Fine-scale habitat preferences influence within-river population connectivity: a case-study using two sympatric New Zealand Galaxias fish species

<ol> <li><p>Understanding the ecological factors governing population connectivity in freshwater systems represents an ongoing challenge for aquatic biologists.</p> </li> <li> <p>We used genetic analysis to test the hypothesis that fine-scale habitat preferences can influence within-river connectivity patterns in freshwater-limited fishes.</p> </li> <li> <p>Genetic variation among sampling sites within systems was compared for co-distributed fish taxa <em>Galaxias gollumoides</em> (low-velocity pool/swamp habitat) versus an undescribed species, <em>Galaxias</em> ‘southern’ (high-velocity riffle habitat), using <em>ϕ</em> statistics. These analyses incorporated 1624 bp of mtDNA from 429 galaxiid specimens, encompassing 95 localities across four river systems in Southland, New Zealand.</p> </li><li> <p>Although similar numbers of haplotypes per river were detected in both taxa, spatial partitioning of this genetic variation within rivers was significantly stronger for <em>G. gollumoides</em> (mean <em>ϕ</em>_ST = 0.539) than for <em>G. ‘</em>southern’ (mean <em>ϕ</em>_ST = 0.142). Within-catchment genetic divergence values were also higher for the former species.</p> </li> <li> <p>These finding suggest that a combination of main-channel habitat continuity and high water velocity facilitates relatively high connectivity in <em>G. ‘</em>southern’, whereas <em>G. gollumoides</em> populations are genetically isolated in fragmented low-velocity habitats. We conclude that population connectivity can be strongly influenced by habitat preference, leading to profound differences in the phylogeography, diversity and conservation status of freshwater fish lineages.</p></li> </ol>