This is my save for my fortress set to trap giant sperm whales. The tanks work, I have confirmed it.

There are four levers by the tanks. One to open the outer flood gates. One to open the hatch covers. And one to open the inner floodgates. When you drop the hatch covers, these inner floodgates need to be closed or you will not have enough water from the top to fill the bottom and your whales will -all- die.

The last lever is unassigned. It is to remotely open cages placed in the upper tank after it is flooded.

The bridges to the outside are currently closed. There is lots of food for the dwarves, but the sea and land are fished and hunted out. There are also lots of animals you will need to kill to clear them from the population. These were mostly to pass the time while I prepared this.