(2) The shellfish may be diseased, or be seriously contaminated, by living in dirty water. (3) Decomposition may set in after the shellfish have been removed from the water—particularly if they have been kept too long in a warm place. It has been found recently that shellfish that have been deliberately fattened on sewage can be effectively cleansed in such a way as to get rid of ingested sewage bacteria. This process has been carried out successfully on a commercial scale at Conway by the Ministry of Agriculture and Fisheries. Danger from infected shellfish may also be safely avoided by boiling them. When shellfish are gathered at the right season of the year and from suitable localities, they are a perfectly safe and wholesome food. Of the many species of edible fishes that are known and used, the number is by no means complete, and new species are added from time to time. Thus, in 1916, the United States Bureau of Fisheries introduced a new edible fish (_Lopholatilus chamaeleonticeps_), which they christened the tile fish. After this fishery had been in existence for twelve months, the known catch of tile fish amounted to over 10,000,000 lbs., valued at more than $400,000. In 1917, the same Bureau introduced the dog-fish under a new name. As people were prejudiced against the name “dog fish,” the Bureau altered it to “gray fish,” “which is descriptive, not preoccupied, and altogether unobjectionable.” The fish is now caught in large numbers, and forms the basis of a very flourishing canning industry. Attempts have been made recently to utilize as food the edible portions of the shark (which is closely related to the dog fish) and the porpoise. The food value of most fishes varies very much according to the condition of the fish when it is caught—that is whether it is spawning or not. Further, it may be considerably modified by the changes that take place subsequently in the composition of the flesh during the processes of curing, cooking or preserving. Generally speaking, all marine fish annually pass through a well-marked series of seasonal changes, the stages of which appear to depend upon changes in the temperature, salinity and alkalinity of the sea. These changes are directly connected with the development of roe and milt, with the fluctuation in the percentage of oil and fat in the liver and body tissues, and also with the rate of growth. Thus the chemical composition of the fish, and hence its food value, varies greatly according to the season at which it is caught. Norwegian brisling (“Skipper Sardines”) are caught in the summer just before spawning time. At this time the fat content is high; in winter the fat content is low, and the fish possesses small commercial value. The gradual change in the composition and food value (in calories per pound) of the herring as spawning time approaches is well shown in Table III. (Prof. J. Johnstone, Trans., Liverpool Biolog. Soc., Vol. xxxiii (1919), p. 106.) TABLE III