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reduce the number of larvae produced. Other things being equal, a drop in production of larvae would decrease recruitment. The condition of the physical habitat also is strongly influenced by the hydrologic regime because of relationships between flow and extent of habitat, or between flow and sediment transport, for example. Water quality, another abiotic factor, could cause mortality via pollutants, or reduce recruitment by more subtle effects like delay of spawning due to colder water temperatures. In general, however, water quality effects other than temperature have been studied litfle. Biotic factors are most likely related to predator-prey relationships, biodiversity, and changes in productivity. In the present altered environment, predation and competition for food and/or space by nonnative fishes also may be factors. Environmental factors that regulate the abundance of a life history stage, or a population, are considered "limiting factors." There are abiotic and biotic factors that regulate growth and mortality, and the relative importance of these limiting factors may vary in time (e.g., with season or with life history stage) or space (habitat occupied by a particular life history stage at a particular time of year). Especially for species that are endangered, and thus rare, it may be difficult to define rigorously the factors limiting population size. A certain amount of inference based on best professional judgment therefore becomes necessary. At least some of the endangered fishes require large geographic areas and have a complex life cycle (including spatial separation of life history stages). It is essential to understand there is not one set of limiting factors to overcome if recruitment is to be improved. Each life stage must be addressed. .; Basin-Wide Limiting Factors Abiotic Abiotic limiting factors maybe viewed as those factors that affect the suitability of the physicochemical components of habitat or alter productivity Physical habitat parameters such as water flow, depth, passage, etc., have been changed in most locations of the Colorado River and its tributaries. Water quality in some locations in the. basin has been degraded due to various causes, but historic records are scant. Rivers are often used to remove unwanted debris, and the UCR basin is no exception to this. Dead cattle, sheep, chickens, and other domestic and agricultural wastes have been dumped into UCR basin streams (all have been observed by HMT). In addition, -most of the UCR basin is an oil producing region, and refuse oils and other toxic substances have been found its streams, to the detriment of fishes. One report that documented contaminated water samples taken from the Green River that contained 50% oil .also recorded heavy losses of fishes, "...particularly Colorado River salmon (Ptychocheilus Lucius)..." (Anonymous 1953). Oil spills have occurred more recently, including a spilt from a broken pipeline at a Yampa River crossing near Craig, Colorado. In 1989, 13,200 gal. of crude oil escaped from a broken 6" pipe (Garner 1989). This spill occurred during the annual Colorado pikeminnow spawning migrations and may have interfered with olfactory cues (Woolf 1989) or destroyed eggs or larvae, because 16