Studies on the Dunmore Herring Stock 2. The Otoliths of the Dunmore Herring By

Previous work by a number of authors led to the conclusion that herring could be separa-ted into summer-autumn spawning and winter-spring spawning components by means of differences in the appearance of their otoliths. The otoliths of Dunmore winter spawning herring were examined and found to include a wide range of otolith types consistent with both summer-autumn and winter-spring spawning fish. Various separations were made on the material in an attempt to isolate these two components. In all cases each separated component had a very high mean vertebral number of the order occurring in other stocks of winter-spring spawning herring, but quite outside the range of low values known to occur in the stocks of summer-autumn spawning herring in this region. It was concluded that all the Dunmore winter spawners were members of a self-contained stock and had in fact all been spawned at the same time of year. The wide divergency of otolith types amongst these herring is suggested as being due, in the main, to the way in which the larvae are distributed away from the Dunmore spawning grounds. This could lead to considerable variations in both temperature and food, affecting the brood as a whole during the critical periods when such parts of the otolith as the nucleus and first hyaline zone are being formed