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 individuals provided evidence of genetic structure among 14 cod (Gadus morhua) populations spanning the range of the species in the northwest (NW) Atlantic. Using DA and Ds, measures of genetic distance, as well as F-ST and R-ST measures of population structure, differences are revealed among populations 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 genetic structure at spawning-bank scales consistent with variation in oceanographic features and in the spatiotemporal distribution of spawning, each of which may represent barriers to gene flow among geographically contiguous populations inhabiting a highly advective environment. The differences described are consistent with postdispersal spawning fidelity to natal areas,a behaviour that may be facilitated by topographically induced gyre-like circulations that can act as retention mechanisms. Significant degrees of substructure among neighbouring and contiguous cod populations may be most easily explained by the associated oceanographic features and processes that conceivably form the template for the evolution of the structure. We suggest that bathymetric and hydrodynamic structure represents a rational starting point for developing hypotheses to examine the processes that lead to the genetic structuring of marine fish species.