Response of fish biota to dams in the Lower Colorado River Basin: empirical findings and utility for predicting responses to climate and water use change

Abstract: The biologists have done their jobs. We know the life cycles and habitat requirements of endangered western fishes... [A]successful management program could be devised and implemented for the Grand Canyon region. [But, c]urrent politics stand in the way, just as surely as politics of the 1960s aided and abetted our efforts to learn enough to save this fauna. - W. L. Minckley 1991

Development of water resources in the Colorado River Basin over the past century has changed a dynamic, seasonal river into a highly managed system. Conversion of lentic habitat to reservoirs has altered seasonality of flow, temperature, and turbidity. These changes have played out in the fish fauna, where native fish species have declined and non-natives have increased. Relative to the past, the key aspect of the new environment is competition and predation, especially at juvenile stages, which is likely mediated by more stable flow regimes and increased resource limitation from reduced floodplain-river interactions. A dataset (SONFISHES) assembled by W. L. Minckley and covering 150 years of fish occurrence in the Lower Colorado River (below Glen Canyon Dam) has enabled characterization of patterns in extirpations and range contractions among native fishes and expansions among non-native fishes. Several studies have combined these data with measures of extinction risk to explain how range fragmentation and species traits correlate with observed or threatened extinctions in native species. Another study analyzed range shifts within a strategy- space of potential fish life histories (originally introduced by K. O. Winemiller and K. A. Rose) to understand how human activity has created and removed ecological niches. Together, these studies support the view that alterations to the river environment have caused observed changes. Although there is a convincing link between changes in the fish biota and the modified environment, I argue that additional work is needed to make this knowledge useful for predicting responses to further modification. In particular, to predict future changes in fish biodiversity requires a model that quantitatively relates alterations in environmental factors to persistence in the strategy-space of fish life histories. Only with such a model can we know the expected magnitude of change in fish fauna for a projected change in environmental conditions. Unfortunately, even without such a model we can qualitatively predict the fates of threatened “big-river” fishes in the absence of political will to intervene. As the quotation above suggests, this will has been lacking since at least the 1990s.