The work of F. S. Russell on plankton animals as indicators of different water masses, particularly the species Sagitta setosa J. Müller and S. elegans Verrill, and of Atkins, Harvey and Cooper on plant nutrients, have pointed the conclusion that the water in the English Channel, in the neighbourhood of Plymouth, was consistently richer in nutrients and plankton in the 5 or 6 years prior to about 1930 than it has been at any time since. (See Russell, 1935, 1936a, 1939; Cooper, 1938; Armstrong & Harvey, 1950; Harvey, 1950.) The comparative infertility of the sea in this area in the 1930's, and until the present time, has been shown not only by the low winter phosphate- maximum, and by the sparse plankton characterized by the presence of S. setosa, but also by the reduced numbers of young fishes surviving their early larval stages after the summer spawning (Russell, 1939).