Acoustic Tracking of Southern Flounder (<i>Paralichthys lethostigma</i>) Released in the Charleston Harbor Estuarine System, South Carolina
Hart, Morgan Paige
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Southern flounder is an economically important fish in the southern U.S. Atlantic and Gulf of Mexico coastal regions, but population numbers are declining. Genetic and morphometric studies have shown that the Atlantic population is distinct from the Gulf of Mexico, but there is little evidence of stock structure along the Atlantic coast due to extensive mixing (North Carolina to eastern Florida). Several studies have reported that sexually mature southern flounder move out of estuaries into offshore waters to spawn in the fall, but specific spawning areas and movement patterns along the Atlantic coast are not fully understood. The aim of this study was to investigate this by tracking the movement and fate of individual southern flounder using acoustic telemetry. First, several methods of acoustic tag attachment were tested to optimize fish survival and tag retention. We found that most fish (95 %) survived tagging at water temperatures below 25oC, but survival was poor (40-85 %) at higher temperatures. Also, fish with surgically implanted tags had better tagging recovery and tag retention than those with externally attached tags. After optimizing tag attachment, 118 acoustically tagged southern flounder (>275 mm total length) were released into Ashley River (Charleston, South Carolina) and their movements were tracked using a combination of fixed receivers and active tracking. Movement results show that 96% of the fish have been detected, 88% fish stayed within the river of release, and 12% moved into the Atlantic Ocean in the fall months. Movement distances between release location and the last date of detection varied from 0 to 263 km, with the largest distances (> 100 km) only occurring during January and December. The probability of an acoustically tagged southern flounder moving into and being detected in coastal waters increased significantly with its TL at the time of release. Among those fish not detected in coastal waters, some were detected continually within the Charleston Harbor estuarine system, whereas others ‘disappeared’ (i.e. they were not detected) during winter months but later reappeared during spring. Small-scale movements (<2 km) of fish within the Charleston estuaries (within the Ashley River and from the Ashley River into the Charleston Harbor and the reverse) were most prevalent in the fall and spring months, while summer movements were the most limited. Specific spawning areas are still unverified, but my study suggests that fish move into coastal waters off Georgia and/or Florida during the time of year when spawning occurs.