Estuarine-coastal connectivity and partial migration of southern flounder in the Gulf of Mexico

Southern flounder (Paralichthys lethostigma) from an estuarine complex in the Gulf of Mexico were tagged with acoustic transmitters (n = 60) from 2016 to 2018 to assess estuarine-coastal connectivity during fall/winter spawning runs. Both egress and residency behaviors were observed for adult southern flounder caught in the Galveston Bay Complex (GBC), and individuals were classified as migrators or residents based on their maximum distance detected (MDD). Migrators (MDD >5 km; mean 13.8 km) displayed directed egress out of the GBC, with individuals moving through the tidal pass in November and December, peaking in mid to late December. In contrast, residents (MDD <5 km; mean 0.6 km) showed limited movements and were often detected in the same general area throughout the fall, winter, and spring, indicating overwintering in the GBC. Conventional tagging was also performed on over 1300 southern flounder and mean MDD for all recaptured fish was 5.7 km. Mean monthly MDD of conventionally tagged fish was also highest in December and linked to egress from the GBC. Although directed migrations into the Gulf were observed for southern flounder with both tagging approaches, a meaningful fraction of the population displayed sedentary tendencies with MDD less than 1-2 km and retention within the GBC. The coexistence of two migratory contingents with contrasting estuarine-coastal migration behaviors is symbolic of partial migration by southern flounder, which may influence the resilience and stability of the population. Given that the timing and magnitude of peak egress out of this estuarine complex did not align with management regulations intended to protect spawning adults (i.e., peak movement after reopening of fishery), our findings also indicate the need to extend future closures to protect migrators during the primary egress period.