I've bred Macrobrachium scabriculum, which also need brackish water to mature. I never had more than ~15% of any given brood reach maturity. You definitely need a freshwater hatchery for the female to release the zoea into.
I don't have pictures of my old setup, but I'll describe it as best as I can.
I used a large plastic jug, with the bottom cut off and inverted and secured to a solid wood base, with brackets to support the inverted jug, as the nursery tank. I drilled two holes in the base and inserted two hoses. The first hose/tube was attached to an air pump. The second tube was hooked to a reservoir located below the jug. A third hose/tube was hooked to the bottom of a reservoir located above the nursery jug, the tube went into the open top of the jug. On off valves were in each hose, including the air hose. I kept an adjustable spot light aimed at the side of the jug, near the top. I painted the bottom two thirds of the jug black (will explain later why). The top and bottom reservoirs had double the capacity as the nursery jug.
You'll also need a source of marine greenwater. Once you get a colony going, it is easy to maintain. This is the food for the larval shrimp.
Here's how the setup functioned:
The first hose, the air hose, went into the jug from the bottom and extended three inches into the jug, a small air stone attached to the end of the hose. The air pump ran at ~25% of capacity. This caused the bubbles to break up and agitate the water from the middle. This help to create a convection current that plumed up the middle and went down the sides to the bottom.
The second hose only went in flush to the bottom of the jug, and ended in a bucket below the jug. This was used to drain water and detritus out of the nursery.
The third hose, attached to a hole in the bottom of the top reservoir and went into the top of the jug, ending just below the normal waterline. This was used to add water to the nursery.
Here's how to operated the setup:
The nursery jug should be filled with clean healthy water from her tank. You'll need to have the tank start with good low readings of ammonia, nitrates, nitrites, etc...
You'll need to watch the mother for egg maturity. Once the mother is moved to the nursery, no feeding is allowed. A piece of perforated plastic, cut to fit the inside diameter of the jug, is used as a base for the female shrimp to walk on. Have the top reservoir filled with marine water. The bottom reservoir should be empty.
Once the majority of the eggs have hatched, removed the mother and put her back in her home tank. Next carefully remove the plastic base, gently swirl the water to keep the hatchlings from being removed with the support. Then turn on the spotlight and start adding marine water from the top reservoir, increasing the salinity at a slow steady rate. This mimics the zoea floating down stream to an estuary. After the nursery was one third above normal, start to drain the water at the rate it flows in. When the water in the top is near empty shut off the outlet.
The next step was to clean out the top tank and replace it with filtered freshwater. You want this water to be as pure and clean as possible. Use freshwater R/O, if possible. The bottom reservoir was just emptied.
The zoea are attracted to light. The spotlight is used to attract the free swimming zoea, while the nursery is mostly painted black to keep the light only visible at the top few inches of water, keeping the free swimmers from the bottom. Due to the convection flow, the discarded carapaces, excrement and dead will settle at the bottom, and be removed with the water that's drained.
Feed sparingly, only a few drops of greenwater, once or twice daily.
The second day open the drain valve so that you get a slow rate. Open the top reservoir to match the flow. Watch the water line to make sure the level stays constant. You'll also need to monitor the water level in your source reservoir and your waste reservoir.
During the night shut off the light and all water flow, leave the air pump on. In the morning turn the light on five minutes before resuming water flow.
This is essentially what you do until you see tiny versions of adult shrimp, or they all die off. Make sure to keep track of what you do for each new batch of zoea. Make small changes at a time, such as increasing the rate that the water changes from saltwater to freshwater.
Good luck.
