Spatial and temporal variation in the genetic composition of a larval cod (Gadus morhua) aggregation: cohort contribution and genetic stability

Polymorphism at six microsatellite DNA loci among cod larvae sampled repeatedly over a 3-week period from an aggregation on Western Bank of the Scotian Shelf provided evidence of several heterogeneous groups within the aggregation. There was strong evidence of heterozygote deficiency and departure from Hardy-Weinberg expectations for the larval aggregation as a whole (N = 1337) and for all larvae sampled within a single water mass (CW larvae), but not for a subset of these larvae considered to be part of a single cohort on the basis of age at length. These results suggest that both the entire aggregation and the CW subset originated from several distinct spawning events involving spawners with heterogeneous allelic compositions, but that the larvae forming the cohort originated from a single spawning event. Our results establish a link between the ecological match-mismatch hypothesis and the genetic 'sweepstakes' selection hypothesis. There was no evidence that the larvae originated from different populations as measured by (δμ)² distance, R_ST and F_ST estimates among subsets. Additional analyses showed the larval cohort to have greater genetic similarity to adult cod sampled on Western Bank 2 yr later than to adult cod sampled on Banquereau Bank (≃150 km away) also 2 yr later. These results suggest that the genetic composition of cod on Western Bank remains stable over time.