Interannual variation in connectivity and comparison of effective population size between two splittail (Pogonichthys macrolepidotus) populations in the San Francisco Estuary

The discovery of two genetically distinct splittail populations within the San Francisco Estuary, one which spawns in the rivers of the Central Valley and another in the Petaluma and Napa Rivers of the San Pablo Bay, prompted the need to evaluate their degree of connectivity and relative sizes. We genotyped multiple age-0 splittail cohorts using 19 microsatellite loci to assess any spatiotemporal changes in the distribution of the two populations and estimate their effective population sizes (Ne). Genetic population assignments demonstrated that while age-0 splittail are predominantly spatially segregated by populations, substantial geographical overlap may occur during years of high precipitation. However, despite this periodic range overlap, the original observed population structure has persisted for nearly a decade which has included a similarly wet year. This suggests that the present population structure will likely persist in the future due to strong philopatry and/or adaptive differences. We also found that Ne estimates were generally lower for the San Pablo Bay population than the Central Valley population, which is consistent with the relative amount of habitat availability in the two locations and genetic diversity indices. The relative isolation and apparent lower Ne of the San Pablo Bay splittail population indicates a higher vulnerability to extinction. A more consistent monitoring effort of splittail in the Petaluma and Napa Rivers may be necessary in order to better understand the future viability of this less studied population.