Stability in the historical pattern of genetic structure of Newfoundland cod (Gadus morhua) despite the catastrophic decline in population size from 1964 to 1994

We report on evidence of long term stability in the geographic pattern of genetic differentiation among cod (Gadus morhua) collected from 5 spawning banks off Newfoundland and Labrador over a period spanning three decades (1964-1994) and 2 orders of magnitude of population size variation. Six microsatellite DNA loci amplified from archived otoliths (1964 and 1978) and contemporary (1990s) tissue samples revealed fidelity to natal spawning banks over this period. A two level (spawning bank and decade) hierarchical and multilocus AMOVA indicated that 1.55% of the total variation in allele frequencies could be attributed (P = 0.036) to spatial structure while no variance component could be attributed to temporal changes. A finer scale analysis among cod from just 3 of these spawning banks reveals, however, evidence consistent with some post-collapse mixing between cod from two banks. In the context of fisheries management and conservation, the survival of the spatial pattern of genetic differentiation during the population collapse suggests that if recovery eventually occurs it will likely be through population re-growth in situ rather than by migratory influx.