Landscape biogeography and population structuring of a facultatively amphidromous galaxiid fish, Galaxias brevipinnis

Processes responsible for population structuring across spatial and temporal scales represent key components in understanding speciation and evolution. We use a hierarchical approach to investigate the degree and mechanisms of structuring in landlocked and diadromous populations of the facultatively amphidromous fish Galaxias brevipinnis (koaro) across various temporal and spatial scales in southern New Zealand. To deter-mine long-term structuring of koaro, populations in multiple lakes and coastal sites were compared genetically. Short-term population structuring was assessed using otolith microchemistry for a subset of sites, and behav-ioural mechanisms driving population structuring were assessed via larval distributions. Genetic data show that lakes foster divergence of lake-developing koaro populations from each other and from coastal stream popula-tions, but there is relatively little structuring within coast or lake populations. Otolith analyses, however, indicate that on a shorter timescale, most koaro larvae do not disperse widely, i.e. recruitment is local. Thus, lake and coastal populations show a distinct meta-population structure based on catchment, in contrast to the prevailing assumption of widespread dispersal, with implications for management. Most koaro larvae were distributed in river plumes, suggesting that a simple larval behavioural mechanism, e.g. positive rheotaxis, may result in larval retention within catchments and lakes. Larvae not retained in plumes create opportunities for genetic exchange within-lake or among coastal sites. Genetic divergence of lake populations of koaro, resulting from both intrinsic behavior and extrinsic landscape, provides insight into the potential of G. brevipinnis to diversify and speciate when landscape and circumstances align, with implications for the management of this and other facultatively amphidromous species.