Salinity Tolerance of Flathead Catfish: Implications for Dispersal of Introduced Populations

—The flathead catfish Pylodictis olivaris is a large predatory fish that has been introduced widely beyond its indigenous range to the detriment of many native fish communities. It disperses rapidly within and among river systems, but its potential to use saltwater or brackish waters for migration and exploitation of marine prey resources is unknown. We evaluated the salinity tolerance of juvenile flathead catfish exposed to NaCl and synthetic seawater (0–34‰) in direct-transfer acute toxicity tests. The 96-h median lethal concentration (LC50) for fish exposed to NaCl at 188C was a salinity of 10.0‰ with a 95% confidence interval (CI) of 9.0–11.1‰, whereas the 96-h LC50 for fish in synthetic seawater (Instant Ocean) at 188C was 14.5‰ (95% CI, 13.7–15.5‰). To determine whether fish could survive the transition from freshwater to brackish water and then back to freshwater, fish were transferred from freshwater at 188C to water with a salinity of 8, 11, or 14‰ for 24 h and then returned to freshwater for 48 h. Fish exposed to synthetic seawater with a salinity of 8 or 11‰ for 24 h had more than 95% survival for 48 h after transfer to freshwater; however, no fish transferred directly to synthetic seawater with a salinity of 14‰ survived the 24h exposure period. Additional acclimation tests with fish that were exposed to synthetic seawater in which salinity was increased daily by 4‰ yielded an LC50 of 15.8‰. Our results provide evidence that flathead catfish could tolerate exposure to many brackish waters along the Atlantic and Gulf of Mexico coasts of the United States and that the dispersal of introduced flathead catfish populations among rivers may not be limited by estuarine salinities. The flathead catfish Pylodictis olivaris is a large predatory catfish that has been introduced widely beyond its native range, including into coastal rivers of the Gulf of Mexico and the Atlantic slope of the United States. Native to the Mississippi, Mobile, and Rio Grande drainages and to portions of the Great Lakes region and northern Mexico, * Corresponding author: [email protected] 1 The Unit is jointly supported by North Carolina State University, North Carolina Wildlife Resources Commission, U.S. Geological Survey, and Wildlife Management Institute. Received October 27, 2004; accepted March 18, 2005 Published online June 22, 2005 this fish has been introduced into waters of at least 13 states and one Canadian province (Jackson 1999). Populations are easily established, by translocation of as few as 11 individuals, and this species can become the dominant predator in a river as rapidly as 15 years after introduction (Guier et al. 1984; Jenkins and Burkhead 1994). Introduced flathead catfish populations exist today in a majority of coastal river systems on the Atlantic slope of the United States from Georgia to Pennsylvania as well as in those of the Florida panhandle. The rapid dispersal within and among rivers, high individual and population growth rates, and obligate carnivorous food habits of the flathead catfish have raised concern for native fish conser-