea results in two stocks of herring living together on Irish Sea nursery grounds: the resident autumn-spawned juveniles that originate in the Irish Sea, and the winter-spawned juveniles that hatch in the Celtic Sea and drift into the Irish Sea as larvae. Measurements of otolith increment width can be used to distinguish between the fast-growing winter-spawned and the slow-growing autumn-spawned stocks, but this method can be time-consuming. Otolith shape analysis is investigated as an alternative method for discriminating between seasonal spawning stocks. Juvenile herring collected from nursery grounds in the Irish Sea in 2006 were classified as autumn- or winter-spawned using increment width measurements. Otolith shape was defined using shape indices and Fourier descriptors. Juveniles were classified successfully to hatch type with a high degree of accuracy (86-87%) using shape variables. The potential use of otolith shape analysis for identifying Celtic Sea juvenile herring in the Irish Sea and its possible use for other mixed-herring stock assessments are discussed.