Strong regional structure in the population genetics of reef-associated Bluebacked Sprat, Spratelloides delicatulus, along the Great Barrier Reef

Clupeiform bait fishes play critical roles in marine food chains and fisheries worldwide. Despite a paradigm of large stocks in temperate regions, the structure of clupeid populations in complex coral reef environments is not described. This study assessed the population structure of a small reef-based clupeiform, Spratelloides delicatulus , over spatial scales of kilometers to 1000’s of kilometers on the Great Barrier Reef using sequence data from the control region of the mitochondrial D-Loop. Following confirmation of the species, we found two strong stock units within the metapopulation of the GBR. The Northern and Southern stocks were genetically significantly different.. Within these stocks surprisingly strong structure was observed among reefs separated at scales of less than 10km. A few individual reefs appeared to play a disproportionate functional role in the generation and maintenance of genetic diversity, likely influenced by neutral factors while other more homogenous reefs are likely influenced by repeated bottlenecks. The short lives and complex stock structure we have found for sprats on coral reefs contrasts with the findings from traditional stocks of high latitude clupeiforms.