Genetic divergence among northern pike from spawning locations in the upper St. Lawrence River

To assess the ecological consequences associated with the degradation of riparian wetlands that historically provided spawning and rearing habitat for northern pike Esox lucius, annual spawning migrations and genetic structure were used to examine this species' dependence on four specific spawning areas in the Thousand Islands region of the Saint Lawrence River. Tagging and recapture over three consecutive spawning seasons resulted in low return rates (ranging from 1.2% in bays to 8.3% in tributaries), and some exchange was observed between locations. Angler recaptures indicated that northern pike did not disperse widely; 72% were recaptured less than 2.5 km from their tagging location, though the remainder moved distances ranging from 2.5 to 18 km. Six microsatellite loci showed significant allele frequency differences and small but significant genetic differentiation (F_ST) values among spawning sites in close proximity (<5 km). However, two sites that were geographically distant (>20 km) were not statistically different. A spawning site sampled in two consecutive years showed no discernable divergence, providing evidence for temporal stability in allelic frequencies. A plausible explanation for these mixed results is the existence of intraspecific contingents adapted to heterogeneous types of spawning and nursery conditions that promote multiple reproductive strategies.