Effects of Low Salinities on Osmoregulation, Growth, and Survival of Juvenile Gulf Killifish

The Gulf Killifish Fundulus grandis , a popular baitfish, is a euryhaline fish species native to coastal estuaries of the Gulf of Mexico and the eastern coast of Florida. Established culture methods have relied on a saline water source but such methods have not been widely adapted by the commercial sector. Since this species is highly euryhaline, there is potential for freshwater culture via the utilization of established inland facilities. In order to assess the feasibility of inland culture, changes in Gulf Killifish hypo-osmotic tolerance throughout early growth were evaluated. Gulf Killifish were challenged with low salinity treatments of 0 (freshwater), 2.5, 5.0, and 7.5‰ at 2, 7, and 12 weeks posthatch for 4 weeks to determine growth, survival, gill Na ⁺,K ⁺-ATPase activity, and whole-body Na ⁺ content. Gill Na ⁺,K ⁺-ATPase activity and whole-body ion samples were collected at 0, 2, 14, and 28 d posttransfer (dpt) for each age-group. Results indicated Gulf Killifish at 2 weeks posthatch grew and survived at 2.5‰ as well as at higher salinities, but growth was reduced in freshwater. At 7 and 12 weeks posthatch, freshwater survival and growth rates at 0‰ salinity were comparable with other salinity treatments. Gill Na ⁺,K ⁺-ATPase activity increased in Gulf Killifish that were transferred to freshwater at both 7 and 12 weeks posthatch. Whole-body Na ⁺ content was higher in freshwater 28 dpt in the 2-week-posthatch age-group, and 14 dpt in the 7-week-posthatch age-group. There were few differences in Na ⁺,K ⁺-ATPase activity and whole-body Na ⁺ content between salinity treatments in the 12-week-posthatch age-group. These results suggest Gulf Killifish develop hypo-osmotic tolerance at less than 2 weeks of age posthatch and culture in low salinities including freshwater will be more successful with fish older than 7 weeks of age.