A nuclear DNA basis for shelf- and bank-scale population structure in northwest Atlantic cod (Gadus morhua): Labrador to Georges Bank

Variation at five microsatellite DNA loci scored in approximate to 1300 ind ividuals provided evidence of genetic structure among 14 cod (Gadus morhua) populations spanning the range of the species in the northwest (NW) Atlant ic. Using DA and Ds, measures of genetic distance, as well as F-ST and R-ST measures of population structure, differences are revealed among populatio ns at continental shelf scales (NE Newfoundland Shelf, Grand Banks, Flemish Cap, Scotian Shelf, Georges Bank) where regions are separated by submarine saddles, channels and trenches. However, we also provide evidence of genet ic structure at spawning-bank scales consistent with variation in oceanogra phic features and in the spatiotemporal distribution of spawning, each of w hich may represent barriers to gene flow among geographically contiguous po pulations inhabiting a highly advective environment. The differences descri bed are consistent with postdispersal spawning fidelity to natal areas,a be haviour that may be facilitated by topographically induced gyre-like circul ations that can act as retention mechanisms. Significant degrees of substru cture among neighbouring and contiguous cod populations may be most easily explained by the associated oceanographic features and processes that conce ivably form the template for the evolution of the structure. We suggest tha t bathymetric and hydrodynamic structure represents a rational starting poi nt for developing hypotheses to examine the processes that lead to the gene tic structuring of marine fish species.