The potential of migration to replenish a genetically depauperate population: A case study of lake sturgeon on the Grasse River
Lake sturgeon, Acipenser fulvescens, are of ecological importance throughout the Great Lakes region, but are of conservation concern. Dams are one factor that can limit lake sturgeon population persistence by preventing movements into heterogeneous habitats, which are critical during various life stages. Loss of migration into a population can result in loss of genetic diversity in that population due to genetic drift. Restoring migration can potentially restore some of that lost diversity. In the Grasse River in New York (a tributary of the St. Lawrence River), lake sturgeon were isolated behind a dam and the resulting population had lower genetic diversity. Ten years ago, the dam was breached, potentially restoring connection between the Grasse River and the more genetically diverse St. Lawrence River. Our research questions were whether upstream and downstream migration rates changed in the Grasse River following the dam breach and how much migration would be required to restore the genetic diversity of the Grasse River. Samples from the Grasse River (n=60) and samples from the St. Lawrence River (n=82) were analyzed to see if individuals migrated into and out of the Grasse River following the dam breach. Population genetic models were run using the software VORTEX to simulate how much migration would be required into the Grasse River in order to restore its genetic diversity.
Following the dam breach, 2% of the individuals were identified as migrants from the St. Lawrence River into the Grasse River, indicating the potential for upstream migration into the Grasse River. In the period prior to the dam breach (1994-1995), 6% of the individuals in the St. Lawrence River were identified as migrants from the Grasse River. In the period after the dam breach (2006), 11% of the individuals in the St. Lawrence River were identified as originating from the Grasse River. Therefore, downstream migration from the Grasse into the St. Lawrence increased after the dam breach. Our population model suggested that, in the absence of migration, the allelic diversity in the Grasse River lake sturgeon population would decrease from an average of 3.2 alleles/locus to 1.8 alleles/locus over 100 years. However, with as little as 5% migration, the allelic diversity would increase, with an average of 4.8 alleles/locus over 100 years. Therefore, the restoration of migration in this river system could result in the recovery of lost genetic diversity in this previously isolated lake sturgeon population.