Genetic impacts of a commercial aquaculture lease on adjacent oyster populations

Commercial aquaculture leases typically support dense aggregations of shellfish that may contribute to wild populations through their reproductive efforts. Aggregations are thought to facilitate successful spawning and fertilization, potentially producing large numbers of larvae that may recruit to wild populations. We investigated the reproductive contribution of cultured oysters (Crassostrea virginica) to wild populations around a commercial aquaculture lease in Stump Sound, North Carolina. Through the late 1990s, the lease was stocked with oysters that originated from remotely set larvae that were produced in a hatchery in Louisiana using oysters from the Gulf of Mexico, which are genetically distinguishable from North Carolina oysters. We sequenced 359-bp of the mitochondrial 16S ribosomal (16S) gene to identify oysters exhibiting mitochondrial DNA haplotypes characteristic of Gulf Coast-derived oysters. An initial evaluation in 2001 of oysters collected from seven natural beds in and around the lease site showed a significantly elevated frequency of oysters exhibiting the 16S Gulf Coast (GC) haplotype than typically observed in NC oyster populations. When the same sites around the lease were resampled in 2015, the 16S GC haplotype was not detected, suggesting the elevated frequency of this haplotype in 2001 was transient. A variant of the 16S GC haplotype, not observed in the aquaculture stock, was observed at higher frequency in the second sampling period relative to the first. The detection of a mitochondrial haplotype with Gulf Coast ancestry not associated with recent aquaculture activities suggests that the genetic impacts on wild oyster populations may vary with the fitness of cultured oysters to local environmental conditions.