申論題

1. Fish are cold-blooded animals, typically with backbones, gills, and fins, and are primarily dependent on water as a medium in which to live. Fishes are also the most numerous of the vertebrates. Best estimates are from 15,000 to 17,000 recent species. At present, more than 70 percent of the earth's surface is covered with water. For the fish, the most important aspects of water are dissolved oxygen, light penctration, temperature, toxic substances, concentrations of disease organisms, and opportunity to escape enemies. Most species of fish have relatively small home ranges to which they restrict most of their movements. However, some fishes are great travelers and seem to be continually on the move. Very mobile are   pelagic fishes   and others which often travel great distances between fresh and marine waters. Generally, a migration is a more or less continuous and direct movement from one location to another. Such movement is under the control of the fish, genetically determined, and is influenced by environmental factors. Migrations are mostly for   spawning  or feeding and are generally time oriented, although sometimes they take place when fishes actively flee adverse conditions.