9547 - Laserfiche WebLink

Home Browse Search

effects like delay of spawning due to colder water temperatures. In general, however, water quality effects other than temperature have been studied little. Biotic factors are most likely related to predation or competition from nonnative fishes, but may also ~ include food supply. Environmental factors that regulate the abundance of a life history stage, or a population, are considered °limiting factors." There are biotic and abiotic 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. ~ Identifying limiting factors is the first step in developing plans for enhancing recovery of the endangered fishes. The next step is assigning priorities for alleviating the limitations imposed by each of the factors. Ideally, priorities for management actions should be established based on the number of fish that will be added to the population. It probably will not be sufficient to address problems one at a time, because multiple ~ factors may be acting in concert. In the review that follows, limiting factors are examined on two overlapping scales: basin-wide and species-specific. Limiting factors that exist on the basin-wide scale will affect some or all life history stages of one or more of the endangered fishes. These ~ include abiotic factors such as flow and temperature (which are linked to some extent), and biotic factors such as the abundance of nonnative fishes. Basin-wide factors will be reviewed separately because it is easier to understand their origins and the pervasiveness of their effects. It is also necessary to examine limiting factors from a species-specific perspective because it provides the temporal and spatial focus necessary for developing recovery plans efficiently. For each species, limiting factors ~ will be reviewed in the context of life history because limiting factors may be different at each stage, especially where the stages occupy different habitat (e.g., Colorado pikeminnow). Basin wide Limiting Factors Abiotic The construction of dams and diversion structures in the Colorado River basin has converted much riverine habitat into reservoirs and smaller lacustrine habitat. Loss or ~ alteration of habitat has been extensive and is documented elsewhere (e.g., Carlson and Muth 1989, Minckley and Deacon 1991).. This loss is, for practical purposes, irreversible. The presence of these structures and their role in regulating flows have other, albeit less direct, effects on fish habitat. Structures in the channel may constitute 24