Silversides at the Grottos in Cayman are a combination of different species of anchovies, sardines, herring and other similar fish.
They are benthic egg layers (deposit them on the sea floor, most likely in the seagrass beds). Once the eggs hatch they are released into the water column. The larvae spend their time floating around in the water until they are large enough to start forming schools...this is when they tend to migrate closest to the surface and find shelter.
This is what we see, it is their juvenile stage.
The ones that survive to adulthood, swim out to form huge shoals offshore.