The researchers first conducted an experiment to see how juvenile rabbitfish originating from 30ppt control seawater (specific gravity of ~1.023) would cope when acclimated to 0 ppt (freshwater), 5 ppt (s.g. ~1.004), 10 ppt (s.g. ~1.008), and 20 ppt seawater (s.g. ~1.015) for eight days. All fish in 0 ppt freshwater died, but all fish in 5ppt and above survived. No surprises thus far. Saltwater aquarists who have treated fish for marine ich via hyposalinity treatment know saltwater fish can cope well with very low concentrations of salt, although this research clearly indicates that some salts are required for reef fish homeostasis.
Next, the researchers placed juvenile rabbitfish in aquariums with 5 ppt, 10 ppt, 20 ppt, and 30 ppt for six weeks. This is where it gets interesting.
Surprisingly, the fish in 10 ppt gained the most weight! This is because feeding rates increased as salinity decreased while food conversion efficiency remained relatively constant regardless of the salinity. Fish ate more food in 10 ppt waters than 20 ppt, and more in 20 ppt waters than 30 ppt. While these results may not translate to other species of reef fish, it invites the intriguing question: Can lower salinity help us to raise juvenile/captive-bred fish or fish with poor appetite?
Finally, “Plasma osmolality of fish in 20 and 30 ‰ salinity was significantly greater than fish reared at 10 or 5 ‰. A salinity of 13.95 ‰ (411.88 mOsmol/kg) was the point of isosmolality for juvenile S. guttatus.”
The findings are published in Fish Physiology and Biochemistry.