Population dynamics of Gila trout in the Gila River drainage of the south‐western United States

In some small, often unstable, streams of the Gila River drainage, New Mexico, Gila trout Oncorhynchus gilae populations fluctuated numerically seasonally and annually. Few differences were noted in length‐weight and size‐structure comparisons, but Fulton condition index varied significantly. Time of sample and time since disturbance (natural or human‐caused) were often associated with differences in condition. Other factors may include availability of prey, cannibalism, and reproductive condition. The repeatedly sampled McKnight Creek population illustrated the resilience of Gila trout populations to natural disturbances. A scouring flood in 1988 caused a >90% reduction in numbers, but, by 1992, population structure was not substantially different from that of other streams. Likewise, juvenile/adult ratio, density, and per cent large specimens (≥ 200 mm total length) of most other samples were within the ranges for the McKnight Creek population. Gila trout density (no. fish min−1 electrofishing) tended to increase with higher elevation and greater drainage density (stream km catchment−1 km−2) but decreased with larger catchments. Information gained in this study demonstrates that a variety of factors must be considered when evaluating the relative well‐being of Gila trout populations and illustrates the importance of larger, more hydrologically complex drainages to the long‐term survival of Gila trout populations.