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 18 degrees C was a salinity of 10.0 parts per thousand with a 95% confidence interval (Cl) of 9.0-11.1 parts per thousand, whereas the 96-h LC50 for fish in synthetic seawater (Instant Ocean) at 18 degrees C was 14.5 parts per thousand (95% CI, 13.7-15.5 parts per thousand). To determine whether fish could survive the transition from freshwater to brackish water and then back to freshwater, fish were transferred from freshwater at 18 degrees C to water with a salinity of 8, 11, or 14%c for 24 h and then returned to freshwater for 48 h. Fish exposed to synthetic seawater with a salinity of 8 or I 1%c 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 parts per thousand survived the 24h exposure period. Additional acclimation tests with fish that were exposed to synthetic seawater in which salinity was increased daily by 4 parts per thousand yielded an LC50 of 15.8 parts per thousand. 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.