It has been observed that the coarseness and thickness of the baleen is suited to the size of the food which is filtered. For example, the rorqual or Sei whale, which feeds on tiny shrimp species, has a filtering surface which resembles fine wool. The blue whale, which feeds on the largest of shrimps and on fish, has the coarsest filter. Generally, fish occur only in the diets of the blue, finback and other rorqual which swim fast enough to engulf them. Here, the fish are weak-swimming, schooling fishes. The gray whale, unlike the other filter feeders, feeds on bottom-frequenting crustaceans known technically as amphipods. These organisms, occurring principally in shallow water, keep the gray whales close to the shores of Siberia and Kamchatka. In order for a whale to be able to exist on a 3 to 4 month feeding period, it must have not only ample food, but time in which to collect it. Although the whales may not all be far enough north to have a 24-hour day, there is enough twilight to let them feed the clock around. In order to take advantage of the prodigious amounts of food available, whales have a huge four-chambered stomach. It is not at all uncommon to find 5 to 10 wheelbarrow loads of shrimp in the stomach. No one yet knows how fast the food passes through the alimentary tract. The shrimps on which the whales thrive are not uniformly concentrated, but occur at special places where oceanographic processes have enriched the surface waters with cold, nutrient-rich, subsurface water. The principal oceanographic action which enriches the water at the surface is called upwelling. It can be induced by a number of physical conditions. One of the best places to look for upwelling is along the edge of the polar icecaps and along the junction of strong currents. Upwelling is one of the ways nature refertilizes the surface waters which are otherwise deficient in nutrients. Whenever the ocean surface is fertilized, the microscopic plants begin to grow and multiply, discoloring the water to a brownish or reddish color. If the subsurface waters continue to be pumped to the surface, the growth continues and a rich ocean pasture results. The animals which begin to crop this are the shrimps, and when they have thrived and reproduced, there are enough for the whales. The other all-important factor in the fertility of the polar seas is the length of day, which provides ample time for the sun’s energy to be entrapped by plants. By autumn, the days have shortened and night begins to predominate. At this time the microscopic plants stop growing and form resting stages which protect them through the long winter.