Salinity preference of postlarval brown and white shrimp (Penaeus aztecus and P. setiferus) in gradient tanks

Brown and white postlarval shrimp (Penaeus aztecus and P. setiferus) were collected at the beachfront and tested a constant low level red illumination in tanks containing salinity gradients ranging from 0 to 50 ppt and 0 to 70 ppt and in control tanks having uniform salinity. Gradients were stable for as long as 119 hours with maximum deviations in salinity of 3 ppt. Generally, final gradient salinities were identical to initial gradient salinities or differed at a given point by only 1 or 2 pt. Postlarvae of both species sought salinities lower than those generally found in the open Gulf of Mexico. It is suggested that postlarval shrimp orient to bays by utilizing natural salinity gradients that extend seaward from the estuaries. Proposed water diversion projects, such as the Texas Water Plan, would restrict or curtail fresh water inflow to the estuaries thereby altering natural gradients of both salinity and dissolved organics. This might affect the immigration of shrimp and other estuarine-dependent organisms to the estuaries. Brown shrimp postlarval distributions in gradient tanks differed significantly according to the season of the year. Spring postlarvae preferred higher salinities than those tested in the summer or fall. Field studies have shown that brown shrimp postlarvae in the spring are found in the estuaries in salinities considerably lower than those that would be expected from their distribution in laboratory gradient tanks. In contrast to findings for brown shrimp postlarvae, white shrimp postlarval distributions in gradient tanks differed seasonally only at low salinity levels. During the summer, the average median salinity difference between brown and white postlarval distributions was only 3.2 ppt. This suggests that there is less difference in salinity preference between the two species at the postlarval stage than has been suggested for larger stages of the life cycle. Experiments designed to determine whether the red illumination, acclimation salinity and temperature influence salinity preference were inconclusive. There was no evidence of any endogenous tidal rhythms for brown or white shrimp such as has been reported for postlarval pink shrimp. Occasional striking variation was noted between experiments. Further work is required to recognize the causes if this presently unexplained variability in shrimp behavior