Originally, the waters of the desert also could be described as fish deserts. Only three recognized kinds of food fish, as we know them today, inhabited the waters of this region. The trout was limited to the cutthroat species or subspecies. These were found as one subspecies in the Rio Grande drainage and the other subspecies in the Arkansas River drainage. The latter is reduced in numbers but a few remnants remain in the headwaters. The former is being artificially propagated and is being maintained successfully. The trout found in the Gila River drainage, originally thought to be a cutthroat, then a rainbow, has been recently described as a separate species. These trout, like the Arkansas River fish, remain as remnants in the headwaters. For the most part, the native trout have had a difficult time surviving in recent years because of the change in water and land use; the habitat has become exceedingly restricted. Channel catfish were originally found in the Rio Grande drainage and continue to maintain themselves satisfactorily. Water and land manipulation, however, present a serious threat, even to this hardy fish. There are records that the gulf eel once came up the Rio Grande during part of its life cycle. This fish of the ocean has long since disappeared—the fate of many migrating fish when confronted with man-made obstructions in the watercourses. Waterfowl were in great abundance once, in both the upper Rio Grande and the Pecos River drainages. These birds utilized the vast marsh areas of the valleys during spring and fall migrations. It is presumed that there was very little nesting of waterfowl in the early years. The mammals that live in or near the water, principally fur bearers, were in abundance as long as water habitat was abundant and prior to unrestricted trapping. With the exception of the beaver (since restocked and protected), many of the fur bearers once numerous are now extremely limited in numbers or approaching extinction. There is only an occasional sighting of the otter which was once relatively plentiful. Habitat for the various water mammals, birds, and fish is found in the drainage systems. The major system is the Rio Grande that flows through the center of the state; the Pecos River is its tributary. In this system are six of the seven life zones and conditions of habitat suitable to almost any form of water-dwelling life. There is a noticeable habitat change due to several causes, such as ranching, irrigation, urban development, and a general minor change in climatic conditions. Many variations are in direct relation, such as improper grazing, unscreened irrigation diversions, dams, and sewage disposal, to mention a few. There is an indication of a long-term warming of the general Southwest, and though this change is fractional, it does have a basic effect on habitat.