Timing and trends of the Southern Flounder spawning migration in two Texas tidal inlets

Objective The Southern Flounder Paralichthys lethostigma is an ecologically and economically important sport fish species found in the northern Gulf of Mexico and the western Atlantic Ocean. However, Southern Flounder populations have been experiencing a long-term decline despite the implementation of increasingly stringent fishing regulations. Limited knowledge about the fishery likely contributes to the lack of regulatory success. This study aimed to explore the timing of the Southern Flounder spawning migration, demographics of the migration, and their impact on the effectiveness of fall season closures. Methods A series of weekly transect surveys was conducted in the fall (September–December) of 2022 and 2023 at two Texas tidal inlets to estimate Southern Flounder density trends throughout the offshore fall spawning migration period as a proxy for migration timing. Boosted regression trees and generalized additive models were used to determine environmental factors that may initiate movement, and the gonadosomatic index was used to estimate peak spawning time. Results Migration occurred in periodic peaks following 4°C or greater decreases in water temperature, with peak migration overall occurring in late November to early December of both years. Peak migration occurred later during the second year of the study (2023), when water temperature decreases were milder. Smaller females (<1 kg) and most males migrated first, primarily in October, followed by larger females (>1 kg) migrating in November and December. The gonadosomatic index suggested a spawning time from December to early January, and the sex ratio was 1.0:4.7, favoring females. Conclusions These findings suggest that environmental changes, such as the variability of annual water temperatures, may affect Southern Flounder population dynamics and key life history patterns, potentially inhibiting the effectiveness of management regulations. Variable migration timing due to warming waters may result in less successful recruitment and delayed spawning, which may decrease the effectiveness of fixed seasonal closures.