Molecular genetic and stable isotope signatures reveal complementary patterns of population connectivity in the regionally vulnerable southern pygmy perch (Nannoperca australis)

Population connectivity (and isolation) and genetic diversity in declining species are important considerations for species recovery and habitat restoration programs. For lotic species, stream confluences with major rivers may be a determinant of population isolation among streams, although small-scale dynamics in population connectivity within streams may also represent attributes of population functioning (e.g. resilience, recolonisation) that are important for species recovery programs. In this study, we examined the importance of among- and within-stream patterns of population connectivity and genetic diversity in the regionally vulnerable southern pygmy perch (Nannoperca australis) in a series of degraded streams in south east Australia in which population declines have been reported. We used molecular markers (mtDNA and microsatellites) to examine among-stream patterns of population connectivity and genetic diversity, and molecular markers and stable isotopic signatures of nitrogen and carbon to examine within-stream connectivity in the species. Results revealed that populations of N. australis are isolated among streams, and that streams therefore represent functional population units for species recovery and habitat restoration programs. Within-streams, local populations were semi-discrete and connected by various degrees of dispersal, with the degree of connectivity differing between the streams. Genetic bottlenecks were not detected, although the effective population size of N. australis in the more fragmented stream may be lower than in the stream where greater population connectivity was found. Protecting and augmenting refugial habitat of extant populations and restoring corridor functions between refugia within streams will generate the greatest biological response at within-stream scales.