The herring(Clupea harengus L.) deposits its eggs in the coastal waters around the North Atlantic Ocean, North Sea, and Baltic Sea. The salinity on the spawning grounds may vary from about 35%0 to 5%0. For instance, Brandhorst (1959) reports that successful spawning tookplace in the Kiel Canal in salinities down to 5%0, and Ford (1929) records that ripe herring have been found in the Tamar estuary. In a series of experiments Ford found that the eggs of herring could be successfully fertilized and incubated even in a salinity of 4·8%0. McMynn & Hoar (1953) investigated the effect of salinity on the development of the Pacific herringC. pallasii and found it had a wide tolerance, the lower level being somewhere between o and 6‰