If salinity in excess of 6 parts per thousand kills them, I wonder if they could be controlled by seeding the beds with rock salt?? Theoretically the large, heavy rock salt crystals should sink quickly, with minimal dissolving on the way down; then dissolve on the bottom, creating a dense bottom layer with salinity well over 6 parts per thousand; then gradually diffuse into the overlying water, causing negligible increase in salinity to the main water mass?? What do you think? Paul M.