Life in and around Conondale National Park

You mostly find adults and juveniles like the above video.  However, it is common to see juveniles, often in large numbers, but I have never seen newly hatched larvae in the wild. I suspect that they are located along the stream edges that tend to have the lowest water velocities.  Often these habitats are temporary during the wet season and by the time these habitat start to dry out the fish are juvenile and capable of swimming resonable well.  I have often seen eggs of rainbowfishes in the wild spawned in amongst the plants. 

A major feature of the rainbowfish populations in the wet–dry tropics of northern Australia is the large dispersal migrations that occur at the beginning and towards the end of the wet season. At the start of flow in the wet season, fish move from dry season refuges to occupy newly-inundated river channels and floodplain habitats. Most species also spawn around this time. In the later half of the wet season, fish migrate from seasonal floodplains and shallow billabongs and move upstream, recolonising refuges for the dry season.

Migration between discrete habitats is a natural process for most rainbowfishes. Rainbowfishes migrate up and downstream and between different habitats at particular stages of their lifecycle. Migrations are commonly associated with adult spawning migration and juvenile growth dispersal; or recolonising habitats in response to changing condition during flood or drought. Dry creek beds become inundated by major over-bank flood flows with seasonal rains. These flood flows quickly dissipate, leaving a series of unconnected pools. Consequently, the rainbowfishes that use these creeks have become adapted to swimming upstream during flood flows (taking advantage of the width of the stream, and the reduced velocities found at the margins) to spawn. Their progeny are carried back downstream by the receding floodwaters. However, a small percentage of both adults and progeny inevitably become trapped within the residual pools.

Below is an image of M. nigrans juvenile migrating upstream at the end of the wet seeason.