Salinity tolerance and seawater survival vary ontogenetically in Florida red tilapia
Salinity tolerance at 10, 25, 40, 55 and 70 days post-hatching was determined in Florida red tilapia fry spawned at 5 ppt. There was a trend toward increased tolerance with age, with mean survival time following abrupt transfer to 32 ppt increasing from 190 min at 10 days post-hatching, to 3915 min at 70 days post-hatching. Ninety-six-hour median lethal salinity increased from 24.8 ppt at 10 days to >32 ppt at 70 days post-hatching. Tolerance improved markedly from 40 days post-hatching. Survival was compared for fish gradually acclimated to seawater (37 ppt) over 7 days, beginning at 11, 25 or 39 days post-hatching. Survival to 48 days post-hatching improved as acclimation to seawater was delayed, from 20.0% to 55.9% for fish beginning acclimation at 11 and 39 days post-hatching. For fish surviving acclimation to seawater, growth through 105 days post-hatching was not influenced by age at acclimation. The results demonstrated that premature transfer to seawater can impair survival in this fish, and that selection of proper transfer time, based on knowledge of ontogenetic variation in salinity tolerance, can improve survival. Long-term growth in seawater, however, was not influenced by age at transfer.