Effects of alkalinity and water hardness on survival, growth and ionoregulation of the pink shrimp Farfantepenaeus paulensis (Pérez-Farfante, 1967), grown at low salinity
In order to evaluate the effects of alkalinity and water hardness on ionoregulation, survival and growth of the pink shrimp Farfantepenaeus paulensis cultivated at low salinity, two experiments were carried out at the Marine and Aquaculture Station (EMA) - FURG. In Experiment 1, in the short term (96h), hemolymphatic survival and ionoregulation (Na +, K +, Ca2 +, Mg2 + and Cl-) were tested in juveniles of F. paulensis at different salinities (10, 5, 2, 1 and 0, 5) and in 5 treatments: no addition of reagent salts in the culture medium (control), addition of CaSO4 to maintain calcium hardness at levels similar to that observed in salinity 10 (calcium hardness), addition of MgSO4 to maintain hardness magnesium at levels similar to that observed at salinity 10 (magnesium hardness),addition of CaSO4 and MgSO4 to maintain calcium and magnesium hardnesses similar to those seen in salinity 10 and addition of Na2CO3, CaSO4 and MgSO4 to maintain alkalinity and calcium and magnesium hardnesses at levels similar to those observed in salinity 10. In Experiment 2 , the survival, growth and ionoregulation of juveniles of F. paulensis were tested in the long term (30 days) using the same treatments mentioned above and in different salinities (10, 5, 2 and 1). The water variables (temperature, dissolved oxygen, pH, total ammonia, alkalinity and ionic composition) were monitored in both experiments. The results obtained demonstrated that the addition of mineral salts (CaSO4, MgSO4 and Na2CO3) in the water of cultivation significantly increased the tolerance of juveniles of F.exposed to low salinity for a short period of time (96 h). It was also observed that F. paulensis juveniles do not tolerate long periods of exposure to salinities below 2, suggesting that this species does not maintain its physiological ion-regulation functions in perfect conditions at such low salinities, even with the supply of mineral salts to cultivation means. Despite this, the results obtained clearly indicated that the alkalinity and hardness of the water have an important influence on the growth of F. paulensis until salinity 2, since maintaining high levels of these parameters allows the prawns to better maintain the hemolymphatic ion-regulation . According to the conditions of the present study and taking into account the physiology of F. paulensis,it is possible to improve the survival and growth rates of this species in crops with low salinity, provided that some techniques of adding mineral salts to the water of the cultivation ponds are adopted.