Spatiotemporal variation in length-weight relationships of endangered humpback chub: Implications for conservation and management

The largest population of endangered humpback chub Gila cypha inhabits the lower Little Colorado River (LCR) and the main-stem Colorado River near its confluence with the LCR in Grand Canyon, Arizona. At present, fish in both rivers spawn almost exclusively in the LCR. Flows in the main-stem Colorad o River are regulated by Glen Canyon Dam, and water temperature approximate s predam winter temperatures year-round. The LCR continues to provide a rel atively natural hydrograph and seasonal warming patterns. Length-weight rel ationships among adult humpback chub from the lower Colorado River basin sh owed a seasonal pattern of declining condition during spring spawning seaso n followed by recovery of condition during summer through early winter. Fis h from the main stem recovered condition more rapidly after reproduction th an did fish from the LCR and may have benefited from dam-mediated environme ntal changes. Grand Canyon Colorado River fish had the greatest weight at l ength of eight locations sampled in the upper and lower basins. Records sin ce 1978 indicate a decline in condition of lower basin humpback chub coinci dent with a reported decline in population size in Grand Canyon. We recomme nd increased monitoring of all populations, including mark-recapture studie s to provide population estimates during the spawning season and condition monitoring during October-November to determine more subtle changes in fish health.