Near absence of hybridization between sauger and introduced walleye despite massive releases

Abstract

We analyzed 11 microsatellite loci to determine the genetic population structure of saugers and to describe the amount and pattern of hybridization with widely introduced walleyes in the upper Missouri River drainage. We detected significant genetic differentiation among saugers spawning upstream and downstream of the confluence of the Bighorn and Yellowstone Rivers. Samples from upstream of the confluence had significantly lower heterozygosity and allelic richness. We suspect this reflects isolation at the periphery of the species’ range and possibly genetic drift resulting from recent population contractions due to dams, diversions, and water development. We detected only eighteen hybrids out of 925 individuals analyzed. Hybridization appeared recent, as nearly 50% of the hybrids showed significant evidence for having a non-hybrid ancestor within two generations. All but one hybrid were detected in the Yellowstone River drainage, despite a substantially higher rate of walleye stocking in the Missouri River drainage, suggesting conditions in the Yellowstone may be more conducive to hybridization. Rarity of hybridization is unexpected, given both massive walleye releases and previous findings of more frequent hybridization in the study area. Introgression of walleye genes into native saugers does not appear to be an immediate threat.