Genetic evidence for a fall spawning group of Gulf sturgeon ( Acipenser oxyrinchus desotoi ) in the Apalachicola River, Florida

The Gulf sturgeon ( Acipenser oxyrinchus desotoi ) is a large, long-lived, anadromous fish inhabiting the northern Gulf of Mexico. This charismatic fish was hunted to near extinction in the early 1900’s and in 1991 was listed as threatened under the Endangered Species Act. Recovery continues to be challenged by threats such as habitat destruction, fisheries bycatch, and climate change. There are seven known natal rivers, historically thought to each contain a single, spring-spawning group. In recent years, multiple rivers (Suwannee, Choctawhatchee) have been shown to contain a second, fall-spawning group. This study utilizes genetic techniques to investigate the proposed existence of a fall-spawning group in the Apalachicola River, Florida. This river once supported one of the largest Gulf sturgeon populations, but post-exploitation faces a difficult path to recovery. The Jim Woodruff Lock and Dam blocks access to the majority of historic habitat in the system, with the only known remaining spawning grounds located immediately downriver. Tissue samples from juvenile Gulf sturgeon were collected between 2013 and 2022 and used to assess genetic population structure within the Apalachicola River. Analyses suggested two distinct genetic groups (F _ST = 0.085). Based on dates of capture, length frequency distributions, and genetic assignment of spawning adults, these were identified as spring- and fall-spawning groups. Approximately 90% of juveniles collected were assigned to the spring, but only slight differences in genetic diversity were detected between groups. The temperature window for spawning was found to be three weeks shorter on average in the fall than the spring, highlighting the need for additional research into differing environmental or anthropogenic influences on these populations. The discovery of a fall-spawning group of Gulf sturgeon in the Apalachicola River improves our understanding of the representation, redundancy, and resiliency of the species and provides critical information for improved management of this river system.