Having to separate freshwater crabs for commercial fisherman while on the boat, we dump the crabs in ice water, as crabs aren't as aggressive and bite less when cold. There's only so much room on the boat for carrying ice, and the ice melts too quickly in the wooden box filled with water and ice in order to "slush" the crabs. We've been unable to work full days, running all of our crab traps, as we run out of ice before a full-day's work. Any ideas of how to slow the melting process of the ice while in the water used for slushing the crabs? Thanks for your time and knowledge. "
Unfortunately there isn't much you can do to slow down the melting of the ice-water slush besides insulating your container. If possible, you can enclose the wooden box in styrofoam to prevent heat from entering the system.
Another note you should keep in mind is that the water in the slush speeds up the melting of the ice. A better scenario would be to keep a large (minimized surface area) block of ice in one container, and the slush in another. Use an ice pick to break up the block and add pieces to the slush as needed.