The published observations of other workers, on samples of herrings taken from the area under consideration, whether they relate to the study of such morphological characters as the number of vertebræ and keeled scales, or are chiefly concerned with the examination of the growth of the fishes as indicated by their scales, demonstrate very clearly that the eastern, central and western sectors of the area are visited by herrings which differ in character. It will be convenient to summarise the opinions of these workers under the headings of the statistical regions in which the samples were taken. The reader is referred to the chart on page 269, which shows the geographical extent of the regions dealt with.

Although locally caught drift herrings may be landed at Plymouth from the end of September onwards until the beginning of March, the fishery only assumes economic importance during the months of December and January, when a large number of visiting drifters from both Cornwall and East Anglia are making daily fishing trips, using Plymouth as their temporary base.

Comparatively little has been published concerning the habitat and growth of young herrings in the English Channel, especially during the first year of their life. On May 26th, 1927, the first of a series of hauls with a small-meshed Saltash tuck seine (vide Davis, 1, p. 70, for a description of this type of net) was made in the River Tamar and a fine haul of young clupeoids obtained, which included the young of the herring, sprat and pilchard. Since then, regular samples have been taken from the rivers Tamar, Tavy and Lynher, and sufficient numbers of young herrings collected on each occasion to give a reasonable indication of the length-distribution prevailing. Sampling is still in progress and will be continued until one year's observation has been completed, but sufficient data are now to hand to merit an initial report. The localities from which the samples were obtained are indicated in the accompanying chart.

Photosynthesis in the open sea is due mainly to two groups of organisms, the diatoms and the dinoflagellates. In British seas the diatoms are so much more numerous that the chemical changes which are due to photosynthetic activity can be almost entirely ascribed to their influence. Recent work on these chemical changes has shown that they do not extend deeper than about 30 metres. Of the external limiting factors light is among the most important and the depth to which photosynthesis can go on must, of course, depend on the light intensity. In the study of the phytoplankton under natural conditions the results are complicated by the interaction of many other factors such as temperature, viscosity, or lack of food salts, so that it is difficult to come to definite conclusions on the effect of light alone.