ONTOGENY OF OSMOREGULATORY STRUCTURES AND SALINITY TOLERANCE IN ATLANTIC CROAKER Micropogonias undulates

Atlantic croaker is a popular  live bait for many game fish in the Northern Gulf of Mexico .  Aquaculture of this species could supply for demand, particularly during the fall and winter seasons when th e  availability of  wild caught  croaker in the desired size range for bait is limited .  Hatchery culture  is  currently  performed at 30 psu, consistent with conditions encountered by wild croaker larvae during their first few weeks of life . This protocol is challenging in areas where high salinity water is not readily available such as the Mississippi Gulf coast . The purpose of this study i s to examine the  ontogeny of  salinity tolerance and assess the potential for low salinity culture during the larval period .

Broodstock  used in the experiment  were  conditioned  at 30 psu and induced for final maturation and spawning using LHRH slow release implants .  Newly hatched larvae were stocked in 6 1,000- L tanks for larval rearing  at a stocking density of 30 larvae/L . Larval culture was initiated at a salinity of  30 psu .  Salinity was  gradually  lowered to 15 psu  starting  at 22 dph for 3 larval tanks (LS group) while the remaining 3 tanks (HS group) remained at 30 psu until 68 dph. Groups of 30 l arvae were sampled at 1, 5, 12, 18, and 25 dph  and directly transferred  for salinity challenge  to 1-L beakers  filled with  2, 10, 20, or 30 psu water(3 replicate beakers per treatment) . Beakers were assessed for larval mortalities every 6 hours for  up to  3 days, or until all larvae died.  

S urvival rates through larval rearing did not differ significantly between the LS and HS groups (p = 0.44). Larvae were longer in the LS group beginning at 5 dph. There was a significant effect of age, salinity, and the interaction  between these two factors on survival duration post transfer during salinity challenges (Fig. 1) . Older larvae (12-25 dph groups) survived longer. S urvival duration was  shortest at 2 psu  and  longest at 10 psu (iso-osmotic conditions).  The transfer of 25 dph larvae to 10, 20 or 30 psu led to similar survival durations.  While these  results suggest  that  larval  viability may be improved at 10 psu,  the  actual survival in culture tanks may be reduced due to negative buoyancy at that salinity.  Further study of buoyancy and feeding at low salinity is warranted. Larvae from the study  were processed  for immunohistochemistry  to reveal chloride cells through detection of the Na/K-ATPase enzyme and describe their distribution and abundance in different tissues and organs.