A procedure for adjusting Grey mullet (Mugil cephalus Lin.) fingerlings to low-salinity, low-hardness waters for economic and environmentally friendly inland culture

Grey mullet fingerlings were grown in different acclimation solutions for three consecutive years, with the aim of developing a procedure for their adjustment to commercial growth in very low salinity (EC = 0.37 dS/m) and Cl−/Na+-deficient inland water (Dan River water, Northern Israel). Tested acclimation conditions composed of simulative solutions of varying ionic compositions (EC range 1.0-1.1 dS/m) and a solution made up by dissolving quarry dolomite beads into the target (Dan River) water by H2SO4 without and with a small NaCl addition (1.5 mM). The latter alternative was favored because of its low production costs and minimal environmental impact (minute addition of extrinsic Na+ and Cl− ions to the environment). The results obtained in the three experimental sets showed that stocking the fingerlings in a ∼1 dS/m acclimation solution for 1 week, followed by 20% daily dilution by the target water, was appropriate for attaining survival rates higher than 70% in all acclimation solutions tested (growth periods varied in the range 43-77 d). Survival rate in brackish water control experiments was 85-95%. In the absence of an acclimation period the fish survival rate was ∼35%. No significant difference was observed in fish growth rates between treatments conducted in a given year, suggesting that fish that survived the acclimation period would grow in the low salinity water at a rate similar to fish grown in brackish water. The minor differences in survival and growth results between the different acclimation solutions seemed to indicate that the role of specific ionic constituents in the water was inconsequential and that the most significant parameter is the overall water salinity. However, histological analyses revealed that applying the acclimation period with very low Cl− and Na+ concentrations (<∼75 mg/L and ∼45 mg/L, respectively) caused non-reversible adverse effects on the fish, whereas in all of the other treatments recorded adverse effects were of a reversible nature. Considering all the results, the best acclimation solution was defined as dissolution of dolomite by H2SO4 into the target low-salinity water plus addition of 1.5 mM of NaCl. The cost of producing the acclimation solution was shown to amount to only ∼0.03% of the overall culture cost.