Microgeographical variation in ovum size of the blacktail shiner, Cyprinella venusta Girard, in relation to streamflow

– There is a strong, positive correlation between egg size and the discharge variable mean annual runoff among populations of Cyprinella venusta Girard from streams in different drainages ranging across the Gulf Coastal Plain of the southern United States. This variation may have resulted from selection for larger egg size, hence larger offspring size, because of greater environmental fluctuation or a greater number of floods in streams with higher runoff. In the present study, we examined C. venusta from populations inhabiting streams varying in mean annual runoff within a single drainage and found a similar correlation between ovum size and runoff. Therefore, the egg size–runoff relationship exists at a much smaller (i.e., intra‐drainage) spatial scale than previous research (based on widespread inter‐drainage variation) has shown. We also attempted to establish a link between mean annual runoff and streamflow‐mediated disturbance as indicated by predictability of streamflow, monthly coefficients of variation in discharge and number of days with extreme low and high flows. Although the overall predictability of discharge did not differ among streams, the streams with high mean annual runoff were characterized by smaller coefficients of variation in winter, fewer days of extremely low flows and a trend toward higher flows in late summer and early fall. At the intra‐drainage scale, higher coefficients of variation did not accurately reflect the numbers of extreme flows. These results call into question the use of coefficients of variation in discharge as indicators of disturbance. Given that eggs, larvae and small juveniles of C. venusta are present in late summer and early fall, when flows in high runoff streams tend to be high relative to low runoff streams, we conclude that these high flows may act as a selective force causing the evolution of larger egg size. Resumen 1. Estudio previos pusieron de manifiesto una fuerte correlación positiva entre el tamaño del huevo y el caudal medio anual entre poblaciones de Cyprinella venusta Girard, de cuencas de ríos diferentes que corren por las llanuras costeras del Golfo en el sur de Estados Unidos. Se planteó como hipotesis que esta variación hubiera resultado de una selección de mayores tamaños de huevo y de aquí mayores descendientes debido a una mayor variabilidad ambiental o a un mayor número de riadas en ríos con mayor escorrentía. 2. Examinanos poblaciones de C. venusta que habitan ríos que varían en esco