The Spawning of Mackerel, Scomber Scombrus L., and Pilchard, Clupea Pilchardus Walbaum, in the Celtic Sea in 1937–39

Abstract

The distributions of mackerel eggs and young stages, pilchard eggs, the siphonophore Muggiaea atlantica and the chaetognaths Sagitta elegans, S. setosa, S. serratodentata and 5. lyra in the Celtic Sea in March 1939, April, May-June 1937, 1938 and 1939 and July 1937 and 1938 are described, together with notes on the hydrological conditions in April 1937, 1938 and 1939, May-June 1939 and July 1938.Mackerel spawning started in mid-March, rapidly reached a maximum in mid-April, declined gradually through May and June and was very slight at the end of July.The changes in intensity of spawning are accompanied by a continuous shift eastward and slightly northward of the locus of spawning. In March spawning occurred in a small area at the western edge of the Continental Shelf only. In mid-April it was spread over the greater part of the Celtic Sea, with the main activity concentrated in two centres—to the south of Ireland and to the west of the mouth of the English Channel. By May-June it had shifted farther to the east and occupied the central area of the Celtic Sea. In July there was no spawning to the westward of the Scilly Isles.The spawning area of the mackerel appears to be confined to the shallow waters over the Continental Shelf.No mackerel young stages were found in March. Small numbers occurred in April (maximum 174). They were abundant in May-June (maximum 8239), and very scarce in July (maximum 20).The distribution of young stages closely follows the movement of spawning during the season.