Graze, the seaweed their pasture, and through groves
Of coral stray, or, sporting with quick glance,
Show to the sun their waved coats, dropt with gold."
—Milton, "Paradise Lost."
THE GROWTH AND VARIATION OF FISH.
How can you tell the age of a fish? This question is often asked and just so often is the answer unsatisfactory.
A fish is a cold-blooded animal; that is, his temperature is nearly the same as that of the water in which he lives. His circulation is sluggish and his appetite is a variable quantity. He has the capacity to take in large quantities of food at one meal and properly assimilate it; on the other hand he is able to fast for weeks at a time. He has his own notions about eating, and it is quite impossible to induce him to change them, and all this has considerable influence on his rate of growth. It is out of the question to expect him to grow when he is fasting; on the other hand he must draw on the fat he has stored up in his body to furnish him energy for his muscular movements and to carry on the ordinary functions of nutrition. The fish here has an advantage over the warm-blooded animals, for he does not need to generate heat to keep his body at a constant temperature. The amount of food often eaten at one time is quite remarkable. I remember once of taking nearly one pound of sunfish from the stomach of a Large-mouthed Black Bass. This does not indicate that a bass must eat such meals three times each day, it only shows his capacity to make use of a large quantity of food when it is abundant and his stomach feels the need of it. A trout is a good feeder; his stomach and mouth are large, much in size like that of the black bass. From experiments conducted at Neosho, Missouri, by Mr. Page, he found that a young trout did best on a daily ration of solid food equal to about seventy-five per cent of its weight. On this amount the trout would reach an average length of six inches in one year. The average amount of solid food consumed daily by a man is from one and one-half to two per cent of his weight, or more than twice that consumed by our active, growing young trout. As mentioned before, the trout is relieved from generating heat to keep his body at a constant temperature, and at one usually much higher than the medium in which he lives.
As an example of the ability of fishes to go for some time without eating, we need only mention our Pacific salmon. There are five species of these large fishes on the Pacific coast. In the early spring (April) many of the largest species, the Chinook, start up the Columbia river for the purpose of spawning. They reach the headwaters of the Columbia in Idaho early in September. During this journey they eat nothing. We know they do not eat, for of the thousands caught each year for the canneries none are found with food in their stomachs; besides, this organ has become much shrunken. If they did eat on this journey there would not, I believe, be enough animal and plant life in the Columbia to furnish each salmon with more than one meal. Now many of them make the journey against a strong current for more than one thousand miles, and reach an elevation of about eight thousand feet above the sea. When they leave the ocean they are in excellent condition, by the time they have reached their journey's end they are thin and haggard, their vitality is so reduced that soon after spawning they die—literally die of starvation. Their eggs hatch during the winter. By the next winter the young salmon are from four to five inches in length, and by the following fall or early winter they go to the sea, having reached an average length of about ten inches. After leaving the fresh water, which only afforded them a scant subsistence for nearly two years, the generous ocean gives them plenty of sea room and an abundance of food, which in a few years prepares them to repeat the long journey of their parents. We are, in case of most fishes, ignorant of their life histories, as we are of the salmon's. We know the average rate of growth of the salmon for the first two years, but we know nothing more of them until they return to fresh water to spawn.