Comparative Demography of Two Small Stream Fishes in Response to Variable Stream Flow

Populations of western stream fishes are strongly influenced by variation in stream flow driven by local climatic variation.  Drought results in lowered stream flow and corresponding changes in survival and reproduction of stream fishes.  We would expect selection to result in similar vital rates for all stream fishes in a given system.  To test for differences between species we conducted a long-term mark-recapture study on northern leatherside chub (Lepidomeda copei) and speckled dace (Rhinichthys osculus) in a small tributary of the Bear River (Yellow Creek) in southwestern Wyoming.  Our three recapture efforts included one high water year and two extremely low water years.  We calculated survival, transition, and fecundity estimates for “good” and “bad” years and compared resulting demography between years and species.  In good years when populations are increasing, juveniles represent about ½ to 2/3 of the population for both species.  Both survival and fecundity rates declined sharply in bad years for both species.  Overall, speckled dace were demographically more resilient to bad years than northern leatherside chub.  Small changes in incidence and severity of drought (or increased water withdrawals) will likely lead to extirpation of isolated populations of northern leatherside chub.