The present investigation was undertaken at the invitation of H.M. Treasury, in consequence of an application to the government from H.M. Inspectors of Irish Fisheries for a scientific investigation into the life-history of the mackerel. The problem to the solution of which the inspectors attached particular importance was the relation to one another of the spring and autumn schools of mackerel which regularly visit the Irish coasts. In the spring a multitude of large fish approach the south and west coasts of Ireland to breed. In the autumn schools of immature, but usually well-grown, mackerel come around the island. According to one of the inspectors, the Rev. W. S. Green, there is a sharply defined interval between the two visitations of fish.