An illustrated manual for age reading of ling Molva molva L. and tusk Brosme brosme L. by otoliths
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The stocks of ling Molva molva L., blue ling Molva dipterygia (Pennant, 1784) and tusk Brosme brosme L. in the northeastern Atlantic support significant coastal and offshore bank and slope fisheries in several countries. In 1992, Iceland, the Faroes and Norway initiated a three-year project which should provide improved knowledge of the biology of the three species (Bergstad and Hareide 1996; Magnusson et al. 1997). In that project it was focused specifically on age determination methods which could form a basis for future stock assessments and further studies of biology. For all the three species there was a need for reconsidering the ageing methods applied in earlier studies, and on that basis, perform further technical development and inter-calibration of age readings. This manual summarises results of efforts made for two of the species, ling and tusk, in the period 1993-1996. The manual illustrates the agreed practices which should provide reasonably precise age estimates. Data from several inter-calibration exercises carried out during the nordic project were presented by Bergstad et al. (1997). The only structure useful for determining the age of ling and tusk are the sagittal otoliths, and the otoliths are extracted by the same procedures as used for other gadoids (e.g. Williams and Bedford 1974). This manual aims at assisting age readers who are inexperienced with the reading of ling and tusk otoliths. It should be used as an initial guideline and reference only. In order to achieve precise and stable age determination, there is a need for inter-calibrations of readings by different readers, and repeated reading by the same reader.