Influence of salinity on survival, growth, plasma osmolality and gill Na+–K+–ATPase activity in the rabbitfish Siganus rivulatus

Rabbitfish (Siganidae) are an Indo-Pacific family of teleosts considered very suitable for aquaculture. The rabbitfish Siganus rivulatus has established populations in the Eastern Mediterranean and in now part of the commercial fishery in some Mediterranean countries. In the present work, we studied the salinity tolerance of S. rivulatus and the effect of various salinities on plasma osmolality and gill Na+–K+–ATPase activity (NKA). Three salinity experiments were performed. In the first experiment, fish were maintained at nine salinities between 10 ppt and 50 ppt for 3 weeks and survival and plasma osmolality evaluated. In the second experiment fish were maintained at salinities of 25, 30, 35 and 40 ppt for 6 weeks to study survival, growth, blood osmolality and gill NKA. In the third experiment, fish were maintained at salinities of 10, 15, 20 and 25 ppt for 8 weeks and again survival, growth, blood osmolality and gill NKA were estimated. Results of the first experiment show that S. rivulatus can survive for 3 weeks at salinities ranging from 10 ppt to 50 ppt and that they can maintain a relatively stable blood osmolality (between 398 and 435 mmol kg− 1). In the second experiment we observed no differences in survival and growth of S. rivulatus in all treatments. However, gill NKA was lowest at 35 ppt and increased at salinities above and below 35 ppt. In the third experiment, survival was similar at all salinity treatments but growth decreased slightly at 10 ppt. Gill NKA increased as salinity decreased. Results show that S. rivulatus is a highly euryhaline fish and a very strong osmoregulator. Furthermore, although iso-osmolality is at 14.6 ppt, NKA activity is lowest at 35 ppt, which is the salinity in the Indo-Pacific where the fish evolved but not in the Mediterranean where juveniles for the present experiment were collected. Thus, we conclude that 35 ppt is the optimum salinity for S. rivulatus.