Acoustic Tagging Update from Sabine Lake

Since December 2020, Texas Parks & Wildlife Department (TPWD) and Texas A&M University at Galveston have been engaged in an acoustic tagging project to record fish residency within multiple estuarine and nearshore habitat types and monitor movement among estuaries along the Gulf of Mexico. The Sabine Lake System Acoustic Array (SLSAA) is part of the Integrated Tracking of Aquatic Animals in the Gulf of Mexico (iTAG), a network of more than 60 bay-scale and nearshore acoustic receiver arrays placed across much of the U.S. Gulf of Mexico coast. The SLSAA now consists of 31 receivers strategically placed throughout the Sabine Lake Estuary and nearshore Gulf. The arrangement of receivers is designed to monitor fish movements and residency within and between a variety of estuarine habitats, such as the Salt Bayou Marsh to the west of Sabine Lake, the Sabine Lake Oyster Reef, the Sabine and Neches Rivers, the Intracoastal Waterway, and multiple bayou distributaries connecting Sabine Lake to the Sabine National Wildlife Refuge to the east; receivers also monitor fish movement between Sabine Lake and the Gulf through Sabine Pass (Image 1). This study will provide valuable information that will shed light on the behavior of different fish species contributing to a better understanding of estuarine ecosystems by anglers and management agencies.

Fish tagging began in the fall of 2021, with a focus on Southern Flounder. In the fall, adult Southern Flounder migrate from the bays to Gulf to spawn. Of the total number of flounder tagged from September to early November 2021, those that were observed migrating to the Gulf of Mexico did so between November 12^th and December 13^th. They began aggregating in tidal passes and moving into the Gulf when the ambient water temperature decreased below 68°F. This behavior is consistent with studies that have suggested that in order to survive and quickly develop into healthy juvenile flounder, larvae require a water temperature between 59 to 73°F, but do best in water with a temperature range between 62 to 65°F. Additionally, if water temperature is towards the high end of that range (>65°F), then a greater proportion of the juvenile flounder will become male fish. This is because flounder produce larvae that are genetically approximately 50% female and 50% male. However, if exposed to environmental stressors, such as warm water, then some of those genetically female fish will develop into males. Ultimately, this results in a disproportionate abundance of adult male (sperm) to female (eggs) gametes produced during future spawning events.

The preliminary data from 2021 and future data collected from tagged fish will continue to provide insight into the life history of Southern Flounder and other recreationally important species, helping to inform management decisions. For example, the closure of the southern flounder fishery in Texas waters between November 1^st and December 15^th was enacted in 2021 to protect fish undergoing their spawning migration. Providing protection from fishing pressure during the “spawning run” allows for protection of the spawning stock and increases the opportunity for more juveniles to enter estuaries and ultimately recruit into the fishery. Movement patterns in 2021 supported this management decision because they show that all tagged flounder that were observed migrating to the Gulf did so while the closure was in place. Many tagged fish pinged multiple receivers as they moved through the system. The inferred path of a southern flounder that was tagged on September 24^th, 2021 in Salt Bayou appears to have stayed in Salt Bayou until late November when it staged near Keith Lake Fish pass from November 15^th to 26^th, before migrating through Sabine Pass on Nov 28^th and 29^th. We have also observed a number of individual flounder returning to Sabine Pass from the Gulf following spawning. Further investigation into the timing of fish movements such as these will allow for a more robust understanding of the migration and residency patterns of Southern Flounder.

During year two of the SLSAA project, the number of different species tagged was expanded to include Spotted Seatrout, Black Drum, Red Drum, Bull Shark and Alligator Gar. The project was able to leverage both TPWD routine sampling and enthusiastic local anglers to tag several individuals of each of these species. Other interesting preliminary results gathered from SLSAA tagging data thus far include Bull Shark and Black Drum entering Sabine Lake that were tagged in Galveston Bay; Spotted Seatrout tending to return to previously visited spots within a narrow range of movement; and slow methodical movements of Alligator Gar along the entire length of Sabine Lake from Sabine Pass to the Sabine River.

In year three, TPWD will be expanding this study to include another acoustic array in the Cedar Lakes region and will concentrate on tagging Alligator Gar in both the Cedar Lakes and Sabine Lake areas. Following Hurricane Harvey, populations of Alligator Gar along coastal Texas increased dramatically, providing a good opportunity to tag a large number of fish and record their movements to see if individuals are resident in their estuary or if inter-estuarine exchange occurs. Expansion of the arrays and coordination between TPWD and other research agencies and institutions is possible because the same types of receivers and tags are used by all of the iTAG acoustic arrays throughout the Gulf of Mexico. Since the battery life of tags ranges between 1 to 3 years depending on the size of the fish (larger fish can accommodate larger tags with longer battery life), we anticipate gathering data and information for many years from tagged individuals, which will be really cool. Stay tuned.