Studies may be concerned primarily with the amount of organic material represented in tons per year of fish over certain fishing grounds, the poundage yield of oysters on cultivated beds, or the season's yield in barrels of oil from whales. These are evaluations of commercially important yields and though they do have a direct relation to the total organic production in certain sections of the sea, yet they are too far removed from the original source of synthesis of organic material to give anything more than a very general picture of production of the sea; for we do not know the ratio between the volume of fish, and so forth, and the volume of plants which, through various animal transformations, have supplied the food for growth and other vital energy-consuming processes of these and the intermediary animals. The yield of commercially sought animals is also subject to change through the influence of man and may therefore not give a true value representing their place in the balance of nature. Nevertheless, in semiclosed areas it is possible to arrive at figures which illustrate in a broad way the relationship in production of plants and herbivorous and carnivorous animals. Petersen (1918) has made certain calculations of the quantitative food relations of important animals in coastal waters where eelgrass, Zostera, furnishes the main source of primary food. This relationship is diagrammatically shown in fig. 249, from his report. As a working hypothesis he has assumed that each unit of weight of herbivorous animal substance requires ten units of plant substance in its formation, and each unit of weight of carnivorous organism feeding directly upon herbivorous forms requires ten units of the latter, hence one hundred plant units are needed to produce each unit of a carnivorous animal such as is represented by the plaice, for instance. But many carnivorous forms such as the cod feed only upon carnivorous animals, as shown by the radiating arrows in the figure, and this habit entails another similar loss as the length of the chain of transformers is increased, with the result that it has required a thousand units weight of Zostera to produce one unit weight of cod or
From these calculations, which can give only rough approximations, we can at least obtain some perception of the vast quantity of plant substance involved in production of a small amount of carnivorous animal tissue, and in the light of this the world's production of fish, which is in the neighborhood of 13 million tons annually, takes on a new significance. Failure in production in the initial link of the food chain must be recorded in the series of sequences leading up to the predators of highest rank.
Quantitative food relations of certain animals dependent on Zostera for the primary food. (After Petersen.)