As we had expected, the liver-fat of the red cod differs from that of the flesh, and here also there are interesting relationships, for the mean molecular weight of the liver fatty acids differs from that of the flesh by 52—which corresponds to four carbons; also the amount of unsaturation is proportionate to the molecular weight: thus the ratio of 334 (mean molecular weight) to 139 (the iodine value) is practically the same as 282 to 113. It has been shown by Leathes and others that in mammals the liver-oils are always less saturated—i.e., have a higher iodine value—than the body-fats. Here there seems to be the opposite condition; but the figures may be accounted for by assuming that the unsaturated fatty acids of the body-fats had had four carbon atoms split off, and that in the process of splitting some of the unsaturation had disappeared; otherwise, owing to the smaller size of the fatty-acid molecule in the liver-oil, the iodine value would have been higher than it is.

In the examples of simultaneous analyses of fat of food and fat of consumer of the food given by Rosenfeld none is in such close agreement as are our figures. We would have been glad, however, if he had examined the “consumers” at another season of the year, when the nature of their food was less obvious, or even different, for it would have been interesting in comparison with our results in the case of red cod B. These, as already stated, were caught six months earlier, or later, in the life of the cod—unfortunately we cannot say which with certainty, for the scales were not examined, but from the weight of flesh it is probable that the red cod B were either younger fish or fish of the same age as A but in poorer condition.

In these “winter” fish the main characteristics of the fats remain very much the same as in the “summer” fish—e.g., the iodine values and mean molecular weights of the fatty acids are similar. The percentage of fat in the flesh is 0.1 higher in the winter fish, but this is within the experimental error, and there may have been wastage of the protein part of the flesh. Incidentally, our analyses confirm those of Mrs. Johnson (Part 2 of this series) in finding that red cod is poor in fat.

There are, however, certain differences to be noted:—

Although these points of difference are small when looked at singly, they all indicate a certain degree of depletion of the reserves, and, taken in conjunction with the empty state of the stomach, point to a condition of semi-starvation.

The explanation which we think best fits in with the data we have obtained is that the main food of this fish for the whole year consists of whale-feed. During the summer and autumn the red cod gorges itself on this food, and fattens so far as its liver is concerned. In winter it retires to deep and colder water. Here, owing to lessened metabolism, and possibly lessened food-supply, it lives largely on its reserves till the whale-feed season again occurs. During the time of plenty the character of the fat acquired has a direct relationship to the fat of the food.